Photon 2 RTL872xD ADC Input Impedance

Firmware
1

I was looking for the input impedance for the RTL872xD, and saw that there were two possible values.

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When the Photon 2 does an analog read, does it operate in bypass mode (4 MΩ)?
Also, do we know what the resistor divider mode does? I do want to measure a resistor divider, but if the only difference is 8x less impedance then it probably won't make much of a difference for me. Especially not if it's not available in the Device OS.

2

Resistor divider mode is only used for VBAT_MEAS to reduce the voltage to the 3.3V ADC limit, that is to say there is just a fixed voltage divider before the ADC Input.

If you have a voltage divider before your input, it will work better if it's a relatively small resistance across both legs. Because of the high impedance of the RTL872x ADC, you may get inaccurate readings if you have a large resistance on the input.

1 Like
3

Got it, thanks.

How small would you recommend R1 to be? I was planning on using 10 kΩ.

4

10K should be fine.

It's just if you're trying to minimize leakage current and use like 100K or 1MΩ resistors that you'll run into trouble.

5

Yeah I’m planning on only supplying 3.3V when I need to measure it. R1 = 10k and R2 is something less than ~470k.
Thanks!

6

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