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Subject: Re: Adjustable precision diode

Date: 05/10/02 at 7:20 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: Adjustable precision diode posted by Bob Jeffords on 05/09/02 at 11:27 PM:

Hello Bob,

One way you might be able to do this is to use a dual op-amp plus a few resistors. The first op-amp is used as an inverting amplifier, gain = -1. You create the offset by adding another resistor between the summing node (the minus input of the op-amp) and one of the supply rails. The second op-amp in the package is used to make another inverting amplifier (gain = -1) that takes the output signal from the first op-amp and inverts it. The overall effect through the circuit is to amplify the signal by -1 x -1, i.e. it isn't changed except for the addition of the offset at the summing node of the first stage.

Connect the offset resistor to the positive supply rail to get a positive offset at the output of the second op-amp, or to the negative supply rail for a negative offset. The larger the resistor value, the smaller the offset.

If you are using surface mount parts, a dual op-amp in an SO8 package plus the resistors should fit within 0.5" square. Accuracy of the circuit will depend mostly on the accuracy of the resistors defining the gain of the op-amp stages.

Hope this helps,

David Ashby

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