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op amps wont zero

I have a pair of TL071s running as follower with gain. The second 071 uses 1k trimpot on 1-5 to offset. Inverting input has100 ohms to ground and feedback from 6 thru 470K fixed and 100K trimpot. With gain set to lowest the circuit stabilizes at near 3volt on pin 6. I would like it to be at 1/2 of 5 volts so it will match the zero of the data logging section which follows the op amps. I have tried feeding positive and negative voltages via a 1 Meg pot to both inputs and can never get it to stabilize except as above. The only option I can think of is a pot across the output to ground to bring this to the 2.5 volts I would like. I this my only solution? Tango2

Reply to
Tango2
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Eh ? A follower has no gain !

You mean a DC gain of ~ 5000 x ? You're using the wrong op-amp for that kind of gain.

Graham

Reply to
Pooh Bear

Sorry, the first 741 is a buffer to input. 2nd has gain of approx. 3000 where pins 1 & 5 are used for the offset null. Funny it works thorough. Yes, in fact I've used a 741 here too. Perhaps my explanation was too abbreviated.

The problem is that when offset null adjusted so it is stable (i.e. not zig-zagging all over the screen) that is what should be at my zero level. Here I am outputting to a screen which gets its input from an A/D. The software has been tested and DOES show that for 127 value from A/D the trace is at zero on the screen. Thus for 0-5 volt range the zero is really 2.5volts top of scale is 5 volts and 0volts is at bottom. The problem is that the output to the A/D when the Op Amp is stabilized, the actual voltage is 3.2 volts. Hence, I am considering a voltage divider across to 2nd Op Am to ground and tap this trimpot to get the voltage to 2.5 volts.

This circuit is being used by at least one other geologist with a different logging configuration where they too are recording earthquakes. They are using 741s but I opted for 071 to minimize device noise. Tango2

Reply to
Tango2

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