Please confirm my use of a 5133

[Fro2]

I have a NI USB-5133 Digitizer.  According to the manual for the device it can measure up to 40 Vpk-pk, and the maximum input overload is 30 Vpeak thus 60 Vpk-pk.

I have attached an adjustable source voltage which has an unknown output value to the digitizer and used the feature within the digitizer to measure the input voltage.  I turned up the adjustable source voltage and monitored the value displayed on the computer that the digitizer was reporting.

As I turned up the dial I noted the dial setting as well as the displayed voltage.  I stopped turning up the voltage once the digitizer reported 41 Vpk-pk.  I then estimated what the dial needed to be to exceed the 60 Vpk-pk.

I tabulated the results.

Dial	Measured	Computed
11	5	5.30
20	12	12.01
30	20	19.45
40	27	26.90
50	34	34.34
60		41.79
70		49.24
80		56.68
84		59.66
90		64.13

The measured and computed values above are Vpk-pk.

Based on the collected data one should never exceed a dial setting of 84 otherwise one could damage the device.  In fact one should not even exceed 60 because the displayed value may not be accurate.

After doing this, something just did not seem right.  I had an uneasy feeling which I can’t explain.  This was too easy to do.  Maybe I am forgetting something.  Do you think the method I used to determine the maximum dial setting is valid?

Some may suggest to attach a voltmeter to the adjustable source voltage as well or in lieu of the digitizer to measure the voltage.  I could not do that because the actual source is a pulse and when the voltmeter was connect to the source alone it did not even see the pulse.  I did try it, I even tried to use the AutoHold feature on the Voltmeter but that did not work so I was left to use the digitizer.

I kind of feel uneasy to use a device to display a voltage and to use the same device to determine how high it can go before damaging it.

Suggestions…

Someone please respond to put my mind at easy.

[Nathan-P]

Fro2,

The method you used is ok, but is not the best, and I agree with your statement:

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Fro2 wrote: 

Based on the collected data one should never exceed a dial setting of 84 otherwise one could damage the device.  In fact one should not even exceed 60 because the displayed value may not be accurate.

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A couple of other things to consider:

• All of your calculations assume that the Voltage Source has a linear output, and may not be.  
• If your using the USB-5133 to determine the absolute value of the voltage output, you will need to take into account the accuracy specification of the scope.  The higher the voltage (and range), the less accuracy you will see in your measured result.

In order to be confident with the max dial value, you will want to also take into your calculations the worst possible accuracy for your measurements to lower your maximum dial voltage.  And the accuracy is only valid when operating within voltage levels.

Of course the best is to use a high-accuracy, high-range scope to determine the absolute voltage levels output by the voltage source.  It is far better to measure the dial position that corresponds to 60 Vpp signal, than just calculating it, since then you can be sure.

I hope this helps.

[Fro2]

Thanks for the feedback.  I did confirm that the Voltage Source does have a linear output.

You wrote "Of course the best is to use a high-accuracy, high-range scope to determine the absolute voltage levels output by the voltage source.  It is far better to measure the dial position that corresponds to 60 Vpp signal, than just calculating it, since then you can be sure."  I do not know why I did not think of this.  In fact the plan is to replace a scope with the digitizer.  So why don't I just use that scope that is being replaced to measure the output of the device and determine what the maximum dial position should be.  The scope I have is a LeCroy 9350.  It has a 250V max input and is 500MS/s. I could use the scope to create the table up to 70 Vpeak which is higher than the overload voltage for the 5133.  That way I will know what the dial setting will be for the overload voltage.

Also someone suggested that I could put a crowbar circuit, such as the one shown below, on the input of the digitizer and set it to trigger at the 40 Vpk-pk point, the max range of the 5133.  That way the digitizer definitely would not get destroyed and the displayed measurement would be within the measureable range of the digitizer.