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Bad output from a device - claims from manufacturer
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populusnigra
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- Joined: Fri Mar 17, 2017 6:25 pm
Bad output from a device - claims from manufacturer
I'm using my Picoscope 4444 with the 441 differential probe for measuring the output of a device which is supposed to generate a square wave signal with a 1/4 of a volt positive offset. The device runs on batteries.
Unfortunately my Picoscope 4444 shows the signal has a negative offset and I've contacted the manufacturer and he claims my scope has a problem. He says I got bad readings because:
1. I haven't established a zero line and for that I've to calibrate my scope otherwise I would get a "mix shift".
2. I'm not using probes with near-zero capacitance, probably I'm using probes with a fairly amount of capacitance.
Both two claims don't have sense to me since I'm using a differential scope with a differential probe like the 441 which doesn't need to be calibrated. Am I doing anything wrong? I would like to hear your comments on these two claims.
Here I attach the psdata file if someone needs to take a look.
Thanks
Unfortunately my Picoscope 4444 shows the signal has a negative offset and I've contacted the manufacturer and he claims my scope has a problem. He says I got bad readings because:
1. I haven't established a zero line and for that I've to calibrate my scope otherwise I would get a "mix shift".
2. I'm not using probes with near-zero capacitance, probably I'm using probes with a fairly amount of capacitance.
Both two claims don't have sense to me since I'm using a differential scope with a differential probe like the 441 which doesn't need to be calibrated. Am I doing anything wrong? I would like to hear your comments on these two claims.
Thanks
- Attachments
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- varigamma_device.psdata
- (1.31 MiB) Downloaded 244 times
Re: Bad output from a device - claims from manufacturer
Hi populusnigra,
At the risk of stating what you've probably already checked, what happens when you swap the Probe connections to the device over? If you look at page 3 of our document on "Choosing the right PicoTechnology active differential probe" (see here: https://www.picotech.com/download/manua ... -probe.pdf) where use of an Active Differential Probe is discussed (the same principle applies to the Passive Probe on the Differential Channel of the PicoScope 4444) you can see that, if all of the connection polarities are correct (i.e. the connections to the device and the polarity of the device/battery) you should see a positive going pulse train. So, an easy test to check if you have polarity issues in the way that you're performing the measurement is to just measure the voltage across a battery using the Differential Probe (if the battery voltage measurement is correct, then there's some polarity reversal at the points where you are measuring on the device).
Regarding the answers from the manufacturer (this section was re-written for simplicity and clarity):
1/ (I believe that the manufacturer has some mixed messaging here, but are, kind of, on the right track). "Mix Shift" is a statistical term (described at the bottom of this post, if you're interested) and I can't see its relevance here, so I'll assume they meant "Voltage Drift".
There are 2 types of Voltage Drift, that I can think of, worth discussing when measuring voltages on a floating DUT (such as a battery powered device) with Differential Probes. The first would be where the voltage measured gradually increases or gradually decreases (typically in seconds, minutes, or hours) due to input bias currents on non ground referenced inputs, allowing the signal to gradually float outside of the common mode input range. However, in this case the signal wouldn't have coincidentally stopped at -10V, giving you a perfectly inverted signal. Instead it would have continued giving you a DC level of -20V.
The second kind of drift does involve establishing a correct "zero Line", or zero'ing any input channel offsets, due to the gradual drift of components (typically in weeks or months) relative to the internal voltage reference. It's certainly something worth doing, before a measurement if you have established that there is a voltage offset that will give you errors in your measurement. However, any zero offset error is typically only a very small fraction of the Input Range (not -10V). If your PicoScope 4444 had a -10V offset on a ±20 V Input Range then it would be ridiculously out of spec, (and what we would class as faulty and in need of repair). So, could you confirm what the offset is for the signal on channel A at ±20 V, by telling us what the average voltage level is when the input is unconnected.
If there is a channel offset, but it's not outside of the specification (less than 200mV) then you can correct it by using the 'Zero all ranges' button, at the bottom of the Channel Options window (opened by clicking the Channel letter Icon on the left of the menu, see here for more details: https://www.picotech.com/library/newsle ... 2017161481).
2/ The capacitance at the Probe tip only affects the frequency response of the Probe (and the signal integrity) when making a measurement (and adds frequency specific loading of the circuit at the measurement point). It certainly wouldn't be introducing any drift in level or reversal in polarity.
Mix Shift
A parameter associated with a statistic population can be stable (not varying), when the same parameter for 2 differently categorized samples of that population are varying. Each sample as a proportion of the population can be referred to as a "Mix", so the varying parameter of the Mix is a "Mix shift".
So, for instance, you could have (a) a population of 'all the spectators at a ball game', (b) a parameter of 'the percentage of spectators that buy a drink during a break in the play', and (c) samples of the population that are men and women. So, 2 Mixes would then be the proportion of men and women from the spectators that buy a drink during the break. So, there may be no shift in the proportion of all spectators that buy drinks over the total period of breaks, but there could be a Mix shift for men and women (in different directions) over the total period of breaks (and as a supplier of the drinks, you might want to know that for timely, gender specific advertising).
Regards,
Gerry
At the risk of stating what you've probably already checked, what happens when you swap the Probe connections to the device over? If you look at page 3 of our document on "Choosing the right PicoTechnology active differential probe" (see here: https://www.picotech.com/download/manua ... -probe.pdf) where use of an Active Differential Probe is discussed (the same principle applies to the Passive Probe on the Differential Channel of the PicoScope 4444) you can see that, if all of the connection polarities are correct (i.e. the connections to the device and the polarity of the device/battery) you should see a positive going pulse train. So, an easy test to check if you have polarity issues in the way that you're performing the measurement is to just measure the voltage across a battery using the Differential Probe (if the battery voltage measurement is correct, then there's some polarity reversal at the points where you are measuring on the device).
Regarding the answers from the manufacturer (this section was re-written for simplicity and clarity):
1/ (I believe that the manufacturer has some mixed messaging here, but are, kind of, on the right track). "Mix Shift" is a statistical term (described at the bottom of this post, if you're interested) and I can't see its relevance here, so I'll assume they meant "Voltage Drift".
There are 2 types of Voltage Drift, that I can think of, worth discussing when measuring voltages on a floating DUT (such as a battery powered device) with Differential Probes. The first would be where the voltage measured gradually increases or gradually decreases (typically in seconds, minutes, or hours) due to input bias currents on non ground referenced inputs, allowing the signal to gradually float outside of the common mode input range. However, in this case the signal wouldn't have coincidentally stopped at -10V, giving you a perfectly inverted signal. Instead it would have continued giving you a DC level of -20V.
The second kind of drift does involve establishing a correct "zero Line", or zero'ing any input channel offsets, due to the gradual drift of components (typically in weeks or months) relative to the internal voltage reference. It's certainly something worth doing, before a measurement if you have established that there is a voltage offset that will give you errors in your measurement. However, any zero offset error is typically only a very small fraction of the Input Range (not -10V). If your PicoScope 4444 had a -10V offset on a ±20 V Input Range then it would be ridiculously out of spec, (and what we would class as faulty and in need of repair). So, could you confirm what the offset is for the signal on channel A at ±20 V, by telling us what the average voltage level is when the input is unconnected.
If there is a channel offset, but it's not outside of the specification (less than 200mV) then you can correct it by using the 'Zero all ranges' button, at the bottom of the Channel Options window (opened by clicking the Channel letter Icon on the left of the menu, see here for more details: https://www.picotech.com/library/newsle ... 2017161481).
2/ The capacitance at the Probe tip only affects the frequency response of the Probe (and the signal integrity) when making a measurement (and adds frequency specific loading of the circuit at the measurement point). It certainly wouldn't be introducing any drift in level or reversal in polarity.
Mix Shift
A parameter associated with a statistic population can be stable (not varying), when the same parameter for 2 differently categorized samples of that population are varying. Each sample as a proportion of the population can be referred to as a "Mix", so the varying parameter of the Mix is a "Mix shift".
So, for instance, you could have (a) a population of 'all the spectators at a ball game', (b) a parameter of 'the percentage of spectators that buy a drink during a break in the play', and (c) samples of the population that are men and women. So, 2 Mixes would then be the proportion of men and women from the spectators that buy a drink during the break. So, there may be no shift in the proportion of all spectators that buy drinks over the total period of breaks, but there could be a Mix shift for men and women (in different directions) over the total period of breaks (and as a supplier of the drinks, you might want to know that for timely, gender specific advertising).
Regards,
Gerry
Gerry
Technical Specialist
Technical Specialist
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populusnigra
- Active User
- Posts: 21
- Joined: Fri Mar 17, 2017 6:25 pm
Re: Bad output from a device - claims from manufacturer
Hi Gerry,
Thanks for your detailed explanations and for the added mix shift info, but regarding "mix shift" I think what the manufacturer actually meant is that in order to measure it right with the PicoScope I need to "calibrate it" or set the zero position right.
When I swap the probe connections (positive probe of the device to negative probe in PicoScope and negative probe of the device to positive probe in PicoScope) I get a positive going pulse, but that shouldn't be the case 'cause what I get is a square wave with totally negative offset and this simple generator should always generate a square wave with just positive offset (with a minimum 1/4 of a volt to be specific). Previously I measured voltage across a battery and all went well as expected.
1. I confirm that on channel A at ±20 V I get an average voltage level of around 10 mV when the input is unconnected. I've also clicked on 'Zero all ranges', but I still see that huge -9.7 V when the device is on showing there is no positive offset when probe connections are connected as they should be.
2. Yep, I totally agree with this. We shouldn't see any reversal in polarity with probe capacitance.
I've tried measuring a different device from another manufacturer using correct polarity and I get a 30 kHz square wave with positive offset (1/4 volt minimum) as expected. I attach a screenshot and psdata as well. I would certainly like to rule out the first device because it is not working as expected, it doesn't generate the right output.
Thanks for your help.
Thanks for your detailed explanations and for the added mix shift info, but regarding "mix shift" I think what the manufacturer actually meant is that in order to measure it right with the PicoScope I need to "calibrate it" or set the zero position right.
When I swap the probe connections (positive probe of the device to negative probe in PicoScope and negative probe of the device to positive probe in PicoScope) I get a positive going pulse, but that shouldn't be the case 'cause what I get is a square wave with totally negative offset and this simple generator should always generate a square wave with just positive offset (with a minimum 1/4 of a volt to be specific). Previously I measured voltage across a battery and all went well as expected.
1. I confirm that on channel A at ±20 V I get an average voltage level of around 10 mV when the input is unconnected. I've also clicked on 'Zero all ranges', but I still see that huge -9.7 V when the device is on showing there is no positive offset when probe connections are connected as they should be.
2. Yep, I totally agree with this. We shouldn't see any reversal in polarity with probe capacitance.
I've tried measuring a different device from another manufacturer using correct polarity and I get a 30 kHz square wave with positive offset (1/4 volt minimum) as expected. I attach a screenshot and psdata as well. I would certainly like to rule out the first device because it is not working as expected, it doesn't generate the right output.
Thanks for your help.
- Attachments
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- mini-zapper.psdata
- (1.21 MiB) Downloaded 231 times
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- Different device generating the right output. 30 kHz square wave with positive offset (1/4 volt minimum)
Re: Bad output from a device - claims from manufacturer
Hi populusnigra,
I would try and communicate the issue to the manufacturer of the Varigamma device as it is clearly reversing the polarity of the signal by the time it gets to the output. Ordinarily, you could argue that you could just use it in the reverse polarity state, by switching the contacts over, but there is a danger that there could be some hidden consequence of operating in that state (which the Manufacturer would be able to confirm or deny), so you should try and resolve this avoiding anyone who doesn't know the inner workings of the device (including the supplier, if you didn't buy it directly from the manufacturer) if you can.
Regards,
Gerry
I would try and communicate the issue to the manufacturer of the Varigamma device as it is clearly reversing the polarity of the signal by the time it gets to the output. Ordinarily, you could argue that you could just use it in the reverse polarity state, by switching the contacts over, but there is a danger that there could be some hidden consequence of operating in that state (which the Manufacturer would be able to confirm or deny), so you should try and resolve this avoiding anyone who doesn't know the inner workings of the device (including the supplier, if you didn't buy it directly from the manufacturer) if you can.
Regards,
Gerry
Gerry
Technical Specialist
Technical Specialist
-
populusnigra
- Active User
- Posts: 21
- Joined: Fri Mar 17, 2017 6:25 pm
Re: Bad output from a device - claims from manufacturer
Hi Gerry,
Yes, indeed, I will communicate this issue to the manufacturer hoping they wil be able to fix it. Thanks for your advice and all your help.
Best regards,
Juan
Yes, indeed, I will communicate this issue to the manufacturer hoping they wil be able to fix it. Thanks for your advice and all your help.
Best regards,
Juan