I looked for this answer but was unsuccessful so I want to ask the experts here. I have configured an op amp in non-inverting mode (the first in a chain of 3 stages). It is for amplifying extremely low level noisy signals so I'm using lower resistor values. The input resistor is a 100ohm and the feedback resistor is 3K ohms. I'm feeding the circuit on the bench with a sine wave from my signal generator which has an output impedance of 50ohms and a minimum level of 4mV. I need to go lower to about 5uV minimum. The generator increases in 1mV increments.
My question is how should I configure a voltage divider in order to divide this signal down to 5uV (minimum) and maintaining a 50ohm impedance match. I need to measure the output of the entire amplifier chain while knowing what level is going in versus what level is coming out. Hope this makes sense and hope I've included all needed info. Thanks for the help.
FJ
Best Answer
Let's call the top resistor of a voltage divider R1 and the bottom R2. The output impedance is R1 // R2, and the gain is R2/(R1+R2). Set these two equations equal to what you want. Now you have two equations and two unknowns. Solve accordingly.
Note that R1 encompasses the 50 Ω of the source, so the actual top resistor (what you add directly in series with the signal generator) will be 50 Ω less than what you solve for R1. At your very low gain, this will make little difference though.