Today I pulled the ETM ring heater driver to investigate why its response to current was so different from the ITM ring heater driver. I found that resistor R44, which controls the gain of the input, was left void on the ETM. The circuit designs had been updated to include a 49.9k resistor, raising the gain by a factor of about 2. I added the 49.9k resistor, returned the ETM ring heater driver to the field, and recalibrated the driver. The calibration was performed by measuring the voltage over a 0.5 ohm resistor on the ring heater driver board (test points TP1 and TP2). The supplied current was increased from 0 to 425 "mA" of DC offset in steps of 25 "mA".
The driver's calibration is much closer to the ITM calibration, and is more consistent with using milliamps for the DC offset on the medm screen. Previously, it was calibrated at 0.0498 mA/ADC count. The updated calibration is 0.02396 mA/ADC count. The attached trace shows the data using the old calibration on the same axes as the data using the new calibration.
I edited the plot to make the axis labeling more transparent. The x-axis is the output on the channel H2:TCS-ETMY_RING_HTR_SEG2_I_MON_OUTPUT (what the MEDM screen thinks it's sending to the ring heater), the y-axis is the current calculated from the voltage measured over the sense resistor R49 on the driver.
Again, the board was modified to bring its gain in line with that of the ITM driver. The initial calibration was done for the unmodified board. The modified calibration is the correct calibration for the modified ETM driver.