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
