Electronics > Metrology

Design considerations for 8.5 digit front end

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Context: I am designing an 8.5 digit ADC with surrounding voltmeter with a novel self-calibration technique as part of my final year project. (about which i will share but not until the project is finished).

As part of the ADC I need an input buffer and believe an ADA4625-1 unity gain buffer should be sufficient. Mostly because this is more about the ADC and getting a +-10V voltage digitized as accurately and precisely as possible. The following project would be about getting other functions such as I and R along with various voltage ranges but for now the ADC is the priority.

does anyone have any suggestions on the input buffer or should it be more than enough for the project at the moment?  Is there anything Im missing? or should this suffice?

Lets says 8.5 digit means a maximum count of +-200,000,000. The step size for 10V would be 50nV. An ADA4625-1 seems to have a offset voltage spec of 80uV typical. Does that seem suitable? Even if you are frequently self calibrating away that offset, just how well temperature controlled would the op-amp need to be tame its offset voltage drift to 50nV between calibrations?

The offset voltage of 80uV I should be perfectly fine. but the tempco can possibly be better. The typical is actually +-0.2uV/C with a maximum of 1.2uV/c

FYI - not targeting 8.5digits, but an attempt to find a simplest way to create an AFE for the high-end single chip ADCs.


The offset (and drift) is one of the lesser problems if the front end does some kind of auto zero by switching between different inputs, including a 0 V (or similar).  The CMRR could be a liniting factor for the linearity.  With typ. 130 dB this not enough to guarantee better than 0.3 ppm INL. However chances are that much of the CMRR is still linear - so from this side it can be just acceptable, though not great, when hunting for possible sources of INL.
Another important parameter is theg gain, as this effects how good the OP-amp can compensate for internal nonlinearity, especially the output cross over. Here the ADA4625 is quite good - though the cross over error could still be a point to whatch for. With the OPA145 I was able to see that type off error. Adding a constant current load can avoid the cross over error by operating the output in a class A range.

For a buffer it is relatively simple to use a bootstrapped supply and this way essentially eliminate the effect of the CMRR.

An alternative buffer would be a Zero drift OP-amp like LTC2057 or OPA189. These usually have very good CMRR and gain, at lest for DC and low frequencies.


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