Question
We have already used the AD7795 in a multi channel thermocouple logger. The
extra channels and sequencer on the AD7124-8 device are attractive. However it
doesn't appear to have chopper stabilisation of the input offsets?
We have seen unacceptable offsets appear on the AD7795 when this feature is
disabled in pursuit of higher throughput.
The offset drift figures look low typ 10nV/DegC however the input referred
offset graphs shows drift with temperature in the tens of uV range. Can you
please explain these graphs ?
Answer
The AD7124-8 (http://www.analog.com/AD7124-8) offers a way more flexible way to
get rid of the input offsets and gain errors. The device offers four
calibration modes to reduce the errors to noise levels. Kindly find the
‘Calibration’ bookmark on page 51 of the datasheet. Here the calibration modes
are explained in detail. The limits of this calibration is explained on the
following page.
The input referred offset graphs for AD7124-8 are there to give an indication
how the device behaves in the different power modes and for different gain
settings. Once the working temperature in the circuit is defined a trade-off
between power and precision can be made.