Ron Wright wrote: > The voltage levels on most DTMF decoders are set by the IC designer. On > decoders like the 8880 and 8870 pin 3 is what the internal op-amp of the IC > is producing and this parameter is determined by the IC designer. As with > any IC one needs to supply the proper levels to the device to allow it to > achieve what is needed.
Pin 1 is the non-inverting op amp input, Pin 2 is the inverting op-amp input, and Pin 3 is Gain Select, described below. From the MT8870 datasheet: "GS: Gain Select. Gives access to output of front end differential amplifier for connection of feedback resistor." "The input arrangement of the MT8870D/MT8870D-1 provides a differential-input operational amplifier as well as a bias source (VRef) which is used to bias the inputs at mid-rail. Provision is made for connection of a feedback resistor to the op-amp output (GS) for adjustment of gain. In a single-ended configuration, the input pins are connected as shown in Figure 10 with the op-amp connected for unity gain and VRef biasing the input at 1/2VDD. Figure 6 shows the differential configuration, which permits the adjustment of gain with the feedback resistor R5." Same pin-out on the 8880. You can build it as a single-ended input or a differential one, and mess with GS... depending on how you built your audio circuit... The input op-amp, has a decent amount of gain available in the op-amp itself, too... because it' meets the specs to pull a tone up from -31 dBm (dBm referenced to 1mV into a 600 ohm impedance input -- the typical telco reference for dBm) and won't fully reject tones until -37 dBm. These chips can hear DTMF tones that are incredibly low amplitude... on quiet circuits. The problem is... radios aren't quiet circuits. The published error rate at -31 dBm is 1 in 10,000 without background noises. They're very good decoders. But depending on how they're hooked up, generic statements like 3 V P-t-P aren't ALWAYS correct... unless the repeater controller is feeding the op-amp in the unity-gain single-ended configuration. Nate WY0X
