I've built an Arduino shield based on the reference circuit in a TI app note for an MSP430-based modem (page 9/pdf page 13 of
http://www.ti.com/lit/an/slaa037/slaa037.pdf). The plan is to allow people to implement experimental software modems on the AVR, so the shield is primarily an interface between the phone line and Arduino analog inputs and PWM outputs. (And before you ask, this shield is intended to be used on private PBXes only--not connected to the PSTN.)
The problem is that during initial testing, I'm getting no audio from the phone line. After taking another look at the TI reference design, I'm beginning to wonder if it's wrong. There are a couple of questions I have about it, and I'm hoping someone can help explain its theory of operation.
First, the two output op-amps have resistors from VREF to their non-inverting inputs. Is there a reason for this? VREF should be a low-impedance source, so I'm not sure what those resistors are doing.
Second, I'm seeing a signal from the phone line on one side of 500 ohm resistor, but not the other. This leads me to believe that the low-impedance op-amp outputs are overriding any incoming signal. I don't see how this hybrid circuit works. Perhaps the 500 ohm resistor is supposed to be on the other side of the 33k/pot voltage divider?
Any insight would be much appreciated.