AD8400 as a remote trim pot

[Joachim Verghese]

Hi,

I've used the AD8402 dual trimpot for sw-controlled tuning and scaling
of VCOs. It's a nice and economical part, but there are a few things
worth mentioning.

The AD8402 was designed to be powered with +5V, and consequently,
signals to the device must remain within the 0...+5V range. For
example, the output of a +/-15V-powered op-amp cannot directly be
connected to the AD8402, since power-up/down spikes from the op-amp
output will eventually destroy the AD8402. In my own designs signals
associated with the AD8402 are connected to +5V-powered CMOS op-amps
only.

Secondly, the end-to-end resistance of the AD8402 is fairly dependent
upon temperature, so be sure to use the device as a potential divider
if you use it for trimming CVs. In this configuration the temperature
coefficient is only 15 ppm/degC, as opposed to 500 ppm/degC in rheostat
mode.

Also, as with all mixed signal design where high frequency clocks
are present, it's highly advisable use separate grounds and supply
filtering for the analog and digital sections.

-joachim