Attaching scope probe to signal fixes pulse measuring circuit

Thread Starter

Keebler

Joined Mar 28, 2020
5
I have a very simple circuit that is measuring the pulses from a Gems FT-110 flow meter. We are checking the accuracy of the flow meter using a bucket test. All of the results are about double what they should be. When I attach the scope probe to the circuit the flow reading as calculated from the pulses generated is now in line with the bucket test. Note that the probe does not even have to be connected to the oscilloscope for this to happen. Another Gems flow meter RFO-2500P does not have the same problem. It has always been accurate with the same pulse counter program.

FT-110: NPN open collector output, 10K pull-up on signal to 5V, connected to input on Arduino Mega
RFO-2500P: PNP output, no pull-up needed, directly connected to input on Arduino Mega

Both use a 5V power supply. Schematics are in the datasheets. This seems like an impedance mismatch issue to me but I can't grok what the solution is. Secondary question is how do I measure this type of signal without affecting the waveform? What kind of setup/probes do I require? Thanks.
 

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AlbertHall

Joined Jun 4, 2014
10,572
Connecting the probe adds capacitance across the output. This will be between perhaps 10pF and 100pF depending on the probe and whether it is X1 or X10.

There are two possible effects of this on the circuit.
1. It will slow down the rise and fall times of the square wave - especially the rise time. The rise and fall times of the signal from the sensor are not specified.
2. It will tend to reduce any stray high frequency signals picked up on the sensor cable. How long is the cable and is it screened?
 

dpetican

Joined Jan 13, 2016
38
Connecting the probe adds capacitance across the output. This will be between perhaps 10pF and 100pF depending on the probe and whether it is X1 or X10.

There are two possible effects of this on the circuit.
1. It will slow down the rise and fall times of the square wave - especially the rise time. The rise and fall times of the signal from the sensor are not specified.
2. It will tend to reduce any stray high frequency signals picked up on the sensor cable. How long is the cable and is it screened?
Thanks. I suspected that as well. Especially since the effect was noticable without the cable attached to the scope. I will test with some caps on Wednesday and also check the cabling on both flow sensors.
 

MrChips

Joined Oct 2, 2009
22,107
Thanks. I suspected that as well. Especially since the effect was noticable without the cable attached to the scope. I will test with some caps on Wednesday and also check the cabling on both flow sensors.
Are you the TS (Thread Starter) Keebler?
That is going to make your membership confusing. Choose the one user name you want to keep.
 

tautech

Joined Oct 8, 2019
144
Are you the TS (Thread Starter) Keebler?
That is going to make your membership confusing. Choose the one user name you want to keep.
This often happens on forums when members forget/lose their passwords and rejoin with another login name however dual logins are prohibited on most forums and members caught doing so can have both accounts banned.

Trouble is, without being logged in you have no way to contact a moderator in order to get this sorted.
Admins should provide some way for contact in these circumstances so passwords can be deleted and the member can login and enter a new PW.
 

Thread Starter

Keebler

Joined Mar 28, 2020
5
This often happens on forums when members forget/lose their passwords and rejoin with another login name however dual logins are prohibited on most forums and members caught doing so can have both accounts banned.

Trouble is, without being logged in you have no way to contact a moderator in order to get this sorted.
Admins should provide some way for contact in these circumstances so passwords can be deleted and the member can login and enter a new PW.
This. I stopped using the old account. When replying I accidentally logged in using the old account. The real problem is the inability/unwillingness of forums (not necessarily this one) to delete user accounts. However, I just noticed that I could have changed the email on the original account which is what I really wanted.
 
They are suggesting 3-30K and you used 10K. Good. they don;t tell you what VCe(sat) is -- bad.
Cable used and how connected can matter somewhat.

So you might need a few pf of capacitance (out to ground) or there could be other issues.

You might need a bypass cap the signal source. +5 to ground.

e.g. twisted pair shielded cables, grounded at only one end might also fix the problem.

Rule:
Generally you ground at the signal source, but the rule can be broken.
Generally you ground at one end - rule can be broken

Look at the signal in x1 on the scope. This may not be conclusive because it lowers bandwidth.
 

Thread Starter

Keebler

Joined Mar 28, 2020
5
Connecting the probe adds capacitance across the output. This will be between perhaps 10pF and 100pF depending on the probe and whether it is X1 or X10.

There are two possible effects of this on the circuit.
1. It will slow down the rise and fall times of the square wave - especially the rise time. The rise and fall times of the signal from the sensor are not specified.
2. It will tend to reduce any stray high frequency signals picked up on the sensor cable. How long is the cable and is it screened?
I tried a couple of pF caps and the 330pF worked better than the 22pF which didn't work at all. At 1X my scope probe is about 82-100pF so I suspect I could go lower but 330 is all I had laying around.

None of the wires have shielding. I removed the sources of possible high frequency cross-talk from the wire harness but that didn't fix anything. It may not be noise on the PCB ground plane because the RFO2500P works fine. Or it could be that the FT-110 is more sensitive. I'm going to ask Gems for the internal schematic of both sensors.

I'm still wondering how I actually measure this signal accurately on a scope if the probe affects the signal. Is there a method? Thanks.
 

tautech

Joined Oct 8, 2019
144
A 10X probe has 1/10the the loading of a 1X probe, so would have less affect on the signal.
Correct although for some high impedance circuits even a 10x probes loading is too high to not affect the waveforms integrity or value and typically an active probe (read $$) is required. Although some study of passive probes available can find some with low loadings well down in the single unit picofarads.
 

tautech

Joined Oct 8, 2019
144
Nope same thing. My probe is 1/10X. Do I go to 100X, 1000X? Where does the insanity stop ;)
Depends entirely in the signals amplitude and the probes on hand and max sensitivity of your scope.
Eg, I can comfortably display a 3V probe compensation waveform with a 1000X probe.
 
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