# Picking correct level of resistance for a voltage divider

I want to measure my DC voltage source (aka batteries), vs my known Dc voltage from my DC-DC regulator. To do this I'll need a voltage divider. I can calculate the resistances I need - but what I'm not sure about is the over all level of resistance I should go for.

If the voltage divider resistance is to low, I'll be burning current for no effect. If it's excessively high.... is there any downside if the current is near zero? Should I just got for 100K ohms and leave it there?

The math is easy. But picking the right target? That's hard.

When you factor in the input impedance of the measurement device, your options will reduce but yes, there will be a range of values that can be chosen and you will tend to choose the option with the highest impedance that still produces acceptably low errors in measurement.

For instance, the ADC inbuilt in such devices as a PIC micro will not want to see a signal impedance more than a few kohm. As you go to greater values than a few kohm, the measurement error will increase and this can be alleviated, to some extent, by adding a capacitor from input to 0V.

So, read the data sheet for your measurement device, restrict your range of impedances and choose the highest set of resistors that give you an acceptable measurement error.

Sometimes you find that there is no overlap and, in cases like this you would use a spare IO line to activate a MOSFET to connect your battery to a potential divider. Then you can use quite low values and get good measurement accuracy and the battery energy spent is only for a short period of time in the bigger picture of things: -

Taken from this stack exchange answer (by me).

• Accepted as answer - both answers the generic case... and links to a very specific answer that's close enough to my situation to make no difference. Commented Nov 19, 2016 at 20:15

The impedance of a voltage divider depends critically on
a) what's driving it and

Given your driver is either batteries or a PSU, then you can go down to very low values successfully.

You don't say what your load is, but I will assume it's a DVM. Now most of these have a 10M input impedance, but I did buy a cheap one recently that was 1Mohm input. If you drove that from a 100k divider, you would have up to 10% voltage error, not very good. Be warned that some ADCs have lower input impedances than that.