instrumentation-amplifieroperational-amplifiervoltage dividervoltage-referencevoltage-regulator
I'm working on using the TI INA333 in a single 3v supply configuration. I need to create a "mid supply" (half V+) for the REF pin.
I have thought of 3 ways to do this:
Here's the block diagram of the INA333:
It's going to be a battery powered device, so the solution that uses the least amount of current would be the best.
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.
It can be used but for most op-amps the maximum current you would be able to drive from the output will be limited to less than 50mA. Op-amps are not miracle workers! On the other hand you can attach an NPN and PNP transistor in a push-pull configuration and get more current: -
(source: northwestern.edu)
Or, you could use an OPA541 - it is an op-amp and can supply up to 10 amps: -
I guess this one is a bit of a miracle worker!
Best Answer
Whatever is connected to the REF pin should have low output impedance. If the output impedance is high, it will break the matching of resistors in the second stage of the InAmp (150kΩ ones in the drawing). Let's go through the list of proposed approaches in the O.P.
Voltage divider. By itself, voltage divider doesn't have a low enough output impedance. A voltage divider should be buffered in order to connect it to REF. Previously, I've posted a schematic and more details in this related thread.
Voltage regulator. Could work. Voltage reg has a low output impedance. Typical regulator fluctuates more than a voltage reference.
Voltage reference. Could work. Some voltage references have low output impedance. If a reference doesn't have low output impedance, you can buffer it.
I don't know what the signal is, and what will be done after the InAmp. However, pay attention to what's ratiometric and what's absolute.