Hi,
[See below "EDIT" before replying to this post]
Are you saying that you added an op amp to the circuit along with a small "injection" resistor?
I am not sure you can get away with that, even though the op amp is a unity gain amplifier. That's because the network function is changed when you add a unity gain amplifier between two successive stages of any network made of resistors and capacitors. It depends partly on the values and the frequencies of interest but looking at the 5.1k resistor and the 22k resistor, 5.1k is too large of a percentage of 22k (about 25 percent roughly) which means adding a unity gain stage probably changes the dynamics too much. It would also depend on the 220nf capacitor but that sounds large enough where we would still think about comparing the two resistors directly.
In other words, the response of the entire circuit could be very different with the unity gain amplifier in there. If it were not for that 5.1k resistor and 220nf capacitor (part of the 22k network) i would not be so hesitant, but because the three make up a network i would not feel comfortable at all using that op amp like that. Now if it was on the other side (right at the 50 current sense resistor) i might not feel as bad about it because 50 ohms (sense resistor, not injection resistor) is a small percentage of 5.1k and 22k.
The whole idea is to be able to inject a signal that "rides" on the normal signal without altering the circuit too much. The more you alter the circuit the less accurate the results are, and in this case, a unity gain amplifier probably modifies the circuit way too much. Not only does it modify the network transfer function because of the unity gain isolation property, it also introduces other frequency altering components which may or may not affect it as much but still has to be considered.
You could test though, doing it both ways and see if you can measure any difference in the simulation.
In case this doesnt make enough sense, to understand this concept better you could look at a passive two stage RC low pass filter. Look at the response when the two stages have equal values for the resistors and capacitors, then add a unity gain amplifier BETWEEN the two stages and notice how the response changes drastically. The impedance presented to the input of the second stage gets much lower, and the impedance presented to the output of the first stage gets much higher, which changes the circuit drastically.
EDIT:
I just double checked and i see that the 5.1k resistor is not 5.1k it is 5k1, which means it is a 5.1 Ohm resistor. Somehow i read that resistor value wrong with the first look.
This presents a different problem because 5.1 Ohm is a very small value. This means that the unity gain amplifier will not change the dynamics as much as if it was really a 5.1k resistor.
This also means that the ideal way to drive this is with a zero ohm source injected between the 50 ohm sense resistor and the 5k1 network. That would probably mean a low impedance transformer with the secondary in series with the 50 ohm sense resistor and the 5k1 network.
If you do it any other way you risk altering the results, but lucky for us today we have the simulators so you could test it both ways and that should tell you if the unity gain amp is ok to use that way. In the simulation you can use a very very small value resistor or even just a sine source in series with the 50 ohm sense resistor and the 5k1 resistor network. Try it both ways and see if you can measure any difference in the gain and phase.
This turned out to be a little more difficult because we dont really have a high impedance on the left side of the 50 ohm sense resistor unfortunately. We need a source with an impedance much less than 5.1 Ohms to do this right.