Selecting op amp to measure 1uV on high impedance - Page 1
Forum section: electronics / projects

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 09, 2022, 03:52:41 pm
content: I need op amp which has input impedance high enough. But i am failing to find such one. Can you suggest me something? I am measuring on 1Mega ohm. Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: January 09, 2022, 04:33:31 pm
content: That's femtoamp metrology territory! Finding, (and affording) a suitable OPAMP*is the least of your problems. Seehttps://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-one/for a primer.* Try googling:femtoamp bias OPAMP

author: Kleinstein
author_rank: Super Contributor
timestamp: January 09, 2022, 04:48:21 pm
content: The requirements are still a bit vague. A few OPs to look at are MAX4238, LTC2051 and OPA379, LMP7721.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 09, 2022, 06:13:08 pm
content: Measuring 1 µV on a high-impedance input? Yeah. I think you may encounter some noise problems.

author: Marco
author_rank: Super Contributor
timestamp: January 09, 2022, 06:15:12 pm
content: Just getting a MAX4238/4239 and seeing what it does if you solder some well cleaned through hole resistors/capacitors across it for about 0.1 Hz bandwidth amplification, with a 1 Meg resistor from the positive input to ground, seems the way to go. If the bias current caused offset is low enough you can use it as is. With pretty much everything else you would need to do some extra nulling.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 09, 2022, 06:47:07 pm
content: I do not like the noise. =)

author: Terry Bites
author_rank: Super Contributor
timestamp: January 09, 2022, 07:21:33 pm
content: It depends on the signal bandwith you are interested in and it depends on the voltage offset and drift you can live with.How high does your input impedance need to be?Zero Drift opamps eg ADA4528 have an Ib of 100pA running on 5V supply.Electrometer opamps like the ADA4530 are ideal but the pcb needs very careful construction and very good shielding. This amp gives a better DC accurcy and drift specs compared to an LMC6041 or similar. My experience has been that its simpler and less time consuming to by the EVAL boards. Building circuits with femto amp devcies is not stright forward and requires special PCB design and serious shielding to get the benefits.

author: magic
author_rank: Super Contributor
timestamp: January 09, 2022, 07:26:36 pm
content: 100pA may be too much bias for 1MΩ source impedance and 1µV resolution, whatever that means.No one mentioned LTC2054 yet? It might just be stable enough to consider its Ib as offset error, provided that source impedance stays constant.Regarding noise, make sure that you aren't asking for less than Johnson noise of 1MΩ over the bandwidth of interest

author: TimFox
author_rank: Super Contributor
timestamp: January 09, 2022, 07:31:32 pm
content: Quote from: MacIntoshCZ on January 09, 2022, 06:47:07 pmPlease clarify: does your application require a 1 megohm input resistance (resistance from amplifier input to common) or a 1 megohm source resistance (Thévenin equivalent output resistance from voltage source)?This distinction has a huge impact on the circuit noise.

author: Marco
author_rank: Super Contributor
timestamp: January 09, 2022, 10:06:23 pm
content: Quote from: MacIntoshCZ on January 09, 2022, 06:47:07 pmAt 0.1 Hz you'd only see a fraction of it.

author: julian1
author_rank: Frequent Contributor
timestamp: January 09, 2022, 11:38:24 pm
content: Is input bias current (for jfet/chopper) constant/linear? Can it be calibrated out?

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 10, 2022, 06:49:57 am
content: Input impedance should not lower the measuring resistance. Probably the lowest resistance i can use is 100k. But i would rather go for 1M.I am new to those fancy tricks with op amps. Looks like fun for a while.

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2022, 07:03:49 am
content: Chopper stabilized type operational amplifiers are not suitable for high impedance measurements because of high input current noise, no matter what their input bias current is. They are always used with low AC impedance in one way or another to suppress their high current noise.Common choices for your application are precision, this means low noise, low input bias current parts including the LT1012 bipolar and OPA140 JFET. I really like the LMC6081 CMOS but it has higher voltage noise so might not be as suitable.If your measurement only requires a high impedance at DC, then a chopper stabilized type can be used with added capacitance to provide a low AC impedance.

author: jonpaul
author_rank: Super Contributor
timestamp: January 10, 2022, 12:07:50 pm
content: Johnson-Nyquist noise will be importnat at 1 M Zohttps://en.wikipedia.org/wiki/Johnson–Nyquist_noise

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2022, 04:56:22 pm
content: Ignoring capacitive effects, the 130nV/SqrtHz Johnson noise from the 1 megohm resistance means that achieving 1 microvolt (170nV RMS) resolution requires limiting the bandwidth to 1.06 Hz if a single pole 6dB/octave filter (1.6 shape factor) is used. 1 microvolt across 1 megohm is also 1 picoamp showing the importance of low input bias current, so the LT1012 will not do here without triming the input bias current. Incidentally, the LT1012 *is* used in 1 megohm input impedance circuits with good results.The low DC bandwidth means flicker noise cannot be ignored.An input shunt capacitance to crush the chopper current noise might work, but that will have to be a low leakage capacitor which would be difficult for say 0.25 microfarads to produce a 1 Hz noise bandwidth. I have not seen good data on chopper input current noise so I am not sure what value to pick. I would certainly try it with different lower valued capacitors.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 10, 2022, 05:29:49 pm
content: The data for the input current noise of chopper OP are not very reliable. Especially if the given values are very low they are often wrong. The current noise may also vary between units and may depend on the input impedance ( MHz range). The 170 fA/sqrt(Hz) given for the LTC2057 seem to be about right.This is more than the current noise of the LT1012 at 1 Hz, but not by much. The LTC2057 is not a type good for high impedance.The BW limiting does not have to be done directly at the input, e.g. with a 0.25 µF capacitor and 1 Mohms. The AZ OPs seem to be OK with something like a few 100 pF or 1 nF. Usually this is not just 1 capacitor to ground, but more like filtering like a pi filter or even more, so that additional input loading does not effect the higher frequency impedance seen by the OP input much.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 10, 2022, 05:33:33 pm
content: Also, on top of the "inherent" noise of the circuit, at those very low amplitudes on high input impedances, EMI is going to become a major concern.

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2022, 06:06:51 pm
content: The example below shows what I mean by low AC impedance. The DC path for each input of the chopper stabilized amplifier goes through a 10 megohm resistor, but the AC input impedance is controlled by a 0.1 microfarad capacitor.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 10, 2022, 09:31:00 pm
content: Good point