Switch to reset charge amplifier

Started by Geoffrey April 3, 2012
Hi there

I have a charge amplifier, used to amplify the signal from a piezo
electric transducer, with 2nF of reference capacitance shunted with
1GOhm. I need to be able to reset the amplifier, preferably with a
solid state device (the circuit has to be very compact, and operate in
a high-vibration environment). One possibility which comes to mind is
to simultaneously short the amplifier output and input to ground. The
input is the inverting input of the amplifier, and is nominally at the
same ground potential as the non inverting input. I am considering the
use of a mosfet as the shorting switch for the input. When the mosfet
is off, its gate will be at ground potential, as will its drain
(virtual ground) and source. Is there a mechanism whereby a leakage
current could still flow from drain to source? Every nanoampere of IDS
gives 1V of offset at the output, and it should be noted that the
circuit must work at 120 deg C.

Any help appreciated.

Best regards
Geoff
Geoffrey wrote:
> > Hi there > > I have a charge amplifier, used to amplify the signal from a piezo > electric transducer, with 2nF of reference capacitance shunted with > 1GOhm. I need to be able to reset the amplifier, preferably with a > solid state device (the circuit has to be very compact, and operate in > a high-vibration environment). One possibility which comes to mind is > to simultaneously short the amplifier output and input to ground. The > input is the inverting input of the amplifier, and is nominally at the > same ground potential as the non inverting input. I am considering the > use of a mosfet as the shorting switch for the input. When the mosfet > is off, its gate will be at ground potential, as will its drain > (virtual ground) and source. Is there a mechanism whereby a leakage > current could still flow from drain to source? Every nanoampere of IDS > gives 1V of offset at the output, and it should be noted that the > circuit must work at 120 deg C. > > Any help appreciated. > > Best regards > Geoff
One cute trick for this is to invert the power supplies to the op amp--with suitable current limiting resistors, of course--and let the ESD protection diodes form a diode bridge that connects the output to both the inputs. (I forget where I heard of that one--I certainly didn't invent it.) Op amps obviously aren't specified for this, so you'll have to test yours or ask the apps folks. (They probably won't know either, but it's worth a try.) Also the gigohm resistor probably doesn't help you at 120C--50 pA of bias current puts you in the shot noise limit. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Geoffrey wrote:
> Hi there > > I have a charge amplifier, used to amplify the signal from a piezo > electric transducer, with 2nF of reference capacitance shunted with > 1GOhm. I need to be able to reset the amplifier, preferably with a > solid state device (the circuit has to be very compact, and operate in > a high-vibration environment). One possibility which comes to mind is > to simultaneously short the amplifier output and input to ground. The > input is the inverting input of the amplifier, and is nominally at the > same ground potential as the non inverting input. I am considering the > use of a mosfet as the shorting switch for the input. When the mosfet > is off, its gate will be at ground potential, as will its drain > (virtual ground) and source. Is there a mechanism whereby a leakage > current could still flow from drain to source? Every nanoampere of IDS > gives 1V of offset at the output, and it should be noted that the > circuit must work at 120 deg C. > > Any help appreciated. > > Best regards > Geoff
Doesn't your amp have a reset input that can be controlled by a logic level? Oh, come to think of it, our Kistler amps need a switch closure. And I think some of the newer hideous ones omit this feature... I'd seriously consider a reed relay to avoid any leakage or charge injection weirdness from using a MOSFET. Then again, a MOSFET might work. It'd take some poring over specs, then going home to dinner and wondering what they mean, then having the lightbulb go in the shower the next morning. You know what I mean? If I go to a job interview, how am I going to explain the way I do design? -- _____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17
Am 03.04.2012 16:53, schrieb Geoffrey:
> Hi there > > I have a charge amplifier, used to amplify the signal from a piezo > electric transducer, with 2nF of reference capacitance shunted with > 1GOhm. I need to be able to reset the amplifier, preferably with a > solid state device (the circuit has to be very compact, and operate in > a high-vibration environment).
a couple of years ago I have seen a charge amplifier for X-Ray detectors which used the FET in the input stage to reset the integrator in a tricky way. The lid of the FET's metal can (I think it was a 2N4416) was was opened. To reset the integrator the crystal of the FET was lit by a pulsed LED until Vout was zero. Cheers, Alexander
On Apr 3, 11:53 pm, "Mr.CRC" <crobcBO...@REMOVETHISsbcglobal.net>
wrote:
> Geoffrey wrote: > > Hi there > > > I have a charge amplifier, used to amplify the signal from a piezo > > electric transducer, with 2nF of reference capacitance shunted with > > 1GOhm. I need to be able to reset the amplifier, preferably with a > > solid state device (the circuit has to be very compact, and operate in > > a high-vibration environment). One possibility which comes to mind is > > to simultaneously short the amplifier output and input to ground. The > > input is the inverting input of the amplifier, and is nominally at the > > same ground potential as the non inverting input. I am considering the > > use of a mosfet as the shorting switch for the input. When the mosfet > > is off, its gate will be at ground potential, as will its drain > > (virtual ground) and source. Is there a mechanism whereby a leakage > > current could still flow from drain to source? Every nanoampere of IDS > > gives 1V of offset at the output, and it should be noted that the > > circuit must work at 120 deg C. > > > Any help appreciated. > > > Best regards > > Geoff > > Doesn't your amp have a reset input that can be controlled by a logic > level?  Oh, come to think of it, our Kistler amps need a switch closure. >  And I think some of the newer hideous ones omit this feature... > > I'd seriously consider a reed relay to avoid any leakage or charge > injection weirdness from using a MOSFET.  Then again, a MOSFET might > work.  It'd take some poring over specs, then going home to dinner and > wondering what they mean, then having the lightbulb go in the shower the > next morning.  You know what I mean? > >
"If I go to a job interview, how am I going to explain the way I do design?" No problem just be honest. "Well I think this is the way I would solve that problem, but it usually takes me a day or two to process things. Can I drop you an email if I have a better idea in the shower tomorrow morning?" George H.
> > -- > _____________________ > Mr.CRC > crobcBO...@REMOVETHISsbcglobal.net > SuSE 10.3 Linux 2.6.22.17- Hide quoted text - > > - Show quoted text -
George Herold wrote:
> On Apr 3, 11:53 pm, "Mr.CRC" <crobcBO...@REMOVETHISsbcglobal.net> > wrote: >> I'd seriously consider a reed relay to avoid any leakage or charge >> injection weirdness from using a MOSFET. Then again, a MOSFET might >> work. It'd take some poring over specs, then going home to dinner and >> wondering what they mean, then having the lightbulb go in the shower the >> next morning. You know what I mean? >> >> > "If I go to a job interview, how am I going to explain the way I do > design?" > > No problem just be honest. "Well I think this is the way I would > solve that problem, but it usually takes me a day or two to process > things. Can I drop you an email if I have a better idea in the shower > tomorrow morning?" > > George H.
Oh I see I omitted a word. What if what I meant to say was: "then having the lightbulb go *out* in the shower the next morning?" Heh heh. -- _____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17
On Wed, 4 Apr 2012 08:40:41 -0700 (PDT) George Herold
<gherold@teachspin.com> wrote in Message id:
<ca148295-6632-48e3-8cc6-76ec6ec0f8b4@w5g2000vbv.googlegroups.com>:

>On Apr 3, 11:53 pm, "Mr.CRC" <crobcBO...@REMOVETHISsbcglobal.net> >wrote: >> Geoffrey wrote: >> > Hi there >> >> > I have a charge amplifier, used to amplify the signal from a piezo >> > electric transducer, with 2nF of reference capacitance shunted with >> > 1GOhm. I need to be able to reset the amplifier, preferably with a >> > solid state device (the circuit has to be very compact, and operate in >> > a high-vibration environment). One possibility which comes to mind is >> > to simultaneously short the amplifier output and input to ground. The >> > input is the inverting input of the amplifier, and is nominally at the >> > same ground potential as the non inverting input. I am considering the >> > use of a mosfet as the shorting switch for the input. When the mosfet >> > is off, its gate will be at ground potential, as will its drain >> > (virtual ground) and source. Is there a mechanism whereby a leakage >> > current could still flow from drain to source? Every nanoampere of IDS >> > gives 1V of offset at the output, and it should be noted that the >> > circuit must work at 120 deg C. >> >> > Any help appreciated. >> >> > Best regards >> > Geoff >> >> Doesn't your amp have a reset input that can be controlled by a logic >> level?  Oh, come to think of it, our Kistler amps need a switch closure. >>  And I think some of the newer hideous ones omit this feature... >> >> I'd seriously consider a reed relay to avoid any leakage or charge >> injection weirdness from using a MOSFET.  Then again, a MOSFET might >> work.  It'd take some poring over specs, then going home to dinner and >> wondering what they mean, then having the lightbulb go in the shower the >> next morning.  You know what I mean? >> >> > "If I go to a job interview, how am I going to explain the way I do >design?" > >No problem just be honest. "Well I think this is the way I would >solve that problem, but it usually takes me a day or two to process >things. Can I drop you an email if I have a better idea in the shower >tomorrow morning?"
Sure, but wait till you get out of the shower first.
On Apr 4, 1:59 am, Alexander <inva...@invalid.invalid> wrote:
> Am 03.04.2012 16:53, schrieb Geoffrey: > > > Hi there > > > I have a charge amplifier, used to amplify the signal from a piezo > > electric transducer, with 2nF of reference capacitance shunted with > > 1GOhm. I need to be able to reset the amplifier, preferably with a > > solid state device (the circuit has to be very compact, and operate in > > a high-vibration environment). > > a couple of years ago I have seen a charge amplifier for X-Ray detectors > which used the FET in the input stage to reset the integrator in a > tricky way. The lid of the FET's metal can (I think it was a 2N4416) was > was opened. To reset the integrator the crystal of the FET was lit by a > pulsed LED until Vout was zero. > > Cheers, > Alexander
That's a neat idea! If the signal is unipolar the OP could put an LED across it and and reset it by shinning a second LED into the first. George H.
Many CT systems use current to frequency converters, the X-ray photodetector arrays have similar large C issues.  This way the I to v converter is always draining the charge a bit.  The IFC had a range of picoamps to 4 ma, resulting in 5 Hz to 10 Mhz, from the oscillator. This gets rid of dumping the integrator.
Picker/Phillips had the patent.

Steve 


On Apr 5, 11:31 am, o...@case.edu wrote:
> Many CT systems use current to frequency converters, the X-ray photodetector arrays have similar large C issues.  This way the I to v converter is always draining the charge a bit.  The IFC had a range of picoamps to 4 ma, resulting in 5 Hz to 10 Mhz, from the oscillator. This gets rid of dumping the integrator. > Picker/Phillips had the patent. > > Steve
My two cents for the design would be an op-amplifier with a high fedility gain for the feedback. A second stage may be included as a limiter, including the diodes in a back to back configuration for reset. eeL
On Tue, 17 Apr 2012 19:13:53 -0700, "Mr.CRC"
<crobcBOGUS@REMOVETHISsbcglobal.net> wrote:

> >>>> Teflon is not a good idea, due to microphonics. >>> Depends on how much signal he has. I'd expect an engine explosion to >>> make lots of signal. >> >> That also depends on how much gets to the sensor crystal. It can't take >> direct exposure to the combustion chamber gasses, for heat and chemical >> issues. Thus attenuation, maybe a lot of it, before the sensor crystal >> "sees" it. > > >Our engine in-cylinder pressure sensors are flush with the head, so see >the combustion chamber environment directly. I think all they have is a >thin metal foil over the sensor material surface. >
We may never know unless someone sections one. ?-)
josephkk wrote:
> On Sun, 15 Apr 2012 14:58:31 -0700, John Larkin > <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >> On Sun, 15 Apr 2012 23:15:07 +0300, Tauno Voipio >> <tauno.voipio@notused.fi.invalid> wrote: >> >>> On 15.4.12 8:05 , John Larkin wrote: >>>> On Sun, 15 Apr 2012 08:10:23 -0700, josephkk >>>> <joseph_barrett@sbcglobal.net> wrote: >>>> >>>>> On Fri, 13 Apr 2012 07:19:40 -0700 (PDT), Geoffrey >>>>> <gmortimer2003@yahoo.com> wrote: >>>>> >>>>>> On Apr 11, 2:49 am, josephkk<joseph_barr...@sbcglobal.net> wrote: >>>>>>> On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> <gmortimer2...@yahoo.com> wrote: >>>>>>>> Hi there >>>>>>>> I have a charge amplifier, used to amplify the signal from a piezo >>>>>>>> electric transducer, with 2nF of reference capacitance shunted with >>>>>>>> 1GOhm. I need to be able to reset the amplifier, preferably with a >>>>>>>> solid state device (the circuit has to be very compact, and operate in >>>>>>>> a high-vibration environment). One possibility which comes to mind is >>>>>>>> to simultaneously short the amplifier output and input to ground. The >>>>>>>> input is the inverting input of the amplifier, and is nominally at the >>>>>>>> same ground potential as the non inverting input. I am considering the >>>>>>>> use of a mosfet as the shorting switch for the input. When the mosfet >>>>>>>> is off, its gate will be at ground potential, as will its drain >>>>>>>> (virtual ground) and source. Is there a mechanism whereby a leakage >>>>>>>> current could still flow from drain to source? Every nanoampere of IDS >>>>>>>> gives 1V of offset at the output, and it should be noted that the >>>>>>>> circuit must work at 120 deg C. >>>>>>>> Any help appreciated. >>>>>>>> Best regards >>>>>>>> Geoff >>>>>>> As a test engineer at an earlier time i am troubled by the conditions you >>>>>>> are subjecting your charge amplifier to. What constraints do you have >>>>>>> that you cannot put your charge amplifier in a benign location, away from >>>>>>> the heat and vibration? >>>>>>> >>>>>>> ?-)- Hide quoted text - >>>>>>> >>>>>>> - Show quoted text - >>>>>> The amplifier will be placed in the Vee of a motor racing engine, the >>>>>> piezo sensor being used to measure the pressure in the combustion >>>>>> chamber. It is possible that a solution involving longer cables could >>>>>> be found, but there are high levels of EMI due to ignition systems and >>>>>> other power electronics. The temperature is a bit of a bother - few >>>>>> charge amps are specified at 125 degrees due to the high leakage and >>>>>> bias currents at that temperature. >>>>> I was wondering. There is no need to subject the charge amplifier to that >>>>> environment. It belongs in the driver compartment. Far less heat, and a >>>>> bit less vibration. Of course you are paying about US$18/ft for the >>>>> special shielded cable for either placement. (Shake and bake >>>>> piezoelectric accelerometer cable from Endevco[, and perhaps Ling or >>>>> Umhholz-Dickie].) >>>>> >>>>> ?-) >>>> What's special about piezo cable? Any teflon coax, or semi-hardline >>>> for super shielding, would work. He'll probably have lots of signal. >>>> >>>> >>> Teflon is not a good idea, due to microphonics. >> Depends on how much signal he has. I'd expect an engine explosion to >> make lots of signal. > > That also depends on how much gets to the sensor crystal. It can't take > direct exposure to the combustion chamber gasses, for heat and chemical > issues. Thus attenuation, maybe a lot of it, before the sensor crystal > "sees" it.
Our engine in-cylinder pressure sensors are flush with the head, so see the combustion chamber environment directly. I think all they have is a thin metal foil over the sensor material surface. -- _____________________ Mr.CRC crobcBOGUS@REMOVETHISsbcglobal.net SuSE 10.3 Linux 2.6.22.17
On Sun, 15 Apr 2012 14:58:31 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 15 Apr 2012 23:15:07 +0300, Tauno Voipio ><tauno.voipio@notused.fi.invalid> wrote: > >>On 15.4.12 8:05 , John Larkin wrote: >>> On Sun, 15 Apr 2012 08:10:23 -0700, josephkk >>> <joseph_barrett@sbcglobal.net> wrote: >>> >>>> On Fri, 13 Apr 2012 07:19:40 -0700 (PDT), Geoffrey >>>> <gmortimer2003@yahoo.com> wrote: >>>> >>>>> On Apr 11, 2:49 am, josephkk<joseph_barr...@sbcglobal.net> wrote: >>>>>> On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> <gmortimer2...@yahoo.com> wrote: >>>>>>> Hi there >>>>>> >>>>>>> I have a charge amplifier, used to amplify the signal from a piezo >>>>>>> electric transducer, with 2nF of reference capacitance shunted with >>>>>>> 1GOhm. I need to be able to reset the amplifier, preferably with a >>>>>>> solid state device (the circuit has to be very compact, and operate in >>>>>>> a high-vibration environment). One possibility which comes to mind is >>>>>>> to simultaneously short the amplifier output and input to ground. The >>>>>>> input is the inverting input of the amplifier, and is nominally at the >>>>>>> same ground potential as the non inverting input. I am considering the >>>>>>> use of a mosfet as the shorting switch for the input. When the mosfet >>>>>>> is off, its gate will be at ground potential, as will its drain >>>>>>> (virtual ground) and source. Is there a mechanism whereby a leakage >>>>>>> current could still flow from drain to source? Every nanoampere of IDS >>>>>>> gives 1V of offset at the output, and it should be noted that the >>>>>>> circuit must work at 120 deg C. >>>>>> >>>>>>> Any help appreciated. >>>>>> >>>>>>> Best regards >>>>>>> Geoff >>>>>> >>>>>> As a test engineer at an earlier time i am troubled by the conditions you >>>>>> are subjecting your charge amplifier to. What constraints do you have >>>>>> that you cannot put your charge amplifier in a benign location, away from >>>>>> the heat and vibration? >>>>>> >>>>>> ?-)- Hide quoted text - >>>>>> >>>>>> - Show quoted text - >>>>> >>>>> The amplifier will be placed in the Vee of a motor racing engine, the >>>>> piezo sensor being used to measure the pressure in the combustion >>>>> chamber. It is possible that a solution involving longer cables could >>>>> be found, but there are high levels of EMI due to ignition systems and >>>>> other power electronics. The temperature is a bit of a bother - few >>>>> charge amps are specified at 125 degrees due to the high leakage and >>>>> bias currents at that temperature. >>>> >>>> I was wondering. There is no need to subject the charge amplifier to that >>>> environment. It belongs in the driver compartment. Far less heat, and a >>>> bit less vibration. Of course you are paying about US$18/ft for the >>>> special shielded cable for either placement. (Shake and bake >>>> piezoelectric accelerometer cable from Endevco[, and perhaps Ling or >>>> Umhholz-Dickie].) >>>> >>>> ?-) >>> >>> What's special about piezo cable? Any teflon coax, or semi-hardline >>> for super shielding, would work. He'll probably have lots of signal. >>> >>> >> >>Teflon is not a good idea, due to microphonics. > >Depends on how much signal he has. I'd expect an engine explosion to >make lots of signal.
That also depends on how much gets to the sensor crystal. It can't take direct exposure to the combustion chamber gasses, for heat and chemical issues. Thus attenuation, maybe a lot of it, before the sensor crystal "sees" it. ?-)
On Sun, 15 Apr 2012 23:15:07 +0300, Tauno Voipio
<tauno.voipio@notused.fi.invalid> wrote:

>On 15.4.12 8:05 , John Larkin wrote: >> On Sun, 15 Apr 2012 08:10:23 -0700, josephkk >> <joseph_barrett@sbcglobal.net> wrote: >> >>> On Fri, 13 Apr 2012 07:19:40 -0700 (PDT), Geoffrey >>> <gmortimer2003@yahoo.com> wrote: >>> >>>> On Apr 11, 2:49 am, josephkk<joseph_barr...@sbcglobal.net> wrote: >>>>> On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> <gmortimer2...@yahoo.com> wrote: >>>>>> Hi there >>>>> >>>>>> I have a charge amplifier, used to amplify the signal from a piezo >>>>>> electric transducer, with 2nF of reference capacitance shunted with >>>>>> 1GOhm. I need to be able to reset the amplifier, preferably with a >>>>>> solid state device (the circuit has to be very compact, and operate in >>>>>> a high-vibration environment). One possibility which comes to mind is >>>>>> to simultaneously short the amplifier output and input to ground. The >>>>>> input is the inverting input of the amplifier, and is nominally at the >>>>>> same ground potential as the non inverting input. I am considering the >>>>>> use of a mosfet as the shorting switch for the input. When the mosfet >>>>>> is off, its gate will be at ground potential, as will its drain >>>>>> (virtual ground) and source. Is there a mechanism whereby a leakage >>>>>> current could still flow from drain to source? Every nanoampere of IDS >>>>>> gives 1V of offset at the output, and it should be noted that the >>>>>> circuit must work at 120 deg C. >>>>> >>>>>> Any help appreciated. >>>>> >>>>>> Best regards >>>>>> Geoff >>>>> >>>>> As a test engineer at an earlier time i am troubled by the conditions you >>>>> are subjecting your charge amplifier to. What constraints do you have >>>>> that you cannot put your charge amplifier in a benign location, away from >>>>> the heat and vibration? >>>>> >>>>> ?-)- Hide quoted text - >>>>> >>>>> - Show quoted text - >>>> >>>> The amplifier will be placed in the Vee of a motor racing engine, the >>>> piezo sensor being used to measure the pressure in the combustion >>>> chamber. It is possible that a solution involving longer cables could >>>> be found, but there are high levels of EMI due to ignition systems and >>>> other power electronics. The temperature is a bit of a bother - few >>>> charge amps are specified at 125 degrees due to the high leakage and >>>> bias currents at that temperature. >>> >>> I was wondering. There is no need to subject the charge amplifier to that >>> environment. It belongs in the driver compartment. Far less heat, and a >>> bit less vibration. Of course you are paying about US$18/ft for the >>> special shielded cable for either placement. (Shake and bake >>> piezoelectric accelerometer cable from Endevco[, and perhaps Ling or >>> Umhholz-Dickie].) >>> >>> ?-) >> >> What's special about piezo cable? Any teflon coax, or semi-hardline >> for super shielding, would work. He'll probably have lots of signal. >> >> > >Teflon is not a good idea, due to microphonics.
Depends on how much signal he has. I'd expect an engine explosion to make lots of signal. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On 15.4.12 8:05 , John Larkin wrote:
> On Sun, 15 Apr 2012 08:10:23 -0700, josephkk > <joseph_barrett@sbcglobal.net> wrote: > >> On Fri, 13 Apr 2012 07:19:40 -0700 (PDT), Geoffrey >> <gmortimer2003@yahoo.com> wrote: >> >>> On Apr 11, 2:49 am, josephkk<joseph_barr...@sbcglobal.net> wrote: >>>> On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey >>>> >>>> >>>> >>>> >>>> >>>> <gmortimer2...@yahoo.com> wrote: >>>>> Hi there >>>> >>>>> I have a charge amplifier, used to amplify the signal from a piezo >>>>> electric transducer, with 2nF of reference capacitance shunted with >>>>> 1GOhm. I need to be able to reset the amplifier, preferably with a >>>>> solid state device (the circuit has to be very compact, and operate in >>>>> a high-vibration environment). One possibility which comes to mind is >>>>> to simultaneously short the amplifier output and input to ground. The >>>>> input is the inverting input of the amplifier, and is nominally at the >>>>> same ground potential as the non inverting input. I am considering the >>>>> use of a mosfet as the shorting switch for the input. When the mosfet >>>>> is off, its gate will be at ground potential, as will its drain >>>>> (virtual ground) and source. Is there a mechanism whereby a leakage >>>>> current could still flow from drain to source? Every nanoampere of IDS >>>>> gives 1V of offset at the output, and it should be noted that the >>>>> circuit must work at 120 deg C. >>>> >>>>> Any help appreciated. >>>> >>>>> Best regards >>>>> Geoff >>>> >>>> As a test engineer at an earlier time i am troubled by the conditions you >>>> are subjecting your charge amplifier to. What constraints do you have >>>> that you cannot put your charge amplifier in a benign location, away from >>>> the heat and vibration? >>>> >>>> ?-)- Hide quoted text - >>>> >>>> - Show quoted text - >>> >>> The amplifier will be placed in the Vee of a motor racing engine, the >>> piezo sensor being used to measure the pressure in the combustion >>> chamber. It is possible that a solution involving longer cables could >>> be found, but there are high levels of EMI due to ignition systems and >>> other power electronics. The temperature is a bit of a bother - few >>> charge amps are specified at 125 degrees due to the high leakage and >>> bias currents at that temperature. >> >> I was wondering. There is no need to subject the charge amplifier to that >> environment. It belongs in the driver compartment. Far less heat, and a >> bit less vibration. Of course you are paying about US$18/ft for the >> special shielded cable for either placement. (Shake and bake >> piezoelectric accelerometer cable from Endevco[, and perhaps Ling or >> Umhholz-Dickie].) >> >> ?-) > > What's special about piezo cable? Any teflon coax, or semi-hardline > for super shielding, would work. He'll probably have lots of signal. > >
Teflon is not a good idea, due to microphonics.
On Sun, 15 Apr 2012 08:10:23 -0700, josephkk
<joseph_barrett@sbcglobal.net> wrote:

>On Fri, 13 Apr 2012 07:19:40 -0700 (PDT), Geoffrey ><gmortimer2003@yahoo.com> wrote: > >>On Apr 11, 2:49 am, josephkk <joseph_barr...@sbcglobal.net> wrote: >>> On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey >>> >>> >>> >>> >>> >>> <gmortimer2...@yahoo.com> wrote: >>> >Hi there >>> >>> >I have a charge amplifier, used to amplify the signal from a piezo >>> >electric transducer, with 2nF of reference capacitance shunted with >>> >1GOhm. I need to be able to reset the amplifier, preferably with a >>> >solid state device (the circuit has to be very compact, and operate in >>> >a high-vibration environment). One possibility which comes to mind is >>> >to simultaneously short the amplifier output and input to ground. The >>> >input is the inverting input of the amplifier, and is nominally at the >>> >same ground potential as the non inverting input. I am considering the >>> >use of a mosfet as the shorting switch for the input. When the mosfet >>> >is off, its gate will be at ground potential, as will its drain >>> >(virtual ground) and source. Is there a mechanism whereby a leakage >>> >current could still flow from drain to source? Every nanoampere of IDS >>> >gives 1V of offset at the output, and it should be noted that the >>> >circuit must work at 120 deg C. >>> >>> >Any help appreciated. >>> >>> >Best regards >>> >Geoff >>> >>> As a test engineer at an earlier time i am troubled by the conditions you >>> are subjecting your charge amplifier to.  What constraints do you have >>> that you cannot put your charge amplifier in a benign location, away from >>> the heat and vibration? >>> >>> ?-)- Hide quoted text - >>> >>> - Show quoted text - >> >>The amplifier will be placed in the Vee of a motor racing engine, the >>piezo sensor being used to measure the pressure in the combustion >>chamber. It is possible that a solution involving longer cables could >>be found, but there are high levels of EMI due to ignition systems and >>other power electronics. The temperature is a bit of a bother - few >>charge amps are specified at 125 degrees due to the high leakage and >>bias currents at that temperature. > >I was wondering. There is no need to subject the charge amplifier to that >environment. It belongs in the driver compartment. Far less heat, and a >bit less vibration. Of course you are paying about US$18/ft for the >special shielded cable for either placement. (Shake and bake >piezoelectric accelerometer cable from Endevco[, and perhaps Ling or >Umhholz-Dickie].) > >?-)
What's special about piezo cable? Any teflon coax, or semi-hardline for super shielding, would work. He'll probably have lots of signal. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Fri, 13 Apr 2012 07:19:40 -0700 (PDT), Geoffrey
<gmortimer2003@yahoo.com> wrote:

>On Apr 11, 2:49 am, josephkk <joseph_barr...@sbcglobal.net> wrote: >> On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey >> >> >> >> >> >> <gmortimer2...@yahoo.com> wrote: >> >Hi there >> >> >I have a charge amplifier, used to amplify the signal from a piezo >> >electric transducer, with 2nF of reference capacitance shunted with >> >1GOhm. I need to be able to reset the amplifier, preferably with a >> >solid state device (the circuit has to be very compact, and operate in >> >a high-vibration environment). One possibility which comes to mind is >> >to simultaneously short the amplifier output and input to ground. The >> >input is the inverting input of the amplifier, and is nominally at the >> >same ground potential as the non inverting input. I am considering the >> >use of a mosfet as the shorting switch for the input. When the mosfet >> >is off, its gate will be at ground potential, as will its drain >> >(virtual ground) and source. Is there a mechanism whereby a leakage >> >current could still flow from drain to source? Every nanoampere of IDS >> >gives 1V of offset at the output, and it should be noted that the >> >circuit must work at 120 deg C. >> >> >Any help appreciated. >> >> >Best regards >> >Geoff >> >> As a test engineer at an earlier time i am troubled by the conditions you >> are subjecting your charge amplifier to.  What constraints do you have >> that you cannot put your charge amplifier in a benign location, away from >> the heat and vibration? >> >> ?-)- Hide quoted text - >> >> - Show quoted text - > >The amplifier will be placed in the Vee of a motor racing engine, the >piezo sensor being used to measure the pressure in the combustion >chamber. It is possible that a solution involving longer cables could >be found, but there are high levels of EMI due to ignition systems and >other power electronics. The temperature is a bit of a bother - few >charge amps are specified at 125 degrees due to the high leakage and >bias currents at that temperature.
I was wondering. There is no need to subject the charge amplifier to that environment. It belongs in the driver compartment. Far less heat, and a bit less vibration. Of course you are paying about US$18/ft for the special shielded cable for either placement. (Shake and bake piezoelectric accelerometer cable from Endevco[, and perhaps Ling or Umhholz-Dickie].) ?-)
On Fri, 13 Apr 2012 07:19:40 -0700 (PDT), Geoffrey
<gmortimer2003@yahoo.com> wrote:

>On Apr 11, 2:49 am, josephkk <joseph_barr...@sbcglobal.net> wrote: >> On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey >> >> >> >> >> >> <gmortimer2...@yahoo.com> wrote: >> >Hi there >> >> >I have a charge amplifier, used to amplify the signal from a piezo >> >electric transducer, with 2nF of reference capacitance shunted with >> >1GOhm. I need to be able to reset the amplifier, preferably with a >> >solid state device (the circuit has to be very compact, and operate in >> >a high-vibration environment). One possibility which comes to mind is >> >to simultaneously short the amplifier output and input to ground. The >> >input is the inverting input of the amplifier, and is nominally at the >> >same ground potential as the non inverting input. I am considering the >> >use of a mosfet as the shorting switch for the input. When the mosfet >> >is off, its gate will be at ground potential, as will its drain >> >(virtual ground) and source. Is there a mechanism whereby a leakage >> >current could still flow from drain to source? Every nanoampere of IDS >> >gives 1V of offset at the output, and it should be noted that the >> >circuit must work at 120 deg C. >> >> >Any help appreciated. >> >> >Best regards >> >Geoff >> >> As a test engineer at an earlier time i am troubled by the conditions you >> are subjecting your charge amplifier to.  What constraints do you have >> that you cannot put your charge amplifier in a benign location, away from >> the heat and vibration? >> >> ?-)- Hide quoted text - >> >> - Show quoted text - > >The amplifier will be placed in the Vee of a motor racing engine, the >piezo sensor being used to measure the pressure in the combustion >chamber. It is possible that a solution involving longer cables could >be found, but there are high levels of EMI due to ignition systems and >other power electronics. The temperature is a bit of a bother - few >charge amps are specified at 125 degrees due to the high leakage and >bias currents at that temperature.
I should think that you'd get a huge signal from that setup. Why not run well-shielded coax to a remote charge amp in a friendlier location? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Apr 11, 2:49 am, josephkk <joseph_barr...@sbcglobal.net> wrote:
> On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey > > > > > > <gmortimer2...@yahoo.com> wrote: > >Hi there > > >I have a charge amplifier, used to amplify the signal from a piezo > >electric transducer, with 2nF of reference capacitance shunted with > >1GOhm. I need to be able to reset the amplifier, preferably with a > >solid state device (the circuit has to be very compact, and operate in > >a high-vibration environment). One possibility which comes to mind is > >to simultaneously short the amplifier output and input to ground. The > >input is the inverting input of the amplifier, and is nominally at the > >same ground potential as the non inverting input. I am considering the > >use of a mosfet as the shorting switch for the input. When the mosfet > >is off, its gate will be at ground potential, as will its drain > >(virtual ground) and source. Is there a mechanism whereby a leakage > >current could still flow from drain to source? Every nanoampere of IDS > >gives 1V of offset at the output, and it should be noted that the > >circuit must work at 120 deg C. > > >Any help appreciated. > > >Best regards > >Geoff > > As a test engineer at an earlier time i am troubled by the conditions you > are subjecting your charge amplifier to.  What constraints do you have > that you cannot put your charge amplifier in a benign location, away from > the heat and vibration? > > ?-)- Hide quoted text - > > - Show quoted text -
The amplifier will be placed in the Vee of a motor racing engine, the piezo sensor being used to measure the pressure in the combustion chamber. It is possible that a solution involving longer cables could be found, but there are high levels of EMI due to ignition systems and other power electronics. The temperature is a bit of a bother - few charge amps are specified at 125 degrees due to the high leakage and bias currents at that temperature.
On Tue, 3 Apr 2012 07:53:52 -0700 (PDT), Geoffrey
<gmortimer2003@yahoo.com> wrote:

>Hi there > >I have a charge amplifier, used to amplify the signal from a piezo >electric transducer, with 2nF of reference capacitance shunted with >1GOhm. I need to be able to reset the amplifier, preferably with a >solid state device (the circuit has to be very compact, and operate in >a high-vibration environment). One possibility which comes to mind is >to simultaneously short the amplifier output and input to ground. The >input is the inverting input of the amplifier, and is nominally at the >same ground potential as the non inverting input. I am considering the >use of a mosfet as the shorting switch for the input. When the mosfet >is off, its gate will be at ground potential, as will its drain >(virtual ground) and source. Is there a mechanism whereby a leakage >current could still flow from drain to source? Every nanoampere of IDS >gives 1V of offset at the output, and it should be noted that the >circuit must work at 120 deg C. > >Any help appreciated. > >Best regards >Geoff
As a test engineer at an earlier time i am troubled by the conditions you are subjecting your charge amplifier to. What constraints do you have that you cannot put your charge amplifier in a benign location, away from the heat and vibration? ?-)