Circuit resetting when touching one multimeter probe to GND - Page 1
Forum section: electronics / beginners

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 20, 2018, 03:47:17 pm
content: I'm trying to measure some voltages in a circuit I built, and whenever I touch my multimeter's (BM869s) probe to ground, it shows -0.26V and the ADC in the circuit resets. Other probe is floating.Anyone have an idea why that's happening?

author: ammjy
author_rank: Regular Contributor
timestamp: June 20, 2018, 03:54:13 pm
content: i am sorry, my reply is not answer.Multimeter have enough impedance for measure. so i think, measure with multimeter is not problem.(if you used multimeter with low impedence mode, please try without low impedence )if you upload circuit diagram also, other people's can more detaily answer to you.

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 20, 2018, 04:02:19 pm
content: The actual circuit is a bit older (no separate 3.3V rail for digital and missing (noncritical) pullups) but should otherwise be identical.The ground plane in the PCB layout is filled in and via stitched, but I've omitted that for the printoutE: right now it's getting power from a Raspberry Pi, and all outputs/inputs are connected.

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 20, 2018, 05:37:03 pm
content: Touching any pin with just the multimeter probe, disconnected from the multimeter, is enough to reset the device.Unfortunately I can't measure if it's due to RESET going low and causing a reset, or a voltage drop below min.

author: ignator
author_rank: Regular Contributor
timestamp: June 20, 2018, 05:45:09 pm
content: Your DMM may be high resistance, but it has an intrinsic capacitance, and when you touch the probes to your circuit, the capacitance undergoes a charge transfer either from or to your circuit.Try putting a 100K ohm resistor in series with each lead of your DMM probe.I've fought this same problem with a high end Fluke DMM in a ATE usage, every time the relay closed, the circuit (in my case it was a comparator with hysteresis) flipped states.

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 20, 2018, 06:05:14 pm
content: That's what I figured. Even connecting a multimeter probe without the multimeter connected had enough capacitance to trigger a reset.

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 20, 2018, 06:22:22 pm
content: Attaching a 100k resistor to one of the probes was sufficient. Thanks!

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 20, 2018, 07:26:40 pm
content: Is there a way to make my circuit more resistant to stray capacitance? It's supremely annoying having to reset the ADC after I screw in a new probe.

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 20, 2018, 09:06:20 pm
content: Looks like the RPi's power supply is weird and causes a crash. Welp?E: measured with another, still crashes. PC USB works but only supplies 4.7V, so it should be *less* good. Weird.

author: bson
author_rank: Unknown
timestamp: June 21, 2018, 01:50:12 am
content: My guess is there is an abnormal amount of switched current returning through GND and your probe effectively adds a notch filter to it, resulting in GND bounce and a loss of adequate supply voltage during the spikes. Adding a 4.7uF decoupling cap to the ADC's supply could help, but better is to find the source of the switched ground current.

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 21, 2018, 02:01:05 am
content: To clarify, it's not just GND (that's just what I thought at first). Touching *any* pin triggers the reset (analog input, digital input, etc).

author: KaneTW
author_rank: Frequent Contributor
timestamp: July 05, 2018, 06:09:09 pm
content: Measured it at the ADC's DVDD. This was triggered by touching GND with an inductance (multimeter probe, sufficiently long wire, etc). Anything I can do about it?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 05, 2018, 06:19:11 pm
content: Welcome to the wonderful world of compliance testing.