High voltage measurement with an Arduino

Question

I'm looking for a way to somehow measure voltages in the range of 60~90V with an arduino analog input. These voltages are biasing a rather sensitive APD exposed to very low light levels, which supplies very small currents (~0.8 uA) for a transimpedance amplifier. The biasing circuit is shown below.

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I though about using a voltage divider with the lower resistor having a bypass Zener to protect from over voltage, but that would require very large resistances (8~10 GOhms, and I'm mounting this on a PCB). What kind of interference could this have on the APD measurement?

• Majenko: Very large resistances on an ADC input can seriously skew the results. It is generally accepted that the impedance of a device connected to an ADC shouldn't exceed 10% of the impedance of the ADC's input.
• Kamil: How acurrate measurements you need?

Answer 1

I though about using a voltage divider with the lower resistor having a bypass Zener to protect from over voltage, but that would require very large resistances (8~10 GOhms, and I'm mounting this on a PCB).

You certainly do not need GOhms. Note that the circuit you have specified uses a 47k resistor on the output. If you replace this with a voltage divider (say, 42k / 5.1k) you will get 3 - 6 volts with a 5k impedance. If this impedance is too high for your ADC, you can always buffer it with an opamp. Note that the 47k resistor shown will only dissipate ~ 170 mW at 90 volts, so it's not as if you'll save a whole lot of power by increasing its value.

And, just for future reference, zeners are not appropriate for microamp limiting. Typically their leakeage current is in this range.

• joaocandre: Indeed I had noticed the resistor at the end, and as such it would be relatively simple to build a voltage divider to measure output. I'd also wager that a buffer between the measurement node and the analog input of an Arduino would serve as protection against overvoltage - rendering a Zener useless either way.