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I am taking a -8 volt signal and using a resistor divider to make it -3 volts by using a 16.5k ohm resistor as r1 and 10k as r2. I am feeding the -3v into a single supply opamp (+5 and gnd) in the inverting configuration with unity gain (using two 10k resistors) in order to obtain 3 volts. Will the resistor divider before the unity gain non-inverting opamp effect my unity gain?
Essentially i am taking a signal that goes from -8 to 0 and i want to turn this into 3v to 0 for input into a ADC.
Best Answer
Yes, the divider definitely will affect the gain calculation, because its equivalent source resistance (Thévenin resistance) of 6.23 kΩ is in series with one of your 10 kΩ gain-setting resistors. The overall gain will be -0.2325, rather than the -0.375 that you're looking for.
If you want an overall gain of -3/8, it would be simpler to forget about the voltage divider and just set up your opamp with 20 kΩ input and 7.5 kΩ feedback resistors.
Also, be sure to use an opamp whose common-mode input range includes ground.