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contact potential of 7012-S card

7001, 7002 and Switch Cards
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Hongya W
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Joined: April 5th, 2017, 1:10 am
Country: China

contact potential of 7012-S card

Post by Hongya W » April 5th, 2017, 1:47 am

Hi,

Our lab want to buy 7012-S matrix card for resistance measurement, and we still have some questions about it.

1. The contact potential of 7012-S is less than 500nV, I am wondering if it can be smaller when we apply AC current. I have noticed that the errors of low-level voltage measurement can be caused by thermal EMFs, source resisitance noise, ground loops and so on, and EMFs can be eliminate by AC current. So, do you think it is possible for the contact potential significantly reduced when we apply AC current? (We hope it can be dozens of nanovolts.)
Or is there any other methods to reduce the impact of noises causing to low-level voltage measurement?

2. Can we only use HI to connect the sample and voltmeter, making the contact a single-pole single-throw switch? (like the - shows)

Thanks for your help!
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brian.d.smith
Keithley Applications
Keithley Applications
Posts: 402
Joined: June 10th, 2013, 6:00 am
Country: United States

Re: contact potential of 7012-S card

Post by brian.d.smith » April 7th, 2017, 12:17 pm

Here is the pertinent excerpt from the datasheet

CONTACT POTENTIAL:
7012-C: <1μV per channel contact pair 7012-S: <500nV per channel contact pair
<3μV typical per single contact. <1.5μV typical per single contact.

It looks like less contact offset using HI/LO pairs than individual HI contacts.

Using an alternating source does not reduce contact offsets. What it allows is averaging of certain types of offsets to a theoretical zero.

In the 6221/2182A instrument combination we call this the "Delta Mode".

The advice I give is to try it both ways and see if there is a measurable difference.

You have to factor in the uncertainties of your driving source and your voltage measure instrument. The combined uncertainties of these two instruments will set the floor of what you can discern concerning offsets. Any measurement below this floor will not have a metrological basis.

Hongya W
Posts: 2
Joined: April 5th, 2017, 1:10 am
Country: China

Re: contact potential of 7012-S card

Post by Hongya W » April 8th, 2017, 1:16 am

Thank you Brian!

Since you mentioned "Delta mode", I find some infomation about it on internet, "This current reversal technique cancels out any constant thermoelectric offsets, ensuring the results reflect the true value of the voltage." So delta mode can cancel constant offsets. I am not very familiar with contact potential, is the contact potential a constant offset, so I can use delta mode to cancel it?

Thank you again for your help!

brian.d.smith
Keithley Applications
Keithley Applications
Posts: 402
Joined: June 10th, 2013, 6:00 am
Country: United States

Re: contact potential of 7012-S card

Post by brian.d.smith » April 10th, 2017, 7:43 am

Hongya W wrote:Thank you Brian!

Since you mentioned "Delta mode", I find some infomation about it on internet, "This current reversal technique cancels out any constant thermoelectric offsets, ensuring the results reflect the true value of the voltage." So delta mode can cancel constant offsets. I am not very familiar with contact potential, is the contact potential a constant offset, so I can use delta mode to cancel it?

Thank you again for your help!
Contact potential will be the result of two metals in contact. The relay itself has two metal surfaces that are apart when the relay is open and come together when the relay is closed. From a practical standpoint, there are multiple places in each switch card channel where two metals come together; inside the relay itself and also on the circuit board and the wiring to and from the circuit board to the wiring to your device.

The behavior of a contact is dependent on more than one phenomenon. The chemical/metallurgical makeup of each contact point and any thermal gradient across the the contact pair. The thermal dependency is easy to demonstrate. Straighten out an ordinary paperclip to a straight line. Measure the voltage across the paperclip with a sensitive DMM. touch the paperclip at the end and see the microvolts of the DMM change.

We only specify the maximum potential. We do not guarantee or publish a characterization of the behavior of the potential.

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