Oscilloscope + Generator ground loops and misc stepper stuff - Page 1
Forum section: electronics / beginners

author: Personwithhats
author_rank: Contributor
timestamp: August 19, 2023, 08:07:44 pm
content: Hello,I'm attempting to use a signal generator to generate signals for stepper testing -Then an oscilloscope to measure the voltage drop across a resistor in-line with one of the stepper motor coils.Tektronix 2467B analog scopeHP 3326A signal generatorAgilent E3631A for motor power, Arduino gets power from a laptop (via battery, not charging)DMM4040TMC2208 chipI have several issues atm -1) The Oscilloscope and signal generator are both mains grounded, bench PSU output is isolated, for some reason the DMM4040 also appears main grounded?I thought DMM's would be isolated?2) How can I use the generator to make a signal without having to deal with the oscilloscope possibly making an earth loop?Right now I'm stuck to using signal from arduino instead but that has bandwidth limitations and a much less consistent signal.3) If I use on-chip resistors I get half a sine-wave, if I use in-line resistor I get a full sine wave??? I've no clue why this one is going on.Motor and everything runs as expected its just my measuring setup is wrong for some reason......

author: Personwithhats
author_rank: Contributor
timestamp: August 19, 2023, 10:43:23 pm
content: 1) Ok nevermind the DMM is not mains grounded, must have been shorting something2) I can measure the current by sticking TWO probes one on each side and now it's like a differential probe, a bit obvious in retrospectStill not sure what's going on with #3 though

author: Terry Bites
author_rank: Super Contributor
timestamp: August 20, 2023, 07:48:18 am
content: The DMM inputs float free of ground.The DMM is probably grounded via a cable braid/ shield somewhere.#3 My mind reading skills are inadequate here but I'll put on the tin foil hat and give it a go.I'm guessing that the driver IC runs on a single supply and so can't provide a negative monitor signal.Earth loop errors occur when power (motor) currents and signal currents are flowing in the same conductor or PCB trace.Is the DMM connecting to ground via the sense resistor? Check your wiring.You can use the scope's math functions to get a non-earth referenced measurement as Personwithhats suggests.Clip the two probe grounds together. CMRR and such. You can sense on the high side or the low side using this arrangement[Invert 2 and ADD] that gives you Ch1-Ch2.Your fancy DMM can directly measure and extract the current data and apply all kinds of cool stats to it. It will work up to 10 kHz.Use the RTFM button to access these funtions.If you set the DMM to 3A, it will have a burden voltage of 150mV.I've only recently retired my lovely 2467. A sad day but she’s BER. I loved her so….

author: Personwithhats
author_rank: Contributor
timestamp: August 20, 2023, 04:50:02 pm
content: For #3 - The issue is that when measuring the voltage drop across the rsense resistors (e.g. 0.11ohm on a tmc2208) I get the top half of a sine wave with no negative. When measuring across an external 0.1ohm resistor that is inline with one of the motor coils, I get a full sine wave.As per Trinamic "Sense resistors should be carefully selected. The full motor current flows through the sense resistors. "So how am I only measuring half of a wave?? xd