Q
What range of potentiometer values do you recommend for the Offset Null
Configuration? The data sheet (Page 14) shows a schematic with a 20kohm used;
but there are no further notes?
A
We recommend the 20k pot. The pot is used to balance the collector current
sources of the input stage and compensates for mismatches in transistor sizes.
A higher value pot will give you a more coarse adjustment (higher offset per
degree of rotation). A lower value pot will give you higher sensitivity but
generally reduce the linearity. The offset adjust mechanism is not explicitly
tested during production therefore we do not provide numbers for resistor
values vs. amount of offset adjust.
Do note that the offset adjust on the AD797 should only be used to remove the
offset due the AD797. It should not be used to remove offset in your system.
System offset should be removed by summing in an appropriate voltage into the
non-inverting terminal.
There are two reasons for this:
Trimming the AD797 for minimum offset, also trims for minimum temperature
drift. We only guarantee enough offset adjustment to compensate for the AD797
offset, the absolute amount of offset adjustment available is likely to vary
significantly from part to part.