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ADL5391 large output drift

Question asked by Machine42 on Oct 18, 2016
Latest reply on Oct 19, 2016 by Machine42

Hi

I have an ADL5391 setup to act as a voltage controlled attenuator for an input signal. I have a single ended input to XP and then a DAC controlled voltage for multiplier on YP and offset adjust on ZP. None of them have any termination to 2.5V or any other level. Each of the XM, YM & ZM inputs are terminated with 50R to a 2.5V Ext Ref. The WP & WM outputs are terminated into a single 200R resister load and I measure the differential across this with a difference amplifier arrangement to convert to a single ended output. The XP input ranges from 2.5V to 4V. YP is at 4V and ZP is at 2.5V.

When I modulate the XP input with a slow 1KHz square wave I am getting a very slow output settle. WP output is overshooting by 13% and then slowly settling. Please see attachment. After the difference amp the WM and WP drifts combine to give 25% settle drift.

Why is this happening?

I have tried monitoring the ZP & YP nodes and these are flat. When measured, none of the inputs show this drastic drift. There is some step change on the XM, YM and ZM inputs but these are small flat square wave changes.

Regards

Luke

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