Small ripple measurement on a larger DC voltage

Updated: Sat 15, 2011 8:56 pm attached another file.

Viewing of small signals on a large DC offset voltage is a limitation on the DC coupled Nano (although unwanted ripple viewing is a common use for o’scopes). Another o’scope is the easiest solution, but the Nano is so small and portable. Another possible solution would be to offset the captured data by a DC voltage and amplify that offset value by a given gain factor to make that ripple signal more easily viewed on the Nano. None of this would impact the capture buffer, since the acquisitions could be stopped while in this mode, and only the screen display would change. The user would be able to choose from menu choices.

This is all predicated on the condition that the ripple signal always remains within the input voltage range of the Nano. The purpose of this post is to present data that will help to determine if this feature will provide expected advantages or not.

Attached are zip files that contains supporting files for initial analysis with a ripple signal riding on a constant voltage. The supporting files will be based up the 3 input stage gain ranges.

You don’t have to know a lot about Excel to view these results. Just unzip the file and load the *.XLS file into Excel and look at the test result waveforms on your computer screen.

To decode the attached file names below, the 1x=1xprobe, 02vd=.2V/Div, 5usd=5us/Div, 120hz=ripple freq, 200mv=ripple amplitude, PriPos=buffer priority, R3=range3.

Range-1 = 1x probe 10mv - 100mv/Siv
Range-2 = 1x probe 200mv - 1v/Div
Range-3 = 1x probe 2v - 10v/Div
Range-4 = 10x probe 10mv - 100mv/Siv
Range-5 = 10x probe 200mv - 1v/Div
Range-6 = 10x probe 2v - 10v/Div
1x2vd5usd120hz200nvPriPosR3.zip (87.9 KB)
1x1vd20msR2.zip (24.4 KB)
1x02vd20ms120hzPriPostR2.zip (34.7 KB)

Limit of 3 attachments exceeded in previous post.

This 10V/Div in Range - 3 proved undetectable by ADC.
1x10vd5us120hz200mvPriPosR3.zip (55.3 KB)

Thanks for taking the time to measure and share!

If you attempt to conclude on these measurements, what would you say is the upper base level voltage from which we could extract useful ripple data?

There is still one thing keeping me from answering that question. I am still trying to figure out why the buffer data has a negative offset as compared to the screen measurements of the same waveform. I could have a process error, but it seems to me that the two differ by about 250mV, but not consistently.

This is easily noticed while looking at the probe compensation signal that is fully on screen, but the buffer data shows negative transitions. This is also the same reason that I haven’t done a calibration video yet. Maybe you could shed some light on this.

I currently suspect that Range-3 X1 probe and Range-3 X10 probe is out of the question due to the extreme front-end range circuit attenuation for those two scales.

I’m not sure I follow this. The 1 kHz square wave shows both overshoot and undershoot with the minimum voltage dipping below ground. Is this what you refer to as a discrepancy?

I recreated my measurements, and Excel reexamination of the attached files confirms that there is no discrepancy. In my mistake I must have been measuring the bottom line of the squarewave (which I had set at the bottom of the screen using YA-GridPos). Apparently I overlooked the negative overshoot at that time.

The attached files support that there is no problem. The present work around is to rename that file using the measurement parameter name as the file name.

Edited to remove errors.
gndpos.zip (50.4 KB)

Now I am free to state that the small ripple measurement would only be effective in the following ranges (if you choose to implement it) but those ranges would most likely be used often.

x1 probe range 1 not likely needed
X1 probe range 2 for sure, and this range would work for many circuits used in today’s electronics
x1 probe range 3 will not work
x10 probe range 1 awaiting further repeat test results to be completed soon
X10 probe range 2 awaiting further test results to be completed soon
x10 probe range 3 will not work

I just realized that the data from the CN1 project contains the answer to your question.

From the data it can be seen that the attenuation ratio of 6.63E-2 (1x-range-2) works just fine, so all other ranges except 1x-range-3 should be good to go with respect to attenuation, because they have less attenuation before the ADC. My unknown is the resolution of the ADC and that would affect the largest useful V/Div in each range. But then again who knows what size of signal is on that DC voltage, maybe it is much larger than mV, so maybe there is not a known upper V/Div limit to consider. The user through trial and error will find that limit for his particular configuration.

[code]Ra Rb Req atten ratio

5.10E+05 5.10E+05 5.00E-01
5.10E+05 3.90E+04 3.62E+04 6.63E-02
5.10E+05 3.30E+03 3.28E+03 6.39E-03
5.10E+05 5.10E+05 5.00E-01
5.10E+05 7.50E+04 6.54E+04 1.14E-01
5.10E+05 6.00E+03 5.93E+03 1.15E-02
[/code]

If you look at the data I posted yesterday, sensitvity (ADC resolution converted to volts) for each range is included at the bottom. This was meant to complement your ripple measurements with calculated figures. This suggests that 1x from 10mV to 1V is Ok as well as 10x from 0.2V to 1V. In other words we should be able to measure ripple (20mV) on DC levels at 7V or less. Above this, sensitivity is an issue.

[code]Probe Scale Range Max Probe Max

1x 10mV, 20mV, 50Mv, 100mV 0.8 1.18
1x 200mV, 500mV, 1V 8 8.9
1x 2V, 5V, 10V 80 92
10x 0.2V, 0.5V, 1V 10 11.6
10x 2V, 5V, 10V 80 86
10x 20V, 50V, 100V 800 838

Sensitivity is as follows:

288.7uV
2.2mV
22.6mV
2.8mV
21.1mV
204.7mV
[/code]

Thanks, now I fully understand those numbers.

Let me attempt to clarify another point of that post. I deviated from the original measurement testing conditions of 200mv ripple when I realized that a user may want to measure a larger signal on top of a DC voltage level, such as a 5vp-p (or greater) signal riding on top of a 24VDC voltage level. In that application the ADC resolution may not present an issue.

This is why I said that maybe all scales (except the first one) should have that capability and let the user trial/and/error the results. If the user ripple signal was large enough (an actual signal, not just ripple), then satisfactory results would be obtained. The worst that should happen to the user is a flat line for that scale if the ADC resolution for that scale was insufficient. Your Firmware User’s Guide could point out that a flat line result means that the ADC resolution capabilities have been exceeded for that function.

I hope this expanded explanation further clarifies this idea.

If you need to measure a small ripple voltage on a larger DC voltage with the DC coupled NANO, just connect a capacitor in series with the input of the NANO. Then you have an AC coupled 'scope.

On the surface that may appear to be a quick solution. Once you delve into the reactance (Xc) of the coupling cap and the input capacitance of the Nano for each range (and that associated reactance), you will find that frequency dependence and the voltage divider reactance of the two capacitors (your coupling cap and the Nano input capacitance) destroy any AC accuracy of the measured wave amplitude.

In the case of ripple on a power supply, ripple amplitude must meet a specified minimum and since the measured Vpp using an external cap kills the Vpp AC accuracy, then this method will not provide reliable results.

If you are just looking for the presence of a small signal, then your method would be quite acceptable.