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
  • echo,

      I'm more of an amps and volts guy, so I have trouble thinking in dBm.  WRT THD, I can relate to that;

    simply used that for an example.

    A couple of comments

    --  For maximum power transfer, you want TX/RX impedances matched.  For maximum voltage transfer,

       you want the source impedance to be zero and the receive impedance to be infinite.  I don't see how

       mismatching at 3 kHz would affect IMD.

    --  Most rail to rail inputs are a p-pair in parallel with an n-pair, so they have crossover distortion.  See figure 10

      in the rev N data sheet.  Also MT-035:

    http://www.analog.com/static/imported-files/tutorials/MT-035.pdf

    -- Sometimes the groups at semiconductor companies don't talk to each other enough.  Looks like the DSP guys didn't check with the op amp guys.  For precision audio I would not use a quad.  See article(s)

    http://www.planetanalog.com/document.asp?doc_id=528114

    http://www.planetanalog.com/document.asp?doc_id=528116

    Also, in the above article, see figs 10 and 11.  A quarter of an inch of pc board trace made 50 dB difference in

    crosstalk.  If you had a vendor do the pc board layout, you may have several problems.

    If you want the ultimate in performance, especially below 100 Hz, bipolars have a much lower 1/f corner.

    I would also allow for +/-5V or +/-15V if necessary.

    --  If you google "capacitor distortion", you will get some interesting stuff:

    http://www.diyaudio.com/forums/solid-state/5107-capacitor-distortion.html

    So on a new board, you may have one big problem or ten little ones.

    Good Luck,

    Harry

Reply
  • echo,

      I'm more of an amps and volts guy, so I have trouble thinking in dBm.  WRT THD, I can relate to that;

    simply used that for an example.

    A couple of comments

    --  For maximum power transfer, you want TX/RX impedances matched.  For maximum voltage transfer,

       you want the source impedance to be zero and the receive impedance to be infinite.  I don't see how

       mismatching at 3 kHz would affect IMD.

    --  Most rail to rail inputs are a p-pair in parallel with an n-pair, so they have crossover distortion.  See figure 10

      in the rev N data sheet.  Also MT-035:

    http://www.analog.com/static/imported-files/tutorials/MT-035.pdf

    -- Sometimes the groups at semiconductor companies don't talk to each other enough.  Looks like the DSP guys didn't check with the op amp guys.  For precision audio I would not use a quad.  See article(s)

    http://www.planetanalog.com/document.asp?doc_id=528114

    http://www.planetanalog.com/document.asp?doc_id=528116

    Also, in the above article, see figs 10 and 11.  A quarter of an inch of pc board trace made 50 dB difference in

    crosstalk.  If you had a vendor do the pc board layout, you may have several problems.

    If you want the ultimate in performance, especially below 100 Hz, bipolars have a much lower 1/f corner.

    I would also allow for +/-5V or +/-15V if necessary.

    --  If you google "capacitor distortion", you will get some interesting stuff:

    http://www.diyaudio.com/forums/solid-state/5107-capacitor-distortion.html

    So on a new board, you may have one big problem or ten little ones.

    Good Luck,

    Harry

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