In a circuit with a LTC1668 driving a differential buffer stage, and using precision (0.1%) resistors, I'm seeing an output gain error of -4% (there's small variation from unit to unit, but the error is always close to -4%). I've seen the same error across three different revisions of the PCB, on 15 different individual assemblies, both hand built and machine built.
The error isn't a problem - I can very easily compensate for it - but I'd like to get to the bottom of its source.
This is the buffer stage:
Not shown is the LTC1668 reference input, which is driven externally by a LTC6655-4.096 (Vref) through a 0.1% 3.32k resistor (Rref).
The buffer stage is the same topology as the buffer in figure 7 of the LTC1668 data sheet, but the component values are different.
The amplifier is an ADA4637-1.
Given all that, with a DAC code of D:
The factor of 8 is taken from the LTC1668 datasheet, the rest is from basic circuit analysis. RV = 49.9R, RI = 1k, RF = 6.81k at the moment, but the exact values aren't important. I've used a couple of values for RF during testing, but the error is always -4%.
Anne know what the explanation is, or seen the same issue?