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.
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.
kground from a few mV signal. I first think about a difference amp, or
le
can get away with a 1:1 resistor network as the only precision part.
le an AD7177-2, 1 ppm INL but that's a $25-ish part.
, it could be pretty cheap and easy using resistor networks and a small tri m. If you need foil resistors, tens of dollars.
fiers at that level, but you could use them if the input glitches are not a n issue.
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.
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
OK got it. Thanks.
George H.
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.
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.
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
Yes, that is even better as the signal is doubled.
piglet
Right! I'll give it a try tomorrow. (doing boring layout stuff today.)
George H.
Yep, that's gonna be hard on the small signals. The 'quiescent output' graph, though, looks good over temperature. That bodes well for resistor divider (and common regulated power supply) compensation. Be sure to keep stress off the wires when testing (there's 'package hysteresis' warnings, related to mechanical stress). That is always a concern in Hall devices.
amp,
ike this. And they produce downloadable acquisition files for USB. Probably something available at HomeDepot :-)
That's ridiculous. This product, which is very economically priced, gets 12
-bit resolution on differential analog input (multiple channels), and has a nalog output channels which can provide a programmable reference for the di fferential conversion. Sample rate max's out at 50ksps and can be streamed into the laptop via USB. It does a bunch of other stuff too, looks like a g ood catch-all gadget for the lab. Input range seems a bit limited for this application, but he should be able to figure something out. No waste of bit s here.
That's a nice concept: get some single-ended gain first, then take out the common-mode.
One can even bootstrap the supplies of the first stage.
I've done this to pick off NMR gradient coil current shunts, where CMRR has to be extreme.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I'm a fan of Labjacks--I have three U6 Pros from various projects.
The waste of bits is that the signal is hundreds of times smaller than the offset. With a 2.5 mV signal on a 2.5V offset, your 12 bits just turned into four. Using its PWM outputs to subtract off the pedestal isn't much of an improvement if you don't know how big it is *a priori*.
Just a noninverting amp solves that problem completely for a buck.
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
It's basically the single-ended version of the first stage of an instrumentation amp.
Yup. I do it all the time in front ends, where the first stage has no positive CMR otherwise.
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
Sorry, the above is mostly BS. A three pin device and I had it hooked up wrong. Fundamental at 84 kHz on the shoulder of
1/f noise that looks to have and even higher corner. All the spectra modified by opamp GBW limit, and on a 'scope FFT* Hamming window.George H.
*The keysight guys took their 'scope back, whatever deal there is hasn't happened yet.. thinking maybe I should buy one of my own, Infivision 1000 2 chan 100Mhz. ~$1k. And woah a four chan. version! What's the price tag?
:
ce amp,
like this. And they produce downloadable acquisition files for USB. Probab ly something available at HomeDepot :-)
e
e
12-bit resolution on differential analog input (multiple channels), and has analog output channels which can provide a programmable reference for the differential conversion. Sample rate max's out at 50ksps and can be streame d into the laptop via USB. It does a bunch of other stuff too, looks like a good catch-all gadget for the lab. Input range seems a bit limited for thi s application, but he should be able to figure something out. No waste of b its here.
I like my lab jack... I mostly just use it as a data logger, which is a waste of a lot of nice kit.
George H.
Huh, OK. It's probably that I'm a circuit wusse, but inverting gain w/o a ground reference bothers me. I can try it. One could HP filter the output.
(In my case) It looks like there is so much 1/f noise in the sensor that nothing else maters too much. (Lasse's back to back seems to work fine, 'in theory' with equal noise 'densities' I should get a sqrt(2) improvement in S/N. That has to apply to 1/f noise too? No? When adding two noisy signals together. The worst I can do is sqrt (2) improvement and the best is a factor of 2, compared to the noisiest source. (well with all sorts of other gain/ BW caveats.)
George H.
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