![]() ![]() If you want to amplify mV, I would use an op amp with a lower input offset voltage. Generally LM324s are not good for amplifying mV dc voltages because of the input offset voltage.ĭont worry - this is teaching you much more about op amps than you can read in a book and professional engineers still get this stuff wrong! ![]() ![]() If you apply a low impedance voltmeter to the junction of 2 resistors, the junction voltage will change as the meter loads the resistor divider. Input bias currents, with the resistor values you are using should not be an issue (it amounts to the input bias current (100nA) x the parallel combination of the 837k and the 2.14k.) This only amounts to a 100uV or so.Īlso, how are you generating the 1mV? If it is from a high value resistive divider network, are you measuring the 1mV with a fairly low input impedance voltmeter? this could also be contributing to the errors you are seeing. So with a 1mV input and a 5mV input offset voltage, your amplifier could effectively be amplifying a 6mV or -4mV signal. This is a *DC error* that is effectively added (or subtracted - and it can be either) from your input voltage. As the contributors above have said, the main reason for your error is input offset voltage. Back to the point of the original OP, he needs to learn about the fundamentals of op amps. ![]()
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