Forums

DC background subtraction

Started by George Herold January 7, 2019
On Monday, January 7, 2019 at 8:44:16 PM UTC-5, Phil Hobbs wrote:
> On 1/7/19 8:27 PM, George Herold wrote: > > On Monday, January 7, 2019 at 8:01:28 PM UTC-5, bitrex wrote: > >> On 01/07/2019 07:52 PM, George Herold wrote: > >>> On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote: > >>>> On 01/07/2019 07:23 PM, George Herold wrote: > >>>>> On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote: > >>>>>> George Herold wrote: > >>>>>> > >>>>>>> > >>>>>>> Perhaps a silly simple question. I need to subtract off a 2.5V > >>>>>>> background from a few mV signal. I first think about a difference amp, > >>>>>>> or instrument amp. > >>>>>>> But I wondered if there is any advantage to a simple > >>>>>>> opamp summing amp. (I'll add in -2.5 V) > >>>>>>> > >>>>>>> > >>>>>> > >>>>>> ** Considered using a capacitor ?? > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> .... Phil > >>>>> > >>>>> Like a sample and hold? It needs to hold the offset > >>>>> sorta forever, so a pot called for. > >>>>> > >>>>> George H. > >>>>> > >>>> > >>>> DC blocking capacitor. Y'know, like a high-pass on the input. > >>>> > >>>> <https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0> > >>> > >>> Oh no we are talking slowly varying signals. Minutes to > >>> an hour maybe.. > >>> > >>> So it's my bosses idea, you put a hall probe sensor inside > >>> a High Tc super conductor toroid. Induce a super current. > >>> (by cooling down with permanant magnet in the toroid.) > >>> And then observe the field go away as you warm it up. > >>> > >>> George H. > >>> > >> > >> Got it, I figured you had not forgotten how to use a capacitor to block > >> DC but just so long as we cover all the bases here and the signal period > >> wasn't mentioned explicitly ;) > > > > Oh dear no, big C, big R and fet opamp.. about 1 second is the longest > > TC I can wait for. (10uF and 100k or 1uF and a meg.) > > I was doing that today to look at power supply noise. > > I think most of the circuits I build are 'scope preamps. :^) > > I've never done the feedback cap thing, (you posted previoulsy), > > looks like a Sallen-Key. > > > > George H. > > > > If you want to amplify the signal by N times and the offset by 1x (so > that you're still biased above ground), that's easy to do accurately > with a garden-variety noninverting amp.
I don't really care where the signal level is, I just want to look at (small) changes. I'm trying to picture how a non-iverting amp does this, but can't. Can you give me more of a hint? George H.
> > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC / Hobbs ElectroOptics > Optics, Electro-optics, Photonics, Analog Electronics > Briarcliff Manor NY 10510 > > http://electrooptical.net > http://hobbs-eo.com
On Tuesday, January 8, 2019 at 5:09:34 AM UTC-5, piglet wrote:
> On 08/01/2019 00:52, George Herold wrote: > > Oh no we are talking slowly varying signals. Minutes to > > an hour maybe.. > > > > So it's my bosses idea, you put a hall probe sensor inside > > a High Tc super conductor toroid. Induce a super current. > > (by cooling down with permanant magnet in the toroid.) > > And then observe the field go away as you warm it up. > > > > George H. > > > > An identical sensor outside the field and take the difference? > > piglet
Oh that's interesting! This goes down a probe, for which there is always a lack of wires... but it might work. The good thing is it would take out any changes in the local B-field. The bad thing is that the sensor has to be heated to ~130 deg K or it stops working. Two sensors may be a little hard to squeeze in but it's certainly worth thinking about. Thanks, George H.
On Tuesday, January 8, 2019 at 8:42:03 AM UTC-5, speff wrote:
> On Monday, 7 January 2019 19:37:01 UTC-5, George Herold wrote: > > On Monday, January 7, 2019 at 7:00:41 PM UTC-5, bitrex wrote: > > > On 01/07/2019 02:31 PM, George Herold wrote: > > > > Perhaps a silly simple question. I need to subtract off a 2.5V background from a few mV signal. I first think about a difference amp, or > > > > instrument amp. But I wondered if there is any advantage to a simple > > > > opamp summing amp. (I'll add in -2.5 V) > > > > > > > > George H. > > > > > > > > > > Generating the -2.5 offset for the summer sounds like a job for a DC > > > servo/integrator, servoing the op-amp output > > > > It's ~1/2 the bias voltage so I'll reference it from that > > with a pot and buffer. -2.5 will add an inverter. > > (sounds easier to try the int amp first.) > > > > George H. > > Divide the bias voltage in half with precision resistors and subtract? > > Unless you go to some kind of flying capacitor deal, I think at best you can get away with a 1:1 resistor network as the only precision part. > > As suggested, an with excellent DC specs might avoid even that- for example an AD7177-2, 1 ppm INL but that's a $25-ish part. > > Depends a lot on your exact requirements, if you can get away with Susumu RM2012B duals (such as an 0805 dual 10K with +/-2ppm/&deg;C TCR matching), it could be pretty cheap and easy using resistor networks and a small trim. If you need foil resistors, tens of dollars. > > If the output changes by 20ppm, that's 50uV. You don't need chopper amplifiers at that level, but you could use them if the input glitches are not an issue. > > > --Spehro Pefhany
Thanks Spehro, I have to look at the signals still, but I'm thinking the drift/ crud on the sensor will be much worse than any other drifts... so I may be barking up the wrong tree. George H.
On 1/8/19 8:51 AM, George Herold wrote:
> On Monday, January 7, 2019 at 8:44:16 PM UTC-5, Phil Hobbs wrote: >> On 1/7/19 8:27 PM, George Herold wrote: >>> On Monday, January 7, 2019 at 8:01:28 PM UTC-5, bitrex wrote: >>>> On 01/07/2019 07:52 PM, George Herold wrote: >>>>> On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote: >>>>>> On 01/07/2019 07:23 PM, George Herold wrote: >>>>>>> On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote: >>>>>>>> George Herold wrote: >>>>>>>> >>>>>>>>> >>>>>>>>> Perhaps a silly simple question. I need to subtract off a 2.5V >>>>>>>>> background from a few mV signal. I first think about a difference amp, >>>>>>>>> or instrument amp. >>>>>>>>> But I wondered if there is any advantage to a simple >>>>>>>>> opamp summing amp. (I'll add in -2.5 V) >>>>>>>>> >>>>>>>>> >>>>>>>> >>>>>>>> ** Considered using a capacitor ?? >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> .... Phil >>>>>>> >>>>>>> Like a sample and hold? It needs to hold the offset >>>>>>> sorta forever, so a pot called for. >>>>>>> >>>>>>> George H. >>>>>>> >>>>>> >>>>>> DC blocking capacitor. Y'know, like a high-pass on the input. >>>>>> >>>>>> <https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0> >>>>> >>>>> Oh no we are talking slowly varying signals. Minutes to >>>>> an hour maybe.. >>>>> >>>>> So it's my bosses idea, you put a hall probe sensor inside >>>>> a High Tc super conductor toroid. Induce a super current. >>>>> (by cooling down with permanant magnet in the toroid.) >>>>> And then observe the field go away as you warm it up. >>>>> >>>>> George H. >>>>> >>>> >>>> Got it, I figured you had not forgotten how to use a capacitor to block >>>> DC but just so long as we cover all the bases here and the signal period >>>> wasn't mentioned explicitly ;) >>> >>> Oh dear no, big C, big R and fet opamp.. about 1 second is the longest >>> TC I can wait for. (10uF and 100k or 1uF and a meg.) >>> I was doing that today to look at power supply noise. >>> I think most of the circuits I build are 'scope preamps. :^) >>> I've never done the feedback cap thing, (you posted previoulsy), >>> looks like a Sallen-Key. >>> >>> George H. >>> >> >> If you want to amplify the signal by N times and the offset by 1x (so >> that you're still biased above ground), that's easy to do accurately >> with a garden-variety noninverting amp. > > I don't really care where the signal level is, I just want to look at > (small) changes. I'm trying to picture how a non-iverting amp > does this, but can't. Can you give me more of a hint? > > George H.
A noninverting gain of N amplifier is also an inverting gain of N-1 amplifier, and the 1 is very accurate when the loop gain is high. Thus if you have an offset of 2.50000V, connecting the input resistor of the op amp (on the inverting side) to your offset generator and the noninverting input to signal+offset, you get Vout = (N+1)(Vos + Vsig) - N Vos = Vos + N Vsig. This is just like the usual split-supply, single-ended noninverting amp, except with the ground reference relabelled. Resistor inaccuracy causes gain inaccuracy, but the offset remains correct. This move is super common in single-supply data acq circuits, for instance. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net https://hobbs-eo.com
On Tuesday, January 8, 2019 at 10:04:07 AM UTC-5, Phil Hobbs wrote:
> On 1/8/19 8:51 AM, George Herold wrote: > > On Monday, January 7, 2019 at 8:44:16 PM UTC-5, Phil Hobbs wrote: > >> On 1/7/19 8:27 PM, George Herold wrote: > >>> On Monday, January 7, 2019 at 8:01:28 PM UTC-5, bitrex wrote: > >>>> On 01/07/2019 07:52 PM, George Herold wrote: > >>>>> On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote: > >>>>>> On 01/07/2019 07:23 PM, George Herold wrote: > >>>>>>> On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote: > >>>>>>>> George Herold wrote: > >>>>>>>> > >>>>>>>>> > >>>>>>>>> Perhaps a silly simple question. I need to subtract off a 2.5V > >>>>>>>>> background from a few mV signal. I first think about a difference amp, > >>>>>>>>> or instrument amp. > >>>>>>>>> But I wondered if there is any advantage to a simple > >>>>>>>>> opamp summing amp. (I'll add in -2.5 V) > >>>>>>>>> > >>>>>>>>> > >>>>>>>> > >>>>>>>> ** Considered using a capacitor ?? > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>> .... Phil > >>>>>>> > >>>>>>> Like a sample and hold? It needs to hold the offset > >>>>>>> sorta forever, so a pot called for. > >>>>>>> > >>>>>>> George H. > >>>>>>> > >>>>>> > >>>>>> DC blocking capacitor. Y'know, like a high-pass on the input. > >>>>>> > >>>>>> <https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0> > >>>>> > >>>>> Oh no we are talking slowly varying signals. Minutes to > >>>>> an hour maybe.. > >>>>> > >>>>> So it's my bosses idea, you put a hall probe sensor inside > >>>>> a High Tc super conductor toroid. Induce a super current. > >>>>> (by cooling down with permanant magnet in the toroid.) > >>>>> And then observe the field go away as you warm it up. > >>>>> > >>>>> George H. > >>>>> > >>>> > >>>> Got it, I figured you had not forgotten how to use a capacitor to block > >>>> DC but just so long as we cover all the bases here and the signal period > >>>> wasn't mentioned explicitly ;) > >>> > >>> Oh dear no, big C, big R and fet opamp.. about 1 second is the longest > >>> TC I can wait for. (10uF and 100k or 1uF and a meg.) > >>> I was doing that today to look at power supply noise. > >>> I think most of the circuits I build are 'scope preamps. :^) > >>> I've never done the feedback cap thing, (you posted previoulsy), > >>> looks like a Sallen-Key. > >>> > >>> George H. > >>> > >> > >> If you want to amplify the signal by N times and the offset by 1x (so > >> that you're still biased above ground), that's easy to do accurately > >> with a garden-variety noninverting amp. > > > > I don't really care where the signal level is, I just want to look at > > (small) changes. I'm trying to picture how a non-iverting amp > > does this, but can't. Can you give me more of a hint? > > > > George H. > > A noninverting gain of N amplifier is also an inverting gain of N-1 > amplifier, and the 1 is very accurate when the loop gain is high. Thus > if you have an offset of 2.50000V, connecting the input resistor of the > op amp (on the inverting side) to your offset generator and the > noninverting input to signal+offset, you get > > Vout = (N+1)(Vos + Vsig) - N Vos = Vos + N Vsig. > > This is just like the usual split-supply, single-ended noninverting amp, > except with the ground reference relabelled. Resistor inaccuracy causes > gain inaccuracy, but the offset remains correct. > > This move is super common in single-supply data acq circuits, for instance. >
OK got it. Thanks. George H.
> Cheers > > Phil Hobbs > > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC / Hobbs ElectroOptics > Optics, Electro-optics, Photonics, Analog Electronics > Briarcliff Manor NY 10510 > > http://electrooptical.net > https://hobbs-eo.com
tirsdag den 8. januar 2019 kl. 11.09.34 UTC+1 skrev piglet:
> On 08/01/2019 00:52, George Herold wrote: > > Oh no we are talking slowly varying signals. Minutes to > > an hour maybe.. > > > > So it's my bosses idea, you put a hall probe sensor inside > > a High Tc super conductor toroid. Induce a super current. > > (by cooling down with permanant magnet in the toroid.) > > And then observe the field go away as you warm it up. > > > > George H. > > > > An identical sensor outside the field and take the difference? >
or different with two sensors in the field, one of them flipped
On Tuesday, January 8, 2019 at 12:28:04 PM UTC-5, Lasse Langwadt Christensen wrote:
> tirsdag den 8. januar 2019 kl. 11.09.34 UTC+1 skrev piglet: > > On 08/01/2019 00:52, George Herold wrote: > > > Oh no we are talking slowly varying signals. Minutes to > > > an hour maybe.. > > > > > > So it's my bosses idea, you put a hall probe sensor inside > > > a High Tc super conductor toroid. Induce a super current. > > > (by cooling down with permanant magnet in the toroid.) > > > And then observe the field go away as you warm it up. > > > > > > George H. > > > > > > > An identical sensor outside the field and take the difference? > > > > or different with two sensors in the field, one of them flipped
Ohh, that's fun too! Thanks for all the nice ideas everyone, and to think I wasn't going to ask this 'simple' question. George h.
On Monday, January 7, 2019 at 7:52:59 PM UTC-5, George Herold wrote:
> On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote: > > On 01/07/2019 07:23 PM, George Herold wrote: > > > On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote: > > >> George Herold wrote: > > >> > > >>> > > >>> Perhaps a silly simple question. I need to subtract off a 2.5V > > >>> background from a few mV signal. I first think about a difference amp, > > >>> or instrument amp. > > >>> But I wondered if there is any advantage to a simple > > >>> opamp summing amp. (I'll add in -2.5 V) > > >>> > > >>> > > >> > > >> ** Considered using a capacitor ?? > > >> > > >> > > >> > > >> > > >> .... Phil > > > > > > Like a sample and hold? It needs to hold the offset > > > sorta forever, so a pot called for. > > > > > > George H. > > > > > > > DC blocking capacitor. Y'know, like a high-pass on the input. > > > > <https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0> > > Oh no we are talking slowly varying signals. Minutes to > an hour maybe.. > > So it's my bosses idea, you put a hall probe sensor inside > a High Tc super conductor toroid. Induce a super current. > (by cooling down with permanant magnet in the toroid.) > And then observe the field go away as you warm it up. > > George H.
If you're talking about that kind of thing, go digital. You'll get much better performance than analog. There are scopes that will do a "rolling" acquisition of very slowly varying signals like this.
On Monday, January 7, 2019 at 7:52:59 PM UTC-5, George Herold wrote:
> On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote: > > On 01/07/2019 07:23 PM, George Herold wrote: > > > On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote: > > >> George Herold wrote: > > >> > > >>> > > >>> Perhaps a silly simple question. I need to subtract off a 2.5V > > >>> background from a few mV signal. I first think about a difference amp, > > >>> or instrument amp. > > >>> But I wondered if there is any advantage to a simple > > >>> opamp summing amp. (I'll add in -2.5 V) > > >>> > > >>> > > >> > > >> ** Considered using a capacitor ?? > > >> > > >> > > >> > > >> > > >> .... Phil > > > > > > Like a sample and hold? It needs to hold the offset > > > sorta forever, so a pot called for. > > > > > > George H. > > > > > > > DC blocking capacitor. Y'know, like a high-pass on the input. > > > > <https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0> > > Oh no we are talking slowly varying signals. Minutes to > an hour maybe.. > > So it's my bosses idea, you put a hall probe sensor inside > a High Tc super conductor toroid. Induce a super current. > (by cooling down with permanant magnet in the toroid.) > And then observe the field go away as you warm it up. > > George H.
There should be some handheld VOMs that will do statistics on signals like this. And they produce downloadable acquisition files for USB. Probably something available at HomeDepot :-)
On 1/8/19 1:53 PM, bloggs.fredbloggs.fred@gmail.com wrote:
> On Monday, January 7, 2019 at 7:52:59 PM UTC-5, George Herold wrote: >> On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote: >>> On 01/07/2019 07:23 PM, George Herold wrote: >>>> On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote: >>>>> George Herold wrote: >>>>> >>>>>> >>>>>> Perhaps a silly simple question. I need to subtract off a 2.5V >>>>>> background from a few mV signal. I first think about a difference amp, >>>>>> or instrument amp. >>>>>> But I wondered if there is any advantage to a simple >>>>>> opamp summing amp. (I'll add in -2.5 V) >>>>>> >>>>>> >>>>> >>>>> ** Considered using a capacitor ?? >>>>> >>>>> >>>>> >>>>> >>>>> .... Phil >>>> >>>> Like a sample and hold? It needs to hold the offset >>>> sorta forever, so a pot called for. >>>> >>>> George H. >>>> >>> >>> DC blocking capacitor. Y'know, like a high-pass on the input. >>> >>> <https://www.dropbox.com/s/he7zy2rdhomfveh/IMG_20190107_193244554_HDR.jpg?dl=0> >> >> Oh no we are talking slowly varying signals. Minutes to >> an hour maybe.. >> >> So it's my bosses idea, you put a hall probe sensor inside >> a High Tc super conductor toroid. Induce a super current. >> (by cooling down with permanant magnet in the toroid.) >> And then observe the field go away as you warm it up. >> >> George H. > > There should be some handheld VOMs that will do statistics on signals like this. And they produce downloadable acquisition files for USB. Probably something available at HomeDepot :-) >
Big waste of bits though. A nice noninverting chopamp with high DC open-loop gain will do an excellent job of preserving those bits. Two 30-dB gain stages, or a 60-dB composite amp with a nice fast 40-dB stage inside the loop is good medicine for that sort of job. An OPA188 for the outer loop and an LM6171A inside would probably be a decent choice, and yield a bandwidth of around 200 kHz. Using the composite amp improves the low frequency characteristics on account of eliminating the loading on the outer amp and increasing the DC loop gain. (Making the inner stage a lead/lag amp, i.e. putting a capacitor in series with the feedback resistor, will greatly increase the DC gain again. 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 hobbs at electrooptical dot net http://electrooptical.net