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Thermal EMFs

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fstevens
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Joined: June 10th, 2010, 10:28 am
Country: United States

Thermal EMFs

Post by fstevens » June 29th, 2010, 10:16 am

What is the most common source of error when measuring low voltage say below 1mV?

Dale C
Keithley Applications
Keithley Applications
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Joined: June 10th, 2010, 6:22 am
Country: United States
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Re: Thermal EMFs

Post by Dale C » June 29th, 2010, 10:49 am

There are many factors that make low voltage measurements difficult. For instance,
various noise sources can hinder resolving the actual voltage, and thermoelectric voltages
(thermoelectric EMFs) can cause error offsets and drift in voltage readings. In the
past, it would have been possible to simply increase the test current until the DUT’s response
voltage was much larger than these errors, but with today’s smaller devices this
is no longer an option. Increased test current can cause device heating, changing the
device’s resistance, or even destroy the device.
The key to obtaining accurate, consistent measurements is eliminating the error.
For low-voltage measurement applications, such error is composed largely of white noise
(random noise across all frequencies) and 1/f noise. Thermoelectric voltages (typically
having 1/f distribution) are generated from temperature differences in the circuit.

Thermoelectric voltages normally have a 1/f characteristic. This means there can be
significant offset, and the more measurements that are made the more drift. Taken
together, the offset and drift may even exceed VR, the voltage across the DUT induced
by the applied current. It’s possible to reduce thermoelectric voltages using techniques
such as all-copper circuit construction, thermal isolation, precise temperature control,
and frequent contact cleaning.
Refer to the low voltage measurement techniques article and the low voltage measurement application notes on Keithley's web site.
Here are the links.
http://www.keithley.com/support/data?asset=50365

http://www.keithley.com/support/data?asset=50287

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