AnsweredAssumed Answered

AD633 Input Offset Problem

Question asked by JCDARDEN on Dec 29, 2011
Latest reply on Jan 5, 2012 by JCDARDEN

Simple AD633 circuit.

1. As AM Modulator per data sheet

2. Signal into Y1 is 1000Hz sine at 2V p-p (via 10uF cap)

3. Signal into Z is connected to Y1

4. Signal into X1 100Hz sine at 8V p-p (via 10UuF cap)

5. Y2 and X2 are grounded

6. Supply voltage is +/- 12V

 

 

Problem ...

 

This circuit simulates perfectly in multiSim.

 

However on the PCB ... the output is garbage.

 

I do see something I do not understand. That is ..

  - Although the X1 and Y1(Z) signals are coupled to the pins via a 10uF cap..

there are heavy DC offsets on the pins.. Vy,Z(pin-1/4) is offset -6V, Vx(PIN-7) is offset -2V

 

The input caps are not leaking .. the voltages on the circuit side of the caps are different.

 

Is ths a bad AD633 or is something wrong with using simple cap inputs to the AD633?

 

Thanks .... JCD

 

UPDATE ... 29 Dec 11

 

OK. So I bridged the input caps with a short wire and the AD633 produced the beautiful canoinical AM modulation output.  This way the AD633 inputs are direct to op amps (with zero volts dc level).

 

I don't know yet if it is pcb leakage or a characteristic of the AD633 but the input pins were accumulating charge as mentioned above and wacking the output. Now everythng is fine. In this circuit .. maybe everywhere .. the AD633 input pins cannot be driven via a capacitor.

 

The mulitSim simulation does, however, work fine with caps for input ????

 

Any comments/analysis will be appreciated.

 

/ ... JCD

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