user253: I'd like to measure the voltage at a test point on a stepper motor driver chip with an arduino. The voltage at this point controls the amperages to the phase coils and is adjusted to trim the driver.
I have a common ground with the driver already, I'd like to wire the drivers v-ref point to an analog input to drive an LCD showing the output amp number so I can adjust the driver.
The issue I am worried about is that with the hardware I have to test with a properly trimmed driver is pushing 0.84A.
Is this safe to do without killing the arduino board? 

user10907: Yes. The only thing you need to be concerned about is the voltage. It needs to be less than the Arduino Vcc.
The voltage sense circuit (analog inputs) typically have a high impedance and should draw a negligible current from the circuit being sensed. I didn't have much luck finding the actual spec for any of the AVR chips, but I did find this answer to the question on Electronics.StackExchange – where they say "hard to tell, the specs are incomplete, but 10 KΩ seems like a reasonable guesstimate/assumption" (I paraphrase). The reasoning is interesting and worth a read.

253: <p>I'd like to measure the voltage at a test point on a stepper motor driver chip with an arduino. The voltage at this point controls the amperages to the phase coils and is adjusted to trim the driver.</p>

<p>I have a common ground with the driver already, I'd like to wire the drivers v-ref point to an analog input to drive an LCD showing the output amp number so I can adjust the driver.</p>

<p>The issue I am worried about is that with the hardware I have to test with a properly trimmed driver is pushing 0.84A.</p>

<p>Is this safe to do without killing the arduino board? </p>
 10907: <p>Yes. The only thing you need to be concerned about is the voltage. It needs to be less than the Arduino Vcc.</p>

<p>The voltage sense circuit (analog inputs) typically have a high impedance and should draw a negligible current from the circuit being sensed. I didn't have much luck finding the actual spec for any of the AVR chips, but I did find <a href="https://electronics.stackexchange.com/questions/67171/input-impedance-of-arduino-uno-analog-pins">this answer to the question</a> on Electronics.StackExchange – where they say "hard to tell, the specs are incomplete, but 10 KΩ seems like a reasonable guesstimate/assumption" (I paraphrase). The reasoning is interesting and worth a read.</p>
