On Mon, 22 Feb 2016 12:03:39 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 02/22/2016 12:25 AM, Tim Wescott wrote: >> On Sun, 21 Feb 2016 20:46:00 -0600, ChesterW wrote: >> >>> On 2/21/16 5:49 PM, John Larkin wrote: >>>> On Sun, 21 Feb 2016 17:23:43 -0600, Tim Wescott >>>> <seemywebsite@myfooter.really> wrote: >>>> >>>>> On Sun, 21 Feb 2016 15:12:58 -0800, John Larkin wrote: >>>>> >>>>>> On Sun, 21 Feb 2016 16:22:20 -0600, ChesterW <iamsnoozin@yahoo.com> >>>>>> wrote: >>>>>> >>>>>>> I need to make a mixer (a multiplier, not an adder) for sinusoid >>>>>>> signals somewhere between about 10 kHz and 1 Mhz. This is for a >>>>>>> lock-in-amp. My main constraint is low cost. I'd use a >>>>>>> microcontroller with an A to D and a hardware multiplier, but I'd >>>>>>> like to keep the cost down by using a cheaper micro. I could use a >>>>>>> simple power-of-2 window in the micro and then only need additions >>>>>>> to demod, but that would give me a wider bandwidth for noise to get >>>>>>> in. I though a single transistor mixer design might give better >>>>>>> results. Anyone know of a good reference? >>>>>>> >>>>>>> ChesterW +++ >>>>>>> Dr Chester Wildey Founder MRRA Inc. >>>>>>> Electronic and Optoelectronic Instruments MRI Motion, fNIRS Brain >>>>>>> Scanners, Counterfeit and Covert Marker Detection Fort Worth, Texas, >>>>>>> USA www.mrrainc.com wildey at mrrainc dot com >>>>>> >>>>>> A single-transistor multiplier would probably be awful. >>>>>> >>>>>> You don't need to multiply sine waves. Just multiply the signal >>>>>> alternately by +1 and -1 and then lowpass filter. >>>>>> >>>>>> That can be done, for example, by alternately multiplexing between >>>>>> SIG+ >>>>>> and SIG- with a good fast analog mux. That will always have residual >>>>>> errors, but could be pretty good. >>>>>> >>>>>> I guess the best lock-in would use a good ADC and do the multiply >>>>>> digitally, in an FPGA. Noise dither maybe. >>>>> >>>>> With today's parts you may be able to do this with a set of fast >>>>> analog switches and an op-amp. Perhaps even a 74<something fast>4051 >>>>> or whatever? >>>> >>>> 74HC4051 would work, but the 1 MHz requirement is tricky. >>>> >>>> Something like FSA3157 or ADG604 or one of those T3USB gadgets might be >>>> better at 1 MHz. >>>> >>>> The classic single-opamp +1/-1 circuit might work with a small phemt as >>>> the grounding switch. >>>> >>>> But no single transistor! >>>> >>>> >>> I certainly like the price of that FSA3157. >>> >>> To be clear, 1 Mhz is not a requirement, I can pick the frequency. I was >>> planning to look for a clean spot in the spectrum maybe around 40 kHz - >>> something where I can get a reasonable over-sample on the A to D if I go >>> with signal processing in an inexpensive micro. >> >> At 40kHz a jelly-bean op-amp for a few pennies, and John's classic* +1/-1 >> circuit would almost certainly work, with whatever is cheapest for an >> analog switch. > >Lock-ins often have pretty stiff performance requirements, so that e.g. >mildly asymmetrical slewing or Ron variations between the +1 and -1 arms >can be a serious issue. > >To the OP: I gather this isn't a super-high performance application, but >how good does it need to be? > >Cheers > >Phil HobbsThere are at least three ways to do the basic +-1 thing... https://dl.dropboxusercontent.com/u/53724080/Circuits/Plus_Minus_Mixer.JPG All have interesting charge injection issues. Some opamps go nuts if you switch at their inputs, or outputs. A really good analog lockin is hard to do. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Single transistor mixer
Started by ●February 21, 2016
Reply by ●February 22, 20162016-02-22
Reply by ●February 22, 20162016-02-22
On Monday, February 22, 2016 at 12:20:04 PM UTC-5, John Larkin wrote:> On Mon, 22 Feb 2016 12:03:39 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >On 02/22/2016 12:25 AM, Tim Wescott wrote: > >> On Sun, 21 Feb 2016 20:46:00 -0600, ChesterW wrote: > >> > >>> On 2/21/16 5:49 PM, John Larkin wrote: > >>>> On Sun, 21 Feb 2016 17:23:43 -0600, Tim Wescott > >>>> <seemywebsite@myfooter.really> wrote: > >>>> > >>>>> On Sun, 21 Feb 2016 15:12:58 -0800, John Larkin wrote: > >>>>> > >>>>>> On Sun, 21 Feb 2016 16:22:20 -0600, ChesterW <iamsnoozin@yahoo.com> > >>>>>> wrote: > >>>>>> > >>>>>>> I need to make a mixer (a multiplier, not an adder) for sinusoid > >>>>>>> signals somewhere between about 10 kHz and 1 Mhz. This is for a > >>>>>>> lock-in-amp. My main constraint is low cost. I'd use a > >>>>>>> microcontroller with an A to D and a hardware multiplier, but I'd > >>>>>>> like to keep the cost down by using a cheaper micro. I could use a > >>>>>>> simple power-of-2 window in the micro and then only need additions > >>>>>>> to demod, but that would give me a wider bandwidth for noise to get > >>>>>>> in. I though a single transistor mixer design might give better > >>>>>>> results. Anyone know of a good reference? > >>>>>>> > >>>>>>> ChesterW +++ > >>>>>>> Dr Chester Wildey Founder MRRA Inc. > >>>>>>> Electronic and Optoelectronic Instruments MRI Motion, fNIRS Brain > >>>>>>> Scanners, Counterfeit and Covert Marker Detection Fort Worth, Texas, > >>>>>>> USA www.mrrainc.com wildey at mrrainc dot com > >>>>>> > >>>>>> A single-transistor multiplier would probably be awful. > >>>>>> > >>>>>> You don't need to multiply sine waves. Just multiply the signal > >>>>>> alternately by +1 and -1 and then lowpass filter. > >>>>>> > >>>>>> That can be done, for example, by alternately multiplexing between > >>>>>> SIG+ > >>>>>> and SIG- with a good fast analog mux. That will always have residual > >>>>>> errors, but could be pretty good. > >>>>>> > >>>>>> I guess the best lock-in would use a good ADC and do the multiply > >>>>>> digitally, in an FPGA. Noise dither maybe. > >>>>> > >>>>> With today's parts you may be able to do this with a set of fast > >>>>> analog switches and an op-amp. Perhaps even a 74<something fast>4051 > >>>>> or whatever? > >>>> > >>>> 74HC4051 would work, but the 1 MHz requirement is tricky. > >>>> > >>>> Something like FSA3157 or ADG604 or one of those T3USB gadgets might be > >>>> better at 1 MHz. > >>>> > >>>> The classic single-opamp +1/-1 circuit might work with a small phemt as > >>>> the grounding switch. > >>>> > >>>> But no single transistor! > >>>> > >>>> > >>> I certainly like the price of that FSA3157. > >>> > >>> To be clear, 1 Mhz is not a requirement, I can pick the frequency. I was > >>> planning to look for a clean spot in the spectrum maybe around 40 kHz - > >>> something where I can get a reasonable over-sample on the A to D if I go > >>> with signal processing in an inexpensive micro. > >> > >> At 40kHz a jelly-bean op-amp for a few pennies, and John's classic* +1/-1 > >> circuit would almost certainly work, with whatever is cheapest for an > >> analog switch. > > > >Lock-ins often have pretty stiff performance requirements, so that e.g. > >mildly asymmetrical slewing or Ron variations between the +1 and -1 arms > >can be a serious issue. > > > >To the OP: I gather this isn't a super-high performance application, but > >how good does it need to be? > > > >Cheers > > > >Phil Hobbs > > There are at least three ways to do the basic +-1 thing... > > https://dl.dropboxusercontent.com/u/53724080/Circuits/Plus_Minus_Mixer.JPGGot a favorite among those? I've got some other (crazier) circuits scribbled in a note book. An instrument amp with a SPDT analog switch that grounds each side in turn. (Input to both sides through a resistor.) (I think someone else suggested that one...) George H.> > All have interesting charge injection issues. Some opamps go nuts if > you switch at their inputs, or outputs. > > A really good analog lockin is hard to do. > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
Reply by ●February 22, 20162016-02-22
On 2/22/2016 11:41 AM, George Herold wrote:> On Sunday, February 21, 2016 at 7:28:23 PM UTC-5, mixed nuts wrote: >> On 2/21/2016 5:22 PM, ChesterW wrote: >>> I need to make a mixer (a multiplier, not an adder) for sinusoid signals >>> somewhere between about 10 kHz and 1 Mhz. This is for a lock-in-amp. My >>> main constraint is low cost. I'd use a microcontroller with an A to D >>> and a hardware multiplier, but I'd like to keep the cost down by using a >>> cheaper micro. I could use a simple power-of-2 window in the micro and >>> then only need additions to demod, but that would give me a wider >>> bandwidth for noise to get in. I though a single transistor mixer design >>> might give better results. Anyone know of a good reference? >> >> Single diodes have been used as mixers for nearly a century. A >> transistor biased near cutoff with a strong signal applied to the base >> at one frequency and a weak signal at another will produce sum and >> difference frequencies with some gain over a simple diode. > (I'm a bit late to the discussion...) > > You mean a ring modulator type of thing?Just a single diode. Tube type diodes were used initially. Some Ge and SiC diodes were used through the 1930s but they weren't highly evolved. The MIT Rad Lab guys developed some point contact Si diodes that were useful to several GHz (google 1N21) - you couldn't find a radar that didn't have one or a very close cousin. One end of the diode went to ground and the other had a local oscillator (a volt or two) and RF connected to it. For low frequencies it was all lumped element and for microwave it was put into a waveguide. The diode ring stuff came along later - some during WWII in military gear but it really wasn't until the early 60s that balanced mixers became common.> > It would be cool (and perhaps exists) if someone made > synchronous FET (analog) switches that just flipped the > (output) polarity back and forth.Done all the time. Even a lowly XOR gate in CMOS actually works rather well - below a volt or so the signal path is fairly linear. Can be used as a frequency doubler as well. INA 0101 (LO) INB 0011 (SIG) -------- OUT 0110 -- Grizzly H.
Reply by ●February 22, 20162016-02-22
On Sun, 21 Feb 2016 20:44:36 -0600, ChesterW wrote:> On 2/21/16 5:12 PM, John Larkin wrote: >> On Sun, 21 Feb 2016 16:22:20 -0600, ChesterW <iamsnoozin@yahoo.com> >> wrote: >> >>> I need to make a mixer (a multiplier, not an adder) for sinusoid >>> signals somewhere between about 10 kHz and 1 Mhz. This is for a >>> lock-in-amp. My main constraint is low cost. I'd use a microcontroller >>> with an A to D and a hardware multiplier, but I'd like to keep the >>> cost down by using a cheaper micro. I could use a simple power-of-2 >>> window in the micro and then only need additions to demod, but that >>> would give me a wider bandwidth for noise to get in. I though a single >>> transistor mixer design might give better results. Anyone know of a >>> good reference? >>> >>> ChesterW +++ >>> Dr Chester Wildey Founder MRRA Inc. >>> Electronic and Optoelectronic Instruments MRI Motion, fNIRS Brain >>> Scanners, Counterfeit and Covert Marker Detection Fort Worth, Texas, >>> USA www.mrrainc.com wildey at mrrainc dot com >> >> A single-transistor multiplier would probably be awful. > > I was afraid I might hear that. > > >> You don't need to multiply sine waves. Just multiply the signal >> alternately by +1 and -1 and then lowpass filter. > > Yep, even inexpensive micros come with +1/-1 multiplier :). > Unfortunately using this method means the DC mixing product will include > noise from all of the odd harmonics. The beauty of a lock-in is the > narrow noise bandwidth, and it is compromised with the +1/-1 method. > > >> That can be done, for example, by alternately multiplexing between SIG+ >> and SIG- with a good fast analog mux. That will always have residual >> errors, but could be pretty good. > > Or I could DC offset the signal and just do the +/- in the micro. The DC > component will cancel out. > > >> I guess the best lock-in would use a good ADC and do the multiply >> digitally, in an FPGA. Noise dither maybe. > > That's my benchmark. I can use something like a microchip dsPic that has > a hardware multiplier and mix the signals in the chip. The customers > product is high volume though, so pennies matter, and the dsPic costs > around $1.60. If I could implement a reasonable analog mixer I could use > a more modest micro for something around $0.50. > > I've taken a look at the raw signal and I've got plenty of noise for > bit-dithering. I only need a TC of a second or two, so even for an 8 bit > ADC digitization noise will not be a factor.If a 40kHz-ish carrier is good enough for you, consider that there are sub-$1 Arm Cortex M0 parts out there that should be able to keep up with 200kHz-ish sampling rates if you don't mind that one task consuming a good portion of the processor's time. If you're lucky there'll even be one with a good-enough ADC. You'd have to do some benchmarking to see if the multiplication is fast enough -- I suspect that it is, though. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●February 22, 20162016-02-22
On 2/21/16 8:39 PM, Bill Sloman wrote:> On Monday, 22 February 2016 11:04:34 UTC+11, Jim Thompson wrote: >> On Sun, 21 Feb 2016 16:22:20 -0600, ChesterW <iamsnoozin@yahoo.com> >> wrote: >> >>> I need to make a mixer (a multiplier, not an adder) for sinusoid signals >>> somewhere between about 10 kHz and 1 Mhz. This is for a lock-in-amp. My >>> main constraint is low cost. I'd use a microcontroller with an A to D >>> and a hardware multiplier, but I'd like to keep the cost down by using a >>> cheaper micro. I could use a simple power-of-2 window in the micro and >>> then only need additions to demod, but that would give me a wider >>> bandwidth for noise to get in. I though a single transistor mixer design >>> might give better results. Anyone know of a good reference? >>> >>> ChesterW >>> +++ >>> Dr Chester Wildey >>> Founder MRRA Inc. >>> Electronic and Optoelectronic Instruments >>> MRI Motion, fNIRS Brain Scanners, Counterfeit and Covert Marker Detection >>> Fort Worth, Texas, USA >>> www.mrrainc.com >>> wildey at mrrainc dot com >> >> For RF, a simple diff-pair makes an excellent mixer... think half of a >> Gilbert multiplier (you don't need the half that eliminates DC from >> the source... tank loads do that). > > The Faulkner and Harding phase sensitive detector works fine at audio frequencies as well (though it was claimed that it worked up to 30MHz). > > E.A.Faulkner and D.W. Harding J.Sci. Instrum. volume 43 page 97-99 (1966) > > http://iopscience.iop.org/article/10.1088/0950-7671/43/2/305 > > Faulkner improved it a bit > > E.A.Faulkner and J.B.Grimbleby Electronic Engineering volume 39 page 565-67 (1967) > > Both references are from my Ph.D. thesis. > > The original Faulkner and Harding phase sensitive detector turned the - off-set - signal into a modulated direct current at the collector of a transistor constant current source, and fed that into a long-tailed pair of transistors whose bases were driven by the demodulating signal - ideally a pair of anti-phase square waves at the frequency of interest. > > The integrated difference in the currents coming out of the two collectors was the demodulated signal - an operational amplifier subtractor could refer that to 0V. I just stuck a moving coil ammeter across the collector resistors. > > You have to add enough voltage offset to the original AC signal to make sure that the current going into the long-tailed pair never drops to zero, and the mark-to-space ratio (which should be exactly 50%) of the demodulating drive determines how effectively this offset is rejected. > > It's three transistors rather than one, but it's a remarkably good circuit - even better if you used a matched transistor pair for the long-trailed pair. >Sounds interesting. I sent off my $33 today. They said I could download the paper .... on Thursday! Haven't the Brits discovered the internet yet :) ChesterW
Reply by ●February 22, 20162016-02-22
On Tuesday, 23 February 2016 03:51:08 UTC+11, Phil Hobbs wrote:> On 02/21/2016 05:22 PM, ChesterW wrote: > > I need to make a mixer (a multiplier, not an adder) for sinusoid signals > > somewhere between about 10 kHz and 1 Mhz. This is for a lock-in-amp. My > > main constraint is low cost. I'd use a microcontroller with an A to D > > and a hardware multiplier, but I'd like to keep the cost down by using a > > cheaper micro. I could use a simple power-of-2 window in the micro and > > then only need additions to demod, but that would give me a wider > > bandwidth for noise to get in. I though a single transistor mixer design > > might give better results. Anyone know of a good reference? > > > > ChesterW > > Single transistor mixers are the pits. You can make a single-balanced > one using a diff pair, e.g. a BC817DS, which at least keeps the LO out > of the output to some degree.http://www.nxp.com/documents/data_sheet/BC817DS.pdf You can do better with something that species how well the two transistors are matched. http://www.analog.com/media/en/technical-documentation/data-sheets/SSM2212.pdf It's probably two order of magnitude more expensive - $A13.63 from my local broad-line supplier, but they've got 14 in stock. The MAT01 is even better, at twice the price, but they've got 77 in stock. There are less tightly specified duals, but I can't be bothered finding them - somebody else will know off the top of their head. -- Bill Sloman, Sydney
Reply by ●February 22, 20162016-02-22
On 22/02/16 22:50, Tom Gardner wrote:> That's very similar to something I built in 1979 using 4066s.One of the Australian electronics rags published a design in the mid-70's for a "synchrodyne" direct conversion AM receiver, which used a 4066 I/Q demodulator; Q was used to lock the local oscillator at 4x the receive frequency, divided down to produce symmetric I and Q signals.
Reply by ●February 22, 20162016-02-22
On 2/21/16 11:25 PM, Tim Wescott wrote:> On Sun, 21 Feb 2016 20:46:00 -0600, ChesterW wrote: > >> On 2/21/16 5:49 PM, John Larkin wrote: >>> On Sun, 21 Feb 2016 17:23:43 -0600, Tim Wescott >>> <seemywebsite@myfooter.really> wrote: >>> >>>> On Sun, 21 Feb 2016 15:12:58 -0800, John Larkin wrote: >>>> >>>>> On Sun, 21 Feb 2016 16:22:20 -0600, ChesterW <iamsnoozin@yahoo.com> >>>>> wrote: >>>>> >>>>>> I need to make a mixer (a multiplier, not an adder) for sinusoid >>>>>> signals somewhere between about 10 kHz and 1 Mhz. This is for a >>>>>> lock-in-amp. My main constraint is low cost. I'd use a >>>>>> microcontroller with an A to D and a hardware multiplier, but I'd >>>>>> like to keep the cost down by using a cheaper micro. I could use a >>>>>> simple power-of-2 window in the micro and then only need additions >>>>>> to demod, but that would give me a wider bandwidth for noise to get >>>>>> in. I though a single transistor mixer design might give better >>>>>> results. Anyone know of a good reference? >>>>>> >>>>>> ChesterW +++ >>>>>> Dr Chester Wildey Founder MRRA Inc. >>>>>> Electronic and Optoelectronic Instruments MRI Motion, fNIRS Brain >>>>>> Scanners, Counterfeit and Covert Marker Detection Fort Worth, Texas, >>>>>> USA www.mrrainc.com wildey at mrrainc dot com >>>>> >>>>> A single-transistor multiplier would probably be awful. >>>>> >>>>> You don't need to multiply sine waves. Just multiply the signal >>>>> alternately by +1 and -1 and then lowpass filter. >>>>> >>>>> That can be done, for example, by alternately multiplexing between >>>>> SIG+ >>>>> and SIG- with a good fast analog mux. That will always have residual >>>>> errors, but could be pretty good. >>>>> >>>>> I guess the best lock-in would use a good ADC and do the multiply >>>>> digitally, in an FPGA. Noise dither maybe. >>>> >>>> With today's parts you may be able to do this with a set of fast >>>> analog switches and an op-amp. Perhaps even a 74<something fast>4051 >>>> or whatever? >>> >>> 74HC4051 would work, but the 1 MHz requirement is tricky. >>> >>> Something like FSA3157 or ADG604 or one of those T3USB gadgets might be >>> better at 1 MHz. >>> >>> The classic single-opamp +1/-1 circuit might work with a small phemt as >>> the grounding switch. >>> >>> But no single transistor! >>> >>> >> I certainly like the price of that FSA3157. >> >> To be clear, 1 Mhz is not a requirement, I can pick the frequency. I was >> planning to look for a clean spot in the spectrum maybe around 40 kHz - >> something where I can get a reasonable over-sample on the A to D if I go >> with signal processing in an inexpensive micro. > > At 40kHz a jelly-bean op-amp for a few pennies, and John's classic* +1/-1 > circuit would almost certainly work, with whatever is cheapest for an > analog switch. > > DigiKey lists a 74VHC1GT66 at 7 cents each in qty 1000, which is pretty > cheap and should do. > > * I invented one of those when I was in grad school. I tend to invent a > lot of stuff that's been around for decades. It's trying. >That's not tying, that's encouraging! ChesterW -- Best Regards, ChesterW +++ Dr Chester Wildey Founder MRRA Inc. Electronic and Optoelectronic Instruments MRI Motion, fNIRS Brain Scanners, Counterfeit and Covert Marker Detection Fort Worth, Texas, USA www.mrrainc.com wildey at mrrainc dot com
Reply by ●February 22, 20162016-02-22
On 2/22/16 11:03 AM, Phil Hobbs wrote:> On 02/22/2016 12:25 AM, Tim Wescott wrote: >> On Sun, 21 Feb 2016 20:46:00 -0600, ChesterW wrote: >> >>> On 2/21/16 5:49 PM, John Larkin wrote: >>>> On Sun, 21 Feb 2016 17:23:43 -0600, Tim Wescott >>>> <seemywebsite@myfooter.really> wrote: >>>> >>>>> On Sun, 21 Feb 2016 15:12:58 -0800, John Larkin wrote: >>>>> >>>>>> On Sun, 21 Feb 2016 16:22:20 -0600, ChesterW <iamsnoozin@yahoo.com> >>>>>> wrote: >>>>>> >>>>>>> I need to make a mixer (a multiplier, not an adder) for sinusoid >>>>>>> signals somewhere between about 10 kHz and 1 Mhz. This is for a >>>>>>> lock-in-amp. My main constraint is low cost. I'd use a >>>>>>> microcontroller with an A to D and a hardware multiplier, but I'd >>>>>>> like to keep the cost down by using a cheaper micro. I could use a >>>>>>> simple power-of-2 window in the micro and then only need additions >>>>>>> to demod, but that would give me a wider bandwidth for noise to get >>>>>>> in. I though a single transistor mixer design might give better >>>>>>> results. Anyone know of a good reference? >>>>>>> >>>>>>> ChesterW +++ >>>>>>> Dr Chester Wildey Founder MRRA Inc. >>>>>>> Electronic and Optoelectronic Instruments MRI Motion, fNIRS Brain >>>>>>> Scanners, Counterfeit and Covert Marker Detection Fort Worth, Texas, >>>>>>> USA www.mrrainc.com wildey at mrrainc dot com >>>>>> >>>>>> A single-transistor multiplier would probably be awful. >>>>>> >>>>>> You don't need to multiply sine waves. Just multiply the signal >>>>>> alternately by +1 and -1 and then lowpass filter. >>>>>> >>>>>> That can be done, for example, by alternately multiplexing between >>>>>> SIG+ >>>>>> and SIG- with a good fast analog mux. That will always have residual >>>>>> errors, but could be pretty good. >>>>>> >>>>>> I guess the best lock-in would use a good ADC and do the multiply >>>>>> digitally, in an FPGA. Noise dither maybe. >>>>> >>>>> With today's parts you may be able to do this with a set of fast >>>>> analog switches and an op-amp. Perhaps even a 74<something fast>4051 >>>>> or whatever? >>>> >>>> 74HC4051 would work, but the 1 MHz requirement is tricky. >>>> >>>> Something like FSA3157 or ADG604 or one of those T3USB gadgets might be >>>> better at 1 MHz. >>>> >>>> The classic single-opamp +1/-1 circuit might work with a small phemt as >>>> the grounding switch. >>>> >>>> But no single transistor! >>>> >>>> >>> I certainly like the price of that FSA3157. >>> >>> To be clear, 1 Mhz is not a requirement, I can pick the frequency. I was >>> planning to look for a clean spot in the spectrum maybe around 40 kHz - >>> something where I can get a reasonable over-sample on the A to D if I go >>> with signal processing in an inexpensive micro. >> >> At 40kHz a jelly-bean op-amp for a few pennies, and John's classic* +1/-1 >> circuit would almost certainly work, with whatever is cheapest for an >> analog switch. > > Lock-ins often have pretty stiff performance requirements, so that e.g. > mildly asymmetrical slewing or Ron variations between the +1 and -1 arms > can be a serious issue. > > To the OP: I gather this isn't a super-high performance application, but > how good does it need to be? > > Cheers > > Phil Hobbs >200 uV rms error is plenty good enough. It's harsh conditions though, unknown and varying EMI, possibly poor power, big temperature swings. I thought I might switch a couple of calibration components into the loop to manage shifts in zero and proportion. ChesterW -- Best Regards, ChesterW +++ Dr Chester Wildey Founder MRRA Inc. Electronic and Optoelectronic Instruments MRI Motion, fNIRS Brain Scanners, Counterfeit and Covert Marker Detection Fort Worth, Texas, USA www.mrrainc.com wildey at mrrainc dot com
Reply by ●February 22, 20162016-02-22
