# Voltage source for Vref or Voffset

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.

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.

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:

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.

• 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? – Nazar Jun 17 '14 at 19:49
• 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. – Nazar Jun 17 '14 at 19:55
• A trimmer should not cause any problems. It's good engineering practice to minimize the range. I'll edit the answer a bit. – Spehro Pefhany Jun 17 '14 at 20:39