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.

Parents
  • Harry,

    Thanks for posting links; I will spend some time reading through those and looking further into possibly using different op amps.

    Brett,

    Thank you for the reply. I created a pass-through program and uploaded it to the DSP; the 2nd order intermod seemed to improve, but the 3rd order intermod was the same if not a little bit worse... I figured out how to do a echo return loss measurement on our TIMS and if I have both ends terminated 2-wire @ 600 Ohms I get ~ -20 dBm ERL but if I unplug the far end it drops to ~ -10 dBm (which I expected it to be worse). I found a document online that had the schematics for a radio to telephone interface circuit that I plan on taking bits and pieces from and adapting into my design to see if it improves at all...

    If you are curious as to what I found: http://www.sunairelectronics.com/web/workspace/uploads/rtu-200-1326998987.pdf

    Again, thank you for your feedback.

Reply
  • Harry,

    Thanks for posting links; I will spend some time reading through those and looking further into possibly using different op amps.

    Brett,

    Thank you for the reply. I created a pass-through program and uploaded it to the DSP; the 2nd order intermod seemed to improve, but the 3rd order intermod was the same if not a little bit worse... I figured out how to do a echo return loss measurement on our TIMS and if I have both ends terminated 2-wire @ 600 Ohms I get ~ -20 dBm ERL but if I unplug the far end it drops to ~ -10 dBm (which I expected it to be worse). I found a document online that had the schematics for a radio to telephone interface circuit that I plan on taking bits and pieces from and adapting into my design to see if it improves at all...

    If you are curious as to what I found: http://www.sunairelectronics.com/web/workspace/uploads/rtu-200-1326998987.pdf

    Again, thank you for your feedback.

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