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