# Differential and single-ended input impedance

I have this circuit and I'm interested in finding both the single-ended and differential input impedance. But is there even a difference? This is also at DC, so we can just ignore the inductors and the cap and assume them as ideal. The single ended impedance was easy enough, it was approximately 5k. But what's the differential input impedance?

It's a bit unusual to hear single-ended and differential. Are you sure you don't mean common-mode and differential?

Differential is measured from one input to the other input. (It's what a signal sees if it flows into one input and out the other input).

Common mode is measured by shorting the two inputs together and measuring between that node and ground. (It's what a signal sees when it appears on both inputs).

Single ended would mean between one input and GND, but that is a meaningless measurement for this kind of circuit since you don't send a signal into this circuit that way.

• ... No I mean single-ended and differential. So is the differential input impedance just 215 ohms? Sep 26, 2020 at 3:27
• @kefffin About 215 Ohms. Or more specifically, 215||20K. Single ended would mean between one input and GND, but that is a meaningless measurement for this kind of circuit since you don't send a signal into this circuit that way. Sep 26, 2020 at 3:38
• hmm I wouldn't say its meaningless. 215||20K, really? I wouldn't think the 10k's matter. No current would flow from input to another through those, it would go ground. I see that as an open essentially. Sep 26, 2020 at 4:03
• @kefffin Not in a differential signal. Apply 1V to one input and -1V to the other input. Where does the current go? Sep 26, 2020 at 4:05
• I guess each source will have current flowing through its corresponding 10k...but still not seeing it Sep 26, 2020 at 4:17