Calibrating of USB Oscilloscope 4424

by **Erne** » Fri Jan 06, 2012 3:06 pm

Hello together,
is it necessary (and possible) to calibrate the input channels of the scope?
If i measure on both channels the same dc-voltage (8V), i get a difference of A-B=17mV (measured with dc average on math channel A-B). Afterwards i interchange the probes and do the same measurement with the same result. With that the reason for the difference is not a voltage drop of one of the probes. What can be the reason, and what to do to avoid this?

best regards
erne

by **Hitesh** » Tue Jan 10, 2012 1:31 pm

Hi Erne,

We normally recommend calibration on a yearly basis:

http://www.picotech.com/calibration.html

You are free to choose where to calibrate the device - if you would like us to calibrate the device, please e-mail support@picotech.com

With regards to the offset, it should be 1% of the full scale range e.g. +/-5uV for a +/- 50mV range.

Regards,

by **Erne** » Fri Feb 10, 2012 7:25 am

Thank you for the hints...

best regards
Erne

by **Hitesh** » Fri Feb 10, 2012 2:39 pm

Hi Erne,

I appreciate I may not have answered the question fully as I thought it was just a calibration issue. We've taken a further look into this and the suggestion is to setup a Custom Probe for each of the channels as required in order to scale the values.

The steps to take are as follows:

Use a 50 Ohm terminator or a short on each channel to measure the offset from 0V in PicoScope.
Apply the 8V DC input on each channel and measure the voltage.
Reverse the input to achieve a -8V signal and measure the voltage on each channel - this will help to see if the difference is the same at the positive and negative end.
Subtract the offset from your readings for each channel if it is 1mV or above.
If you have obtained negative readings, you can average the magnitude with the positive readings to obtain an average reading.
If your reading is outside 1% of the range, then you will need to scale the value.
Divide the average reading by 8V to obtain your scale value.
Next within PicoScope, click on the Channel options icon for the channel you need to scale, in our case it was Channel B.
Next click the '...' icon next to the Probe drop-down menu.

Channel options - Probe selection
In the Custom Probes window, click 'New Probe...'
Click 'Next >' when the Custom Probe Wizard appears.
Leave the 'Probe Output Units' as volts.
In the Scaling Method dialog, enter your scaling value (m) and offset as required.

Scaling method - set for 99% scaling.
Click 'Next >' and 'Next >' again on the Range Management dialog.
Enable the Filter as required and click 'Enter', then click 'Next >'.
In the Custom Probe Identification dialog, enter a name for the Custom Probe and a short description.
Click Finish.
A new probe will appear in the Library.

Custom probes dialog.
Select the probe and click 'OK' to begin using it.
Repeat for the other channel as necessary.

I have attached an example Custom probe that I created which scales the value to 99%. You can use the Import function in the Custom Probes dialog to load this into PicoScope.

ChB_Probe_99_pc.psprobe: 99% scale probe; (988 Bytes) Downloaded 221 times

Hope this provides suitable clarification.

by **scju** » Thu Jun 08, 2017 6:24 am

Good morning,

is this kind of calibration also applied by using 3th-party-software, like LabVIEW ?

Greetings,
Julian

by **Martyn** » Thu Jun 08, 2017 7:29 am

You could apply the same techniques from within LabVIEW, but you would need to code this yourself.

by **scju** » Thu Jun 08, 2017 9:25 am

Hey Martyn,

thanks for your reply. So for a "real" calibration it is necessary to use the offered calibration service ?

Greetings,
Julian

by **bennog** » Fri Jun 09, 2017 6:29 am

Julian,

The difference you see is 17mV on a +/- 10V scale of the scope
this is a error of 0.085% this is well below the specified tolerance of 1% of the scope.

And also near the LSB of the scope in this range it is 5 mV for the LSB

Just my 2c

Benno

by **Martyn** » Fri Jun 09, 2017 6:30 am

A true calibration will provide you with a table of errors to show exactly where the scope sits in the published specification, for all ranges and on all channels. You can then use the table of errors to adjust any measurements you take.

If you plan to use your product in an industrial or scientific environment where it is necessary to have documented traceability, or you just want your Pico product to be ‘fine-tuned’ to give the best possible performance, then we can provide you with a calibration service referenced to National Standards. This means that you will be given documented proof of the accuracy of your Pico product at the time it is despatched to you. This documented proof is in the form of a calibration certificate that has the errors recorded. For example, the product may have a published accuracy of ±3%, and after calibration the error might be recorded as –1%. You can then allow for this –1% error when taking measurements.

Zeroing offsets before performing tests is also valid, and switching on devices and allowing them to warm up for a period of time is advisable before applying the zeroing process.

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