Other Parts Discussed in Thread: TMP422
We have several boards with remote temperature value problems. For some, the value takes several minutes to change, for instance, at power on it reads 0C, after some minutes it finally drifts up to 22C (correct value). Others read extreme negative values - for example, -50C for a room temp measurement. The local sensor is consistently correct.
We are using 2N2907A transistors, in the transistor configuration, on cables about 24 inches from the PCB with the TMP-422. We are in extended temperature mode (and accounting for the 64C offset) and sampling at 1 second. Our schematic is exactly Figure 21 in the datasheet, with Rs = 50 ohms and Cdiff = 100pF.
One thing I've noticed is that, on good channels, the DX+ and DX- signals are at approximately 0V, then jump to something like 1.5V and 1V, respectively, during the measurement interval. On the bad channels, the signals are at about 2 or 3V, and drop down to 1.5V and 1V during the measurement interval. This seems wrong to me as it puts a voltage across the transistors all the time.
Any ideas?