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Simple DC AFE for ADC chips with unipolar diff inputs
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iMo:
With ADA4522 chopper (HV type with +/-27V Vcc).
ADA seems to be a more advanced arch than the LT2057.
Simulation of a temperature sweep with worst case (?) resistor's tempcos.
The LT5400-8 tc divider mismatch is 0.5ppm/C.
David Hess:
If you bootstrap the operational amplifier's power supplies to its output, then a low voltage part can be used as a buffer over a +/-20 volt or higher input range.  In addition, this removes error from limited common mode rejection which will otherwise be significant.

There is nothing wrong with using junction transistors for low current input clamps other than having to quality or test them however a relatively inexpensive 2N4117/8/9 JFET is guaranteed to have an input bias current of 10 picoamps or less at 20 volts.  A 2N4117A/8A/9A will be 1 picoamp or less.
iMo:
This is with bootstrapping the OPAMP.
As an example below the low-voltage ADA4528 (5.5V Vcc max, set to 5.1V in the simulation) in use.
The ratio of the base resistors gives the voltage spread at the opamp's Vcc.
Also the resistor's values have to be such the transistors feed enough current into the opamp.

PS: this wiring is prone to oscillation with ADA4528, however. Further analysis is required.

David Hess:
It is a good idea to minimize load changes on the output of a precision operational amplifier.  Self heating creates offset voltage changes which limit open loop gain and common mode rejection.  In this case this will also manifest as poorer linearity.
iMo:
Do you mean to use fets in the bootstrap?
The divider there - what to use? - a capacitive divider ? :)
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