AD8608 driving 600 Ohm Impedance line through isolation transformer

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.

  • Well I removed C112, R41, and C58 from the 2 circuits above and it doesn't seem to improve the intermod test levels. My next plan is to remove C56 and replace it with a 2.2uF capacitor since I read that telephone line impedance is characterized as 600 Ohm resistance with 2.2uF capacitance. Along those lines I also plan on putting a 2.2uF cap in series with R42...

    How were the values for C83 and C56 determined? Are they simply to block any kind of DC from entering the op amp or are they acting in conjunction with the resistors to form a filter (I feel really stupid for asking that because I should know the answer but the digital world has taken over my brain since graduating )... Could the RLC system be resonating and causing of the 3rd order intermod being lower than expected?

    Thanks for any input.

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