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Page 54 of 97
Issue: 1.04
A significant source of error with this measurement topology is the common-
mode rejection ratio of the input amplifier. The common-mode signal at the input
to the amplifier changes between the two measurements and will lead to an error
at the input to the ADC.
In order to eliminate this source of error, the microK uses a substitution topology
in which there is a single point of measurement in the system into which the
SPRT and Reference Resistor are switched alternately.
Reference
SPRT
Amplifier
ADC
Resistor
Substitution Measurement Topology
This adds complexity and cost to the design, but ensures that there is no change
to the common-mode signal between the measurement of the reference resistor
and the thermometer. This approach would normally, however, increase the
demands on the current source. In the conventional topology, the current is
common to both the reference resistor and the thermometer. In the substitution
topology, the voltage at the output of the current source changes between the
measurement on the SPRT and the Reference Resistor and the current source
must accommodate this change without any significant change to the sense
current.
The current source in the microK has a very high output impedance, such that the
sense current will not change significantly between the measurement of the
reference resistor and the thermometer. In addition, the whole measurement
system is actively guarded so that both the voltage at the output of the current
source and the common-mode voltage across the device being measured do not
change.
