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| Very low bias current Op-amp to buffer a Kelvin Varley divider |
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| splin:
I thought this interesting question didn't get much response over on the Test equipment forum so I thought I'd put a link here where it might reach a wider audience: https://www.eevblog.com/forum/testgear/buffer-for-kelvin-varley-divider/ The original poster was looking for a very low input bias (preferably < 1pA) op-amp with very low drift, to buffer a KVD with 70k o/p resistance, whilst not compromising the .1ppm linearity spec of the KVD significantly. Jim Williams published an app note covering this exact application, where the errors due to the amps CMRR and finite gain are also pointed out (140dB CMRR equates to .1ppm of linearity error in a voltage follower configuration). That was in 1999 so perhaps there are some better parts available now? Can anyone recommend a better op-amp than the LTC1052 or LTC1152 for a one-off piece of test equipment where selection is possible to a limited extent - for cheaper devices at any rate. I'm also interested if anyone has any suggestions to my question as to why Jim Williams chose to use the LTC1152, which seems to have better specs, rather than the LTC1052 - see reply #11. |
| Kleinstein:
The LTC1052 uses external caps and has higher bias currents. More modern AZ Ops would be something like AD8551 or LTC2057. In general the lower noise ones have higher bias currents and often also a higher offset. However Bias currents are relatively stable, and little dependent on temperature (unless very high). For ultimate performance some kind of bootstrapping may be useful, as the bias can depend on common mode voltage, so that input resistance is not as high as one might expect for a CMOS chip. Finding something in the 1 pA range is hard (maybe selected LTC1050 / ICL7650), and will be a rather high noise, but low offset version. |
| splin:
--- Quote from: Kleinstein on June 12, 2015, 08:53:18 am ---The LTC1052 uses external caps and has higher bias currents. --- End quote --- My datasheets show that the LTC1052 has lower bias current - 1pA typ @25C, 30pA max compared to 10pA typ, 100pA max --- Quote ---More modern AZ Ops would be something like AD8551 or LTC2057. In general the lower noise ones have higher bias currents and often also a higher offset. However Bias currents are relatively stable, and little dependent on temperature (unless very high). For ultimate performance some kind of bootstrapping may be useful, as the bias can depend on common mode voltage, so that input resistance is not as high as one might expect for a CMOS chip. --- End quote --- The LTC2057's could be a good candidate but the bias current is a bit high at 30pA typ @25C, 300pA max. How practical is it to trim it out? It will change as the common mode voltage changes so bootstrapping would help a lot in this respect. When you say bootstrapping do you mean servoing the supply rails to keep the common mode voltage constant to remove the finite gain (Avol) and CMRR error contributions? What are the disadvantages of doing this - presumably the bootstrap amplifier's noise gets added directly? --- Quote ---Finding something in the 1 pA range is hard (maybe selected LTC1050 / ICL7650), and will be a rather high noise, but low offset version. --- End quote --- The LTC1052C has better specs than the LTC1050 except for Avol, particularly Ib typ of 1pA v 10pA. Any particular reason you suggested the 1050? Of course typical is not guaranteed and both have the same 30pA max spec @ 25C. It may not be possible to find any of those 1pA devices for a reasonable expenditure as they aren't especially cheap parts. |
| Kleinstein:
With bootstrapping the OP I meant adjusting the supply to keep common mode voltage constant. The main disadvantages are the extra effort and the need for extra protection circuit. The protection leads to some extra leakage, but may be needed anyway. There is essentially no extra noise or drift, as the AZ OP is still doing the final amplification and will attenuate noise / drift from the bootstrapping circuit to a large extend (e.g. by common mode rejection or loop gain). One advantage is, that the AZ-OP does not need to be made for the high supply voltage. The choice for AZ OPs at 5 V supply is much larger than at 20-30 V. Also bias may be lower at low supply voltage. The second advantage is, that CM voltage may be chosen for minimal bias. I have not seen directly analog compensating for the bias, but it does not look impossibly. The classical solution with same impedance at both inputs however does not work, as the sign is different. Even with a 100 M resistor 10 pA of bias would only be 1 mV of voltage. So a rather higher resistor may be needed. |
| splin:
--- Quote from: EmmanuelFaure on June 13, 2015, 02:18:58 pm --- --- Quote from: Kleinstein on June 13, 2015, 12:06:47 pm ---I have not seen directly analog compensating for the bias, but it does not look impossibly. --- End quote --- It seems now you do : :) http://electronicdesign.com/analog/single-supply-op-amp-input-bias-current-cancellation --- End quote --- You have to wonder if the author actually built that circuit and tested it or just did a theoretical design. I wonder how stable it is and how low you can reliably cancel the input bias current? I see that Linear's LT1013 datasheet includes a 'Triple Op Amp Instrumentation Amplifier with Bias Current Cancellation' circuit on page 18 with the claim of typical input bias current < 1nA (compared to the typical Ib value of 12nA). Not sure how well such schemes would work with a FET/CMOS amp needed for a KVD buffer though. The alternative to trimming out Ibias is to use a non-autozero op-amp with Ib < 1pA and periodically trim out the offset voltage, which is relatively easy to do by shorting the input, especially if the buffer is already driving a suitable meter/ADC. CMOS amps tend to be a bit noisy, but the OPA140 might be a reasonable choice with .5pA typ, 250nV pk-pk .1 to 10Hz noise and 1uV/C drift? The problem then is which <1pA leakage analog switches/Fets would be suitable if you wanted to automate it? It would seem that most of the devices used in the HP3458A front end are no longer manufactured. |
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