AD or TI eval boards as a nanovolt preamp? - Page 1
Forum section: electronics / general

author: J-R
author_rank: Super Contributor
timestamp: February 04, 2025, 12:52:38 am
content: Recently I was contemplating either an off-the-shelf nanovolt preamp (EM, Keithley, etc.) or a dedicated nanovolt meter (K181, etc), but saw these cheap eval boards and was wondering if they would be worth giving a try instead:AD:https://www.mouser.com/ProductDetail/584-EVAL-INAMP-82RZTI:https://www.mouser.com/ProductDetail/595-INAEVM-ALT-SO8Specific amps I'm planning to use with the eval boards:https://www.mouser.com/ProductDetail/584-AD8429BRZhttps://www.mouser.com/ProductDetail/595-INA849DRThoughts?

author: laichh
author_rank: Regular Contributor
timestamp: February 04, 2025, 02:19:19 am
content: Hi J-R,If the input bias current is a concern, thenhttps://www.ti.com/tool/JFE2140EVMis the way to go.

author: J-R
author_rank: Super Contributor
timestamp: February 04, 2025, 10:13:23 am
content: Quote from: laichh on February 04, 2025, 02:19:19 amHow would this perform in comparison to the Keithley 181?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 04, 2025, 11:01:08 am
content: The boads are not really made for nV performance. The first 2 are for INA chips. These are usually non AZ type amplifier that can have reasonable low dift, but still not good for low DC votlage.The JFE2140 board is a low noise JFET amplifier. The DC performance can be good for a FET type, but still not a good DC amplifier.To get close to the Keithley181 DC performance, one would need to start more with a chopper stabilized zero drift OP-amp, maybe OPA388 or ADA4522.

author: laichh
author_rank: Regular Contributor
timestamp: February 04, 2025, 02:58:11 pm
content: Hi J-R,Not sure about Keithley 181. But in Keithley 2182A, they use a pair of matched Toshiba 2SK170 to form the pre-amplifier, so performance wise I would expect the TI JFE2140 to be very similar to Keithley 2182A, or at most +20% worst than 2182A.Do note that the JFE2140EVM evaluation module's input is AC coupled (band-passed). If you want to measure down to DC, you must bypass the AC coupling capacitor.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 04, 2025, 03:48:41 pm
content: The keithley 2182 uses a pair of SK170 JFETs in the amplifier (the A version uses 2 pairs in parallel). The main part to make it a DC accurate amplifier is however the chopping part around the JFETs. The chopping part is what makes the amplifier special and for best accuracy one may want synchronization between the chopper part and the ADC, as they use the integration by the ADC to supress the residual ripple from chopping.The Ti elaluation board is only the simple amplifier with good AC performance, but relatively poor DC performance even without the AC coupling capacitor.For a DIY build chopped amplifier similar to the K2182 one would normally not use the Ti PCB, buit integrate the small circuit part as well.

author: Overspeed
author_rank: Frequent Contributor
timestamp: February 04, 2025, 07:31:06 pm
content: HelloEM amplifier A23 or A2X module perhaps ? made in UKRegardsOS

author: Hedrix
author_rank: Contributor
timestamp: February 05, 2025, 02:06:57 pm
content: The answer will depend on what you are trying to measure.If you need few-nanovolt DC measurements, the Eval-boards cant really compete, same for almost all amplifier ICs (IIRC there are someopamps that reach 30nVpp, but with drawbacks like power consumption and bias currents)The commercially available Amplifier have some impressive datasheet specs but are not perfect in reality either.The performance stated is usually under near ideal conditions and is easy to be negated by power supplys, PCB routing, EMI etc.Also those need a signal with very low source impedance and they are DC (to low frequency AC) only. (though that applies for nanovoltmeters as well).That gives you a low-noise amplified signal, but then youll still need to digitize that.A commercial nanovoltmeter will be more expensive than the amps alone but wont come with the headache of designing all of the auxiliary stuff themodules will need. And in the nV-region everything is a source of headaches.if you want to measure low-voltage AC, then some of the eval boards might work well enough, and there are a few good designs/resourcesaround on the internet for DIY low-noise BJT/JFET amplifiers.DIY'ing such an Amplifier for DC is much more difficult and there are few existing designs and resources in general on the Internet.

author: J-R
author_rank: Super Contributor
timestamp: February 05, 2025, 11:22:13 pm
content: My present target is probably the 10nV area, but not necessarily the single digit nanovolts. The DMMs I have now essentially bottom out at a few microvolts.

author: laichh
author_rank: Regular Contributor
timestamp: February 06, 2025, 03:45:28 am
content: Hi J-R,I would recommend the following:Pre-Amplifier Opt 1:TI JFE2140EVM(JFE2140 JFET + OPAx202 composite amplifier, with necessary modification)Pre-Amplifier Opt 2:ADI EVAL-ADA4523-1(may drop-in alternative op-amp in MSOP-8 package such as OPA210IDGKR)Data Acquisition:TI TIDA-010945(PGA855 PGA + ADS127L21 ADC)

author: Overspeed
author_rank: Frequent Contributor
timestamp: February 06, 2025, 10:45:12 am
content: Hellojust locate this solutionhttps://quantasylum.com/collections/frontpage/products/qa351-uv-dc-voltmeternot a lot technical information but price is quite lowRegardsOS

author: Hedrix
author_rank: Contributor
timestamp: February 06, 2025, 12:12:51 pm
content: for 10-ish nVpp the eval boards wont really do it alone, but with some modification you could get there, i.e. building an amplifier out of multiple parallel JFE2140.(but still needs digitizing and good power supply)or maybe if 30nVpp are acceptable than a used Keithley 181.

author: guenthert
author_rank: Frequent Contributor
timestamp: February 07, 2025, 11:30:52 am
content: I like to caution that used Keithley 181 perform not universally great. Some seem to exhibit very low noise, mine is more around the performance claimed in its specifications (or a even a bit more). Perhaps I should have another look at its internals, but there was also an in-series upgrade: later models have an additional shield.Not long ago, there were a few 182 on the secondary market for a surprisingly low price. Those might be a suitable compromise in price and performance.

author: CurtisSeizert
author_rank: Regular Contributor
timestamp: March 21, 2025, 02:36:00 pm
content: I built up an NVM with JFE2140s, and I will confirm that it takes quite a lot of work to turn a JFE2140 hybrid amp into a DC accurate circuit. Just placing the components on the main board took most of a day. If you want something cheap and easy, you can get good voltage noise performance with ADA4523-1 run at low supply voltages, which minimizes offset and thermal issues. Generally any evaluation board is not going to be suitable for tens of nV performance no matter how good the circuit because thermal gradients are very problematic at that level, so it will need a case. It will also need good isolation, so something that is meant to be powered with a wall wart is not really going to do the trick.