Get a handle on low-voltage measurements
You can minimize low-voltage measurement problems by following a few simple procedures.
By Martin Rowe, Senior Technical Editor -- Test & Measurement World, 10/1/2010 12:00:00 AM
Measuring low voltages (microvolts and lower) opens the door to errors from unwanted sources. Offset voltages in measuring equipment, thermoelectric voltages, and induced voltages from outside EMI (electromagnetic interference) can produce unacceptable errors. You can minimize those errors by following a few simple procedures.
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Dale Cigoy, applications engineer at
Keithley Instruments, has helped engineers
make low-voltage measurements
for many years. Cigoy has written a paper,
“Troubleshoot low-voltage measurement
problems,” in which he explains why these errors
occur and what to do about them.
Offset voltages, for example, add unwanted
DC errors to a measurement
setup. Many test instruments let you
compensate for offset voltages, but you
need a baseline measurement from which
to subtract out errors. Cigoy recommends
that you connect your instrument
to the unpowered circuit under test and
let the instrument’s temperature settle,
typically 1 to 2 hr, and then measure the
offset voltage. Only then can you minimize
errors from the offset voltage.
The paper also covers thermoelectric
voltages caused by temperature gradients
in dissimilar metals. Cigoy explains that
providing good thermal coupling can
minimize temperature gradients.
Ground loops in circuits can cause
measurement errors because of current
flow due to a difference in potential
within the loop, creating an unwanted
voltage drop in series with the source
voltage. Loops also open the door to
electric and magnetic fields that can
couple into circuits. Multiple equipment
grounds often form loops due to the
placement of wiring in a measurement
system. The smaller the loop, the better a
circuit will reject errors from electric
and magnetic fields.
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Your explanations, together with the digital simulations are found to be adaptive for secondary school students in the upper grades. The simplicity of the statements is distinguished and the ability to comprise results can be of great value to college students with little scientific backgroud.
Samih Jamaluddin - 2010-11-10 18:03:06 EDT
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