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Very low bias current Op-amp to buffer a Kelvin Varley divider

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Gyro:

--- Quote ---Which is still 10 times as much as an auto-zero
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Again...only to demonstrate the >25 times improvement that was achieved by supply bootstrapping and Ib / Vos compensation of the basic op-amp. Study / learn / use techniques or not as you wish.

splin:

--- Quote from: XFDDesign on June 16, 2015, 03:51:49 pm ---The Analog Devices AD8616 might be a good candidate if you apply some bootstrapping. 0.2pA typ, 1pA max. The typ CMRR is only 100dB so you would have to wrap some bootstrapping around it to get the extra 40dB and use it as a buffer stage.

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Its a bit noisy at 2.1uV pk-pk and 1.5uV/C (typ) to 7uVC (max) would be a problem especially if the KVD is used at 1V or less.

Oddly figure 9, Ibias v temperature, shows approx 10pA @ 25C. Yet another duff datasheet 'typical' graph. Presumably the .2pA typ/1pA max spec is correct.

Its the first time I recall seeing a Voffset v common mode voltage chart in a datasheet (but I probably wasn't looking). Its pretty horrible - see attached. Around 3.5V as far as I can tell, one unit appears to exhibit an input offset change of at least 300uV over a change in common mode voltage of 250mV or so. That equates to a CMRR of  only 833 or 58dB! How does that allow them to specify 80dB minimum with Vcm = 0 to 4.5V? Another faulty chart? Wrong units? Is CMRR specified using a box method - ie. 450uV/4.5V = 80dB?

The headlined offset voltage is 65uV but the spec, in the Vs=5V table is actually 23uV typ, 60uV max - for Vs at 3.5V and Vcm = 0.5V and 3V. That's cherry-picking! At 5V its 80uV typ and 500uV max.

splin:

--- Quote from: poorchava on June 16, 2015, 05:47:16 pm ---As far as I know LMC6001 offers lowest guaranteed bias current at 25fA.

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The LMP7721, LTC6268/9 and LTC6268/9-10 specify 20fA max at 25C, but they are all 'by design' unlike the LMC6001 which is claimed to be 100% tested.

splin:

--- Quote from: Marco on June 16, 2015, 06:22:07 pm ---Just use a AD8638 or a LTC1052 and throw an aggressive low pass filter at it to get rid of the noise and eat the slightly slow response on a setting change (maybe use some nonlinearity to speed it up a bit for large changes).

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On the other thread you pointed out figure 14 in the AD8638 datasheet showing .1pA Ibias at 25C, Vs = +/-8V. Does that seem likely given that it is specc'd as 1.5pA typ at 25C, Vs = 5V (table 2) and 1pA at Vs = 16V? (and shown as 1.5pA for Vs = +/-2.5V in figure 13). One of them is wrong given they are both supposed to be typical (though they don't specify the measurement setup for fig 14).

1pA is probably good enough, but there is still the question of how many units you'd have to test before you found a typical one.

splin:

--- Quote from: Gyro on June 16, 2015, 01:29:53 pm ---
--- Quote ---That's very interesting. Do you have any links to circuit diagrams?
--- End quote ---

I do if you're prepared to squint a bit! I've never found a copy online, I meant to scan the whole manual sometime but my trial copy of Scan-n-Stitch expired (they're A3 schematics).

Attached are the input board schematic, including i/p protection (Datron used a combination of JFETs, Zeners and an 88k 8W series resistor chain!), Ib compensation and bootstrapped supplies. Q11 (TO92) is thermally coupled (superglue) to the LM312 (TO99) under a plastic cover. The double sided PCB copper is arranged to be as isothermal as possible around them.  All resistors are carbon film (apart from the high >10Mohm ones which are carbon composition).

As I say, they managed to achieve >10G input resistance, <2pA/'C and 0.2uV/'C using VERY humble parts by today's standards and with no cpu to do auto-zero or correction in these units. My unit holds within 1-2uV from cold to operating temp on the 10mV range (x100 gain) with no appreciable long term drift (well aged by now!)

The next generation - 1065/1061 are all evolutions of the same basic starting point, but with a cpu for compensation, autocal, autozero etc.

I've also attached the relevant page from the circuit description.

Hope it's at least of historic interest anyway.

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Thank you, very interesting. You have to admire the designer's skills given these date to the 70's (60s?). Even the recently released 34470A 7.5 digit dvm is only specified at < 30pA. I guess its considered good enough and not worth improving, or they'd rather sell you another instrument for your 'specialist application'.

I wonder how the 1061 design deals with the LM312's offset variation with input resistance given the latter is in the control of the user?

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