I have searched the internet for hours upon hours to find answers to my questions with no luck... Below are 2 circuits that are a part of a project I am working on.... These are the interface circuits between my system and a balanced signal TDM box with 600 Ohm termination (old school Voice Freq Channel to T1 multiplexer)... The problem is when I run voice grade tests through the system I am getting poor results and I am almost 100% there is impedance mismatch causing the problems. I don't have much experience with telephony interface circuits (or impedance matching between op amps and transformers) and am seeking help....
Figure 1: The input side of the circuit
Figure 2: The output side of the circuit
These circuits were derived from the ADAU1442-EZ board which is the DSP we are using for this application.
On the input side of the circuit a balanced signal comes into the transformer which has a 1:1 ratio. I have the 680 Ohm resistor to ground but I think I need to put a 2.16uF capacitor in series with that 680 Ohm resistor to match the line impedance, right?
On the output side I think I should remove the 22k Ohm resistor along with the 3.3nF and 4.7uF cap (replacing the 4.7uF cap with a 2.16uF), again to match the line impedance, but I am not 100% about this.
Is this problem a lot bigger than what I think? Part of our circuit also has a built in telephone hybrid (dual transformer design) which when activated bypasses the 1:1 isolation transformers (not shown in the above figures but the op amp circuits are the exact same). That is where I am seeing the worst voice grade test results (with the hybrid in the circuit).
I appreciate any feedback provided.
The DSP, if running a pass-through program, should be essentially transparent from an analog-to-analog performance perspective.
If you are actively implementing filters during your test, however, then it's possible that they are having a negative impact on the performance, depending on their configuration. Did you try running the test with the DSP program bypassed?