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Meter Probe Offset Null Does Not Zero

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Meter Probe Offset Null Does Not Zero

Postby GalenEvans » Mon Sep 05, 2005 12:37 am

Hello,

I cannot get the Meter window to correctly apply the null offset to the displayed value in the latest version.

I am using PicoScope 5.13.1 (downloaded 2 days ago) with an ADC216. When I display a meter in 10 mV, 20mV, 50 mV, or 100 mV ranges, the meter window reads -0.546mV on channel A and +0.196 mV DC on channel B AFTER a null offset is performed - while the inputs are still shorted.

The equivilent DC measure functions on the Scope window go to zero (well, actually 0.0002 mV which is close enough!) after a null offset. So, I know the hardware is actually able to see and measure zero. The measure functions on the scope screen are always within 0.05% F.S. of my calibrator after a null offset, but the meter will always be off by about -0.546mV on channel A and +0.196mV on channel B. So, the offset is staying in the displayed meter measurements.

I have tried selecting Null Offset with meter window active and with scope window active. No difference. I tried both auto-scale and manually selecting scales for the meter and for the scope. No difference. I tried each channel. No difference. I also tried using 2 different shorts constructed of different materials, and the null volts function on a calibrator in case the issue was thermal EMF. No difference. I also tried rebooting the computer (a Compaq laptop running Windows ME). No difference.

I am connected to the ADC216 through the parallel port with no other devices attached. The only software loaded after boot-up is PicoScope.

Can you investigate?

Thanks,
Galen
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Postby Sarah » Tue Sep 06, 2005 9:51 am

Hi

Thank you for your post.

I will pass this on to our software department to look into, it may be that there is a bug in the latest version.

If you wish to roll-back to a previous version you can get one from:

http://www.picotech.com/software-archive.html

Best Regards
Sarah
 

Postby GalenEvans » Tue Sep 06, 2005 4:18 pm

Thanks Sarah.

BTW, I think your marketing group should consider revising the accuracy specifications of the ADC 216. It consistently measures much better than the advertised 1%; actually it is better .25% F.S. except in the 10 and 20mV scales where it is about 0.25% F.S +/- 0.5 mV.

If your specs were in line with the unit's actual performance and there were a published verification protocol (and/or calibration procedure!), it could be sold as a standard for field calibration of sensors. We've been using it for that purpose and it is way better than toting around a "portable oscilloscope." However, we had to verify performance on a precision calibrator before and after every use for months before the quality auditor would accept that it actually is much better than specified.

Thanks again,
Galen
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Postby Guest » Wed Mar 04, 2009 12:44 am

Hi!

I have exactly the same problem with my ADC-216.

I have tried the latest and several older software versions and all the things that GalenEvans mentions in his post.

My question is: Is this how it should be on the ADC-216 or has there been any kind of fix to this problem?

I'd also like to know if Pico offers any kind of calibration or update service for the ADC-216.

Many thanks for your reply!
Guest
 

Postby Robin » Wed Mar 18, 2009 4:32 pm

Hi

What value do you get on the meter after performing the offset null?

We do offer calibration. Email support@picotech.com if you wish to arrange this.

Robin
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