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Excessive AD835 offset with gain = 37

Question asked by Reson8r on Dec 30, 2014
Latest reply on Jan 9, 2015 by Reson8r

I'm using an AD835 as a voltage-controlled amplifier with gain up to +37 (31 dB).  Attached is a simplified circuit diagram.  GAIN ranges from about 25 mV to 1 V, and SIGNAL is an audio signal no more than +/- 1V.  I have an offset trimmer providing +/-100 mV on pin Y2.  The feedback resistors should provide a gain of 37 with reduced BW (which isn't important here).  Using the data sheet's definitions on pg 10, k = .974 and U' = .0273.

 

My problem is excessive DC offset at the output.  Here is an example idle condition (all levels DC): X1 = 307 mV, X2 = 0 V, Y1 = 15 mV, Y2 = trimmed to 0 V, and I see 1.5 V on the output.  The output according to the data sheet eqn (4) and (5) should be (307 mV) * (15 mV) / .0273 + 0 = 169 mV, not 1.5 V.

 

Shorting X1 and X2 together or Y1 and Y2, the output offset only drops to 1.4 V.  That seems to be the minimum output offset I can get.

 

The Z input bias current will cause some output offset, but only about 180 mV, not the 1.5 V I'm seeing.  It's as if there is a large internal offset which the gain provided by the feedback resistors is amplifying.

 

The circuit is functional and providing the gain control I want, but I need to get rid of this offset.  Where is it coming from?

 

Thanks for any help,

Gerrit

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