EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: ocset on May 20, 2020, 08:05:57 pm
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Hi,
We wish to compare two voltages and see which is highest, and get a rough measurement of the difference. We came up with the attached circuit.
Its cheaper than an In Amp….eg we could just use an MCP6002 dual opamp. Though we aren’t sure. We can’t find In Amps with this ease of functionality. Do you know of one off hand?
LTspice sim and pdf schem attached
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It can work. Are you sure you need to amplify that voltage difference?
But wouldn't it be easier to do a straight measure of both voltages? Switch ADC input from first to second voltage in order?
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What sort of accuracy do you need?
The biggest inaccuracies are likely from the offset voltage of the opamp used.
This can be greatly reduced if you can use for example a 30V power supply (which most opamps can handle) combined with a 2:1 voltage divider on the 48V and 52V power rails.
Then after the opamp you can use a 30V to 5V ratio divider to bring the output in range of your ADC.
Now you have a division ratio of approx 23 and then amplify the difference 8x, which also amplifies the opamp's offset 8x. If you can avoid the amplification, you also reduce amplification of the offset error.
The classical differential amplifier with 4 resistors of equal size already gets you there, because the maximum voltage on the opamp inputs would then be half of the maximum of your power supply. I.E: 52V/2=26V, which is well within range of an opamp with a 30V power supply. You may want to add some protection for when the power supply of the opamp fails, filtering etc.
Instead of using a passive divider on the output, you can also easily clamp the opamp output to within your ADC range. (Series resistor and (diodes to GND and +5V).