Measuring the parasitics of my Siglent 1204X-E - Page 1
Forum section: electronics / projects

author: arildj78
author_rank: Regular Contributor
timestamp: June 07, 2019, 08:04:03 pm
content: I've been measuring a LC tank circuit and calculating both the unknown inductance and the unknown capacitance of the circuit. As part of figuring out this I would like to measure the input of the scope to check if it is actually 15pf / 1MΩ.This is my test setup.9V battery with a 10MΩ resistor in series with the positive supply wire. I wrapped the wires close to minimize the inductive reactance of the wires. The wires are then connected to the scope via MiniGrabber probes.I got a standard C charge graph that leveled off at 804mV, indicating aproximately 8.8V at the battery. When I zoom in at the start of the rise I see a short spike that comes down again to 80mV before the standard C charge. Can anyone explain this behaviour? Am I seeing some inductive parasitic and if so, how can this be modeled?Attached is a screenshot from the scope and a picture of the MiniGrabber probes.Arild

author: magic
author_rank: Super Contributor
timestamp: June 07, 2019, 10:54:25 pm
content: The step is probably caused by capacitance of the resistor, which charges up and dumps some charge into the scope's input within nanoseconds.The spike may be an overshoot caused by that sudden rush of current flowing through inductive wires. If you zoom in, there may even be some decaying ringing there.You could perform your measurement by replacing the resistor with a known capacitor (a few pF) and measuring the resulting voltage division ratio (the height of the initial step).

author: arildj78
author_rank: Regular Contributor
timestamp: June 08, 2019, 10:39:17 am
content: I’be tried to model this in LTspice but I can’t figure out the circuit. I do get the spike by inserting some inductance, but I’m not getting the step.When zooming in in the real circuit I see ringing on the step (186MHz).Can you suggest an equivalent circuit to test in Spice?

author: magic
author_rank: Super Contributor
timestamp: June 08, 2019, 04:02:10 pm
content: Something like that:9V → a few nH of cable inductance → 10M in parallel with 1pF → scope input (1M in parallel with 15pF)

author: arildj78
author_rank: Regular Contributor
timestamp: June 08, 2019, 05:08:51 pm
content: I think that was it! I was fiddling with the extra capacitance and inductance inside the scope and not in the resistor in my test setup. I will post a picture as soon as I have tweaked the values to match reality.Arild

author: plurn
author_rank: Regular Contributor
timestamp: June 08, 2019, 05:52:16 pm
content: Quote from: arildj78 on June 07, 2019, 08:04:03 pmI do not know if this is any use to you, but some measurements for you anyway so you can compare them with your results. I got the following measurements at the channel 1 BNC input of my SDS1104X-E measured with a DER EE DE-5000 LCR meter:at 120 mV per division and above:20.29 pF at 100kHz21pF at 100Hz with AC coupling22pF at 100Hz with DC coupling0.898 MΩ impedance at 100kHz0.9996 MΩ impedance at 100Hz1.0007 MΩ DC resistance with AC coupling0.9988 MΩ DC resistance with DC couplingat 118 mV per division and below:18.27 pF at 100kHz20pF at 100Hz with AC coupling25pF at 100Hz with DC coupling0.681 MΩ impedance at 100kHz0.9998 MΩ impedance at 100Hz1.1978 MΩ DC resistance with AC coupling1.0002 MΩ DC resistance with DC couplingUnless specified above, AC or DC coupling setting did not seem to change the measurement. A check of other channels showed fairly similar results to channel 1.Measurements with a Keysight U1272A multimeter for comparison:at 120 mV and above:1.0014 MΩ DC resistance with AC coupling1.0000 MΩ DC resistance with DC couplingat 118 mV and below:1.1988 MΩ DC resistance with AC coupling1.0026 MΩ DC resistance with DC coupling

author: arildj78
author_rank: Regular Contributor
timestamp: June 10, 2019, 07:40:49 am
content: THANKS !That is awesome. Just what I was looking for. It shows why some of my calculations could be off when I assumed 15pF as labeled. It also explains why there's something happening to a my weak signal when switching from 118mV to 120mV while measuring through a normal coax (as opposed to a proper scope probe)I'm not sure if Dave readis posts here, but exploring this could call for an interesting video about measuring techniques and the internals of a scope frontend.ArildNorth of Arctic CircleNorway