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I have a DiffAmp AD8132 following an OpAmp LT1818. A 1.5V voltage source, such as voltage divider, voltage reference, or voltage regulator should be connected to the B input pin of the DiffAmp to compensate for the offset of the original signal. I was wondering if output impedance of a voltage source would be as crucial as for Vref pin of the In-Amp, which is discussed in this thread.

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I plan to connect the output of the OpAmp directly to point A, and a voltage source directly to the point B. If the impedance matter in this case too, then which voltage source would be the best choice? Do I need any additional compensation resistors before the A and B? The Vcm (pin 2) of the DiffAmp is set to 1.5V to provide a common mode signal at the output.

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Yes, low impedance is important.

The differential input impedance (looking into either input) is 696 ohms when the diffamp is in balance, so if you're looking for 0.1% accuracy you need the source impedance to be well under an ohm.

You can use another of the same kind of op-amp (LT1818) to buffer the 1.5V reference, which could be derived from a higher voltage reference with a voltage divider.

This is not the usual application for a differential amplifier, seems like a lot of parts just to remove 1.5V from the input signal.

Edit: For a voltage divider you can use something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Try to make R2 about 1/5 to 1/10 of the pot value, and choose R1 and R3 to get enough range to account for all variations for sure, but not too much more.

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  • \$\begingroup\$ Thank you. My next stage is a differential input ADC. So, if I connect two LT1818s to A and B respectively, my input impedance should be well matched? This seems to be a better choice than using some voltage reference? \$\endgroup\$ – Nazar Jun 17 '14 at 19:49
  • \$\begingroup\$ Also, if I decided to use voltage divider, would a trimmer be 'ok' in terms of noise as compared to two resistors? I sample a video signal. Want to be it clean. \$\endgroup\$ – Nazar Jun 17 '14 at 19:55
  • \$\begingroup\$ A trimmer should not cause any problems. It's good engineering practice to minimize the range. I'll edit the answer a bit. \$\endgroup\$ – Spehro Pefhany Jun 17 '14 at 20:39

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