CP2100A current probe measuring frequency - Page 1
Forum section: electronics / testgear

author: chrisb741
author_rank: Regular Contributor
timestamp: January 23, 2024, 02:02:30 pm
content: Can someone explain this too me.See image, the orange trance is a rolling average from the CP2100A not B probeIt is measuring the gate current to a MOSFET.If the probe is 800kHZ why am I seeing sub 1us current spikes

author: csuhi17
author_rank: Frequent Contributor
timestamp: January 23, 2024, 02:43:42 pm
content: Quote from: chrisb741 on January 23, 2024, 02:02:30 pmI don't see the image.

author: chrisb741
author_rank: Regular Contributor
timestamp: January 23, 2024, 04:50:52 pm
content: Added

author: bdunham7
author_rank: Super Contributor
timestamp: January 23, 2024, 04:57:42 pm
content: The 800kHz BW specification just means that the response will not roll off by more than a certain amount at that point. I don't see a specific number in the MicSig specs, but -3dB is a typical threshold. The device still has a response above that point, it just may be less accurate and will typically eventually roll off but may also have peaks in the vicinity of the upper BW spec. So you'll see those <1µs spikes, but the amplitude could be signficantly off.

author: Martin72
author_rank: Super Contributor
timestamp: January 23, 2024, 05:46:43 pm
content: Hi,QuoteWhat do you expect to see...It's the current through the gate - In the first time the gate current is "high" because of the gate capacity.These are the dips you see, after "charging" the gate capacity the current is very low - too low for the micsig probe.

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 23, 2024, 05:51:20 pm
content: Is that a high-side Mosfet or low side?The CMRR is something users of current probes don't want to hear or think about but it's usually quite bad: a change in voltage in the measured line with no current change, will induce a false measured current at the output of the probe.

author: chrisb741
author_rank: Regular Contributor
timestamp: January 23, 2024, 11:36:00 pm
content: It is an N-channel MOSFET with no gate resistorsBlue is the voltage on the gate, this is sent by an atmega328pYellow is voltage at DrainI dont know what it should measure which is why I decided to have a look and see.That first pulse shows 50mA after 200ns I expected to see something upto 80mA but dont know what it actually should be.Then I realized its bandwidth is 1250ns being much less then its BW limit shouldnt this value be much lower?

author: Martin72
author_rank: Super Contributor
timestamp: January 24, 2024, 12:01:25 am
content: What is the yellow trace ?Could you post the circuit where you did the measure?Which N-Channel type ist it?QuoteWhy ?(BTW, the negative current dip is about 80mA)

author: chrisb741
author_rank: Regular Contributor
timestamp: January 24, 2024, 12:55:28 am
content: But thats -80mA into the controller, I dont know how much it can sink.Given that this is beyond the BW limit of the device would these peak currents be higher?Attached the circuit I used. Trace in OP is with no resistors at the gateThe attached trace here is with the resistors as indicated by the diagram

author: Martin72
author_rank: Super Contributor
timestamp: January 24, 2024, 10:57:32 pm
content: Hi,I do not know what is about the "LED Stripe", I´ve taken randomly a LED in my spice simulation.First of all it´s not a good idea to drive the IRF540N without the resistors (Simu1), the reason why you don´t measure this current value could be that the µC limit it´s output - don´t stress him too long..Second simulation shows it with the 100R/100K on the gate.As I wrote you will only have current dips because of the gate cap charging.For more precise simulation I should know your load.Nevertheless, the bandwith of the current clamp ist not the deciding point.Else you could do:Integrate the resistors in your circuit, measure the voltage over the 100 Ohm resistor(expected value something around 40mV, easy for the scope) by the scope, compare it with the current clamp measure.

author: chrisb741
author_rank: Regular Contributor
timestamp: January 25, 2024, 12:18:47 pm
content: I have wired everything up againSame circuit with same resistors. The load is 1A CC from a SDL1020X instead of the LED stripThe ORANGE current probe is attached between the MCU and the resistorsThe PURPLE Diff Probe DP1007 is attached across 100RThe pulse is On for 1us Off for 1us

author: Noy
author_rank: Frequent Contributor
timestamp: January 25, 2024, 04:03:25 pm
content: Beside the other answers.Micsig CP2100A/B are nearly the same HW. As far as i know there wera / are a few probes which have matching specs even if they are labeled A/B.I think there was although a hacking thread abaout these probes.

author: Martin72
author_rank: Super Contributor
timestamp: January 25, 2024, 10:14:43 pm
content: QuoteIt would be interesting to see a picture of your real setup.I should still have IRF540N, I might rebuild the setup, but with a generator.

author: Kanbus
author_rank: Contributor
timestamp: January 27, 2024, 05:31:43 pm
content: Semi unrelated question.Won't you get unexpected results by using an electronic load? While your mosfet is off, the e-load circuitry will switch hard on to try and maintain the 1A. Then you'll have a dead short while you turn on your mosfet and until the e-load's regulation catches up again.