AVM-2000 - Page 1
Forum section: electronics / general

author: branadic
author_rank: Super Contributor
timestamp: July 17, 2019, 08:02:36 pm
content: So it's time to have a seperate thread about AVM-2000 with range down to 1µV. I found some teardown pictures on bbs.38hot.nethttp://bbs.38hot.net/thread-270-1-1.htmland part of the schematic.Looks like they use OP282 as input opamp. Any more info on that particular unit?-branadic-

author: 2N3055
author_rank: Super Contributor
timestamp: July 17, 2019, 09:08:33 pm
content: Input amp is AD8221 precision instrumentation amp. U8 is a part of input protection or ranging, i don't remember anymore.U9A is part of offset compensation circuit..I can take a look at details if you want...Regards,Sinisa

author: branadic
author_rank: Super Contributor
timestamp: July 18, 2019, 07:06:30 am
content: That would be great, as this is the only modern unit using opamps instead of a bunch of old and partly unobtanium transistors.-branadic-

author: 2N3055
author_rank: Super Contributor
timestamp: July 18, 2019, 08:26:51 am
content: OK,tonight I will gather what I have an post..Regards,

author: guenthert
author_rank: Frequent Contributor
timestamp: July 18, 2019, 04:22:39 pm
content: Quote from: branadic on July 17, 2019, 08:02:36 pm... down to 0.1µV (1µV is the lowest range of the 50 years old Fluke 845).Thanks for the schematics. I think however that the OP Amp and surrounding circuity is only a part of what ensures the performance of the AVM 2000. There's a relay at the input working as a chopper and I'd think much is done in software after ADC.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2019, 04:38:06 pm
content: The circuit used a BJT based instrumentation amp and possibly the relays for some low frequency chopping. The bias current is compensated with DAC and through Gohms resistors. Still this is more like a nV amplifier for relatively low impedance source - it is not really suited for higher impedance source due to the current noise. At 1 Hz the AD8221 is supposed to give some 550 fA/Sqrt(Hz) and some 12 nV/sqrt(H). So the noise resistance and thus impedance with best noise figure is at some 25 kOhms. With more than a few 100 kOhms the current noise would be a problem. So the input impedance is more like comparable to DMM.This is different from the classical null meters like Keithley155 : they are more suited for higher impedance sources due to less current noise and higher voltage noise.Chopping / AZ with relays (likely reed relays) can also be tricky, the choice of relay can be rather important.

author: 2N3055
author_rank: Super Contributor
timestamp: July 18, 2019, 08:18:42 pm
content: In attachment surprisingly useful application note that speaks about operating principles, and operation and maintenance manual, with full schematics, operating principles and troubleshooting.It is before it was acquired by Tegam.Regards,

author: Echo88
author_rank: Frequent Contributor
timestamp: July 18, 2019, 08:34:56 pm
content: Thank you very much.

author: 2N3055
author_rank: Super Contributor
timestamp: July 18, 2019, 08:49:56 pm
content: Quote from: Echo88 on July 18, 2019, 08:34:56 pmMy pleasure!

author: splin
author_rank: Frequent Contributor
timestamp: July 19, 2019, 12:33:39 am
content: 400pV pp noise?Pah! That's not aknifelow noise Null meter. That's a low noise Null meter:http://www.emelectronics.co.uk/p13a.html24pV pp - provided your source resistance is less than 250 milliohms otherwise you're measuring the thermal Johnson noise of the source. A snip at approx £8500.

author: guenthert
author_rank: Frequent Contributor
timestamp: July 23, 2019, 01:55:49 am
content: Quote from: guenthert on July 18, 2019, 04:22:39 pmI might have misinterpreted a msg on another bbs or projected ideas I had about a Null meter using a cheap MCU. From the schematics it doesn't look like a chopper is used, neither it is mentioned in the description. The (from the high-level schematic) low pass filter with a corner frequnecy of 10Hz (matching the minimum filter time of .1s) doesn't leave much room for a chopper. So this seems to be a DC amplifier (hmmh, they don't seem to specify a drift rate).The relays do allow for input protection, but also polarity reversal and null-ing the input.Sorry, if I have misled anyone.

author: guenthert
author_rank: Frequent Contributor
timestamp: July 23, 2019, 02:02:42 am
content: Quote from: Kleinstein on July 18, 2019, 04:38:06 pmNot sure, how to interpret the current noise figure of the DS. Is this bias or offset current? In any case, the instrumentation amplifier should suppress common mode voltage noise due to bias current noise very well and there's a low impedance path in the form of awhopping 2uC0.1uF capacitor right across the inputterminalsof the i. amp. which should limit the resulting voltage noise due to input offset current noise.

author: Echo88
author_rank: Frequent Contributor
timestamp: July 23, 2019, 01:19:59 pm
content: @guenthert: The manual mentions the following: "This output allows the operator to utilize the AVM-2000 as a high-gain, high-linearity,chopper-stabilized instrumentation amplifier with isolated output."I also was curious what chopper-frequency theyd use for the relays given the 10Hz-filter, but its mentioned nowhere.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2019, 02:23:04 pm
content: The only chopping I see for the input is through the relays. Using polarity reversal is a kind of chopping and the demodulation afterwards can be done after the ADC. There could be some filtering before the relays - this does not interfere with chopping. The signal filtering to a large part is done digital in software. So there is no more analog low frequency LP filter.Using digital filtering can actually be a huge advantage: analog filter nearly always are IIR type and thus give some kind of exponential creeping towards the final value. With a 1 second time constant of some 1/6 Hz cross over this take some 5 seconds to settle to 0.5 %, and longer for a larger step. Digital flittering can be FIR type and thus have a finite response, going down more like nearly linear. So it would be only some 1-2 seconds to 0.01% settling.The current noise is some kind of random bias / offset current. This would to a large part no be suppressed by the CMRR.A large capacitor at the input would only act as an analog filter with all the downsides of possible slow response. A nasty point of the current noise is that it has quite some 1/f contribution, so that filtering is not really effective, as there is always lower frequency noise. At 550 fA the current noise is higher than with most AZ OPs. I would consider this more like deserving a few.

author: guenthert
author_rank: Frequent Contributor
timestamp: July 24, 2019, 05:16:57 pm
content: The capacitor in front of the instrumentation amplifier will be a challenge for any chopper scheme. I was wondering, whether not only the static input bias current of the amplifier, but also the "pump-out" current resulting from charging/discharging said capacitor during polarity reversal could be compensated. Under worst case condition (1mV, 0.1Hz "chopper" frequency) this should be a variable current with an average amplitude of 1nA, no?