DC Voltage Measurement
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7-4
The sketch shows the possible thermal voltage sources in a measurement circuit,
which can exist at an external connection point (Contact 1/2), but can also occur in
the connectors of the measurement unit.
It is always important to carry out all connections with the same material, or at least to
use materials, that produce very small thermal voltages, when they are put in contact
with each other.
The table below shows the different thermal voltages for diverse material
combinations:
Contact Materials approx. Thermal Voltage
Cu - Cu < 0,3µ/°C
Cu - Ag (Silver) 0,4µ/°C
Cu - Au (Gold) 0,4µ/°C
Cu - Sn (Tin) 2-4µ/°C (dep. on composition)
For example, if material 1 consists of a silver lead and material 2 consists of a copper
cable, a connection between contacts 1 and 2 at a temperature difference of just 1°C
already results in a thermoelectric voltage of 400nV. In the smallest voltage range,
with a resolution of 7½ digits (10nV sensitivity), this would result in an error of ± 40
digits.
Noise Effects Through Inductive Interferences
If the measurement leads are located near changing magnetic fields, generated for
example by a nearby high-voltage line, induction will cause an interfering voltage in
series with the measurement voltage.
By using twisted pair measurement leads, inductive interferences by a magnetic field
can be very strongly reduced. One should also take care not to allow the leads to hang
around loosely and move around during measurements, since that can cause error
voltages.
An additional measure for the reduction of noise is to increase the distance to the
magnetic field, or if possible to shield measurements from it.
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