Scope vs benchtop DMM for measuring voltage drop across FET  - Page 1
Forum section: electronics / beginners

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: May 11, 2023, 10:14:41 pm
content: I wanted to measure the voltage drop across the drain to source of a load switch FET to verify that the Rds(on) value was in the right ballpark. First I used a benchtop DMM. Then I tried using my scope to see how well it agreed with the DMM. So I put one scope probe on the drain and another on the source, and put their ground clips on the ground of the circuit. Then I set up the function to subtract the channels to see the voltage drop.Well I found that the value the scope method gave me was way different than the DMM. For example at 10mA load the DMM showed a 0.3mV drop which made sense because Rds(on) should be around 30mOhm, but the scope showed more like a300mV drop. The scope reading also varied significantly with the volts/div scaling. This surprised me because I thought scopes were good when it comes to pseudo-differential readings like this.This is concerning because in the past I've measured inrush current using this method of two probes to measure the voltage drop across a sense resistor.Are scope probes just kind of bad when used in this way?

author: wasedadoc
author_rank: Super Contributor
timestamp: May 11, 2023, 10:33:09 pm
content: I suspect that user error was more responsible than the scope itself for the 300mV difference. Did you first balance the two channel gains to get zero difference when connecting both to the drain? Even doing that, expecting to get close to 0.3 mV is asking a lot.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 11, 2023, 10:34:43 pm
content: IDK, but that must be a very good scope to it could show a 0.3mV drop.

author: bdunham7
author_rank: Super Contributor
timestamp: May 12, 2023, 12:06:08 am
content: Quote from: Jim from Chicago on May 11, 2023, 10:14:41 pmIf you are trying to measure a small difference between two voltages that are much larger relative to your ground point than the difference between them, then small differences in channel gain and probe ratio will drown out your small differential voltage. There's nothing 'pseudo' about the differential set up here, just the quality of the result. Purpose-built differential probes will have similar issues, although they try to minimize them with careful design and adjustment. They still typically would struggle to measure a 300µV difference between two 10VDC sources.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: May 12, 2023, 01:19:29 am
content: A few details missing: Was it a static DC test or under switching? What was the common mode voltage over you observed the 300mV difference?

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: May 12, 2023, 04:11:14 pm
content: Static DC test, p-chan FET where the source is about 48V with respect to ground.

author: TimFox
author_rank: Super Contributor
timestamp: May 12, 2023, 06:08:51 pm
content: In general, one should avoid measuring a small difference between two large values by measuring each value.For a static case, a hand-held DMM (not grounded) measuring the difference directly is a much better method.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: May 12, 2023, 08:14:20 pm
content: Vertical DC accuracy usually in the neighborhood of 2% even for the big name brand scopes. Asking 100mV accuracy over 48V full scale calls for 0.2% accuracy, and as you saw it’s not met. Handheld DMM is much more appropriate: it’s accurate and can float at your target voltage.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 12, 2023, 08:22:44 pm
content: In general, the DMM is more likely to be accurate than the scope but just in case, was the scope probe multiplier (attenuation) set correctly?

author: bdunham7
author_rank: Super Contributor
timestamp: May 12, 2023, 09:13:15 pm
content: Quote from: Electro Fan on May 12, 2023, 08:22:44 pmI don't see how that could help. What settings could you possibly use that would allow you to have 48V on each probe and still allow you to see <1mV difference? Either you are using 20V/div (or 10x probes and 2V/div) and 1mV is way too small to see or you try using 2mV/div and then you overload the input amplifiers. And there's no happy middle ground either. The only simple answers are using an isolated measuring device (DMM, isolated scope, isolated front end) or floating the DUT. Or floating the scope, but let's not go there...