Since the DC-DC converter that was requested as a sample has not arrived yet, I soldered wires to short the pads of the DC-DC converter. Isolation is not important at this point because the board will not be connected to 150 V during validation. Using the continuity mode of a handheld DMM, I verified that none of the power nets are connected. I also checked for shorts between all adjacent pins of the ADC, since I only have one and I don’t want to destroy it. Upon powering the board, all LEDs lit up, and the LDO was correctly outputting 5 V. The ADC’s internal reference is correctly outputting 2.5 48 V. The LTC6820 is outputting 2 V at the IBIAS pin, indicating that it is awake. So far, everything seems to be working.
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Unsurprisingly, the reserved bit didn’t change. The reference voltage decreased to 1.2 V. I’m not sure why the reference stayed at 1.2 V instead of 0 V, but writing to registers seems to work.
I found out that firmware was expecting the start pin to be pulled low instead of high. I’m not sure if this is necessary because the datasheet seems to suggest that data can be read from the ADC in continuous conversion mode, but I still attempted to change it. I couldn’t cut the 5 V trace on the PCB because it continues to the LTC6820. Instead, I decided to try lifting the pin off the PCB, soldering a wire to it, and then soldering the wire to ground. After the wire was soldered, the pin twisted and snapped off. In hindsight, I should have just put Kapton tape on the pad. Now that the pin was gone, I just soldered the wire to the stub that was left and even though it’s delicate, it seems to work. The firmware smoke test required some changes to make it work. Here’s all the changes I can remember at the moment:
The read register command wasn’t sending the byte that indicated how many registers to read. This was an oversight from firmware and was never programmed.
The ADC’s internal reference bias was not enabled. This is required for the current sense amplifier to output the correct value.
After fixing the above problems, the ADC readings were fairly accurate. Here are the inputs and outputs during testing:
Voltage at Input of ADC | Input - 2.48 V Reference | ADC Reading |
|---|---|---|
3.29 V | 810 mV | 805 mV, 806 mV |
2.48 | 0 mV | -0.195503234863 mV |
0 V | -2.48 V | -2.497 V |
These values were measured without any calibration.