# Switch current sense resistor: series or parallel?

I am building a device that measures current in various ranges. For this I want to switch in different current sense resistors. To do this I see two ways: the resistors in series or the resistors in parallel.

Series:

simulate this circuit – Schematic created using CircuitLab

Parallel:

simulate this circuit

They produce slightly different values, but that doesn't really matter all that much, because I have to calibrate it for the actual values anyways.

But I am wondering if there are any inherent advantages or disadvantages of one method over the other for current sense purposes.

• Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. Apr 28, 2019 at 21:33
• Oh, didn't notice that ;-) I'll make an edit
– Jens
Apr 28, 2019 at 21:33
• It's just for future reference. Your circuit is very simple. Apr 28, 2019 at 21:36
• Fairly simple I think, you just need to find the equivalent resistances and compare them each other. Apr 28, 2019 at 22:16
• Read up on Kelvin sensing. Yes, from THE Lord Kelvin. Apr 28, 2019 at 23:47

• @Jens The $R_{DS(on)}$ of analogue switches depends on supply voltage, signal voltage (or: input voltage) and temperature. 100$\Omega$ is only 10% of 1k$\Omega$, so 10% deviation on $R_{DS(on)}$ gives a 1% measurement error. Since they are also cheaply available, I would have picked an analogue switch with a lower ON resistance. Apr 29, 2019 at 9:23