2.5.7 Measurement Speed and Filtering
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To reduce the effect of common mode voltage further, the Model 372 includes an
active common mode reduction circuit. This circuit dynamically adjusts the operation
point of the current source output to minimize the common mode voltage seen at the
measurement input. Active common mode reduction allows the system to operate in
environments that would otherwise saturate the instrument’s differential input
amplifiers. The effect of active common mode reduction is most evident when exci-
tation current is 1 µA or greater, but it should be left on most of the time. Active com-
mon mode reduction should be turned off any time a measurement lead is tied to
measurement common (shield). The reduction circuit will not operate properly in this
configuration because the symmetry of the measurement is lost. Measurements can
be made with one lead grounded if the active common mode reduction feature is
turned off.
2.5.6.6 Matched Impedance Current Source
The Model 372 takes the idea of a differential input one step further with a unique
feature in its current source. The two current source output terminals have the same
source impedance, similar to the voltage input terminals of a differential input that
have the same input impedance. The resistor being measured and all noise sources in
the environment “see” the same impedance on both current source leads when look-
ing back into the instrument.
The circuit improves the measurement in two ways. First, the differential input
remains truly differential and performs well measuring any value of resistor. Tradi-
tional current sources with different impedance on each lead (one lead grounded)
can lessen the common mode rejection of a differential input especially when mea-
suring large resistors. Second, common mode noise sources have the same effect on
both sides of the resistor being measured ensuring that common mode voltages are
not turned into normal mode voltages (voltages are present between measurement
leads). Common mode voltage is rejected by the differential input, but normal mode
error voltage is measured along with the desired signal.
2.5.7 Measurement
Speed and Filtering
The Model 372 has a built-in firmware low-pass filter with a time constant of 200 ms.
This minimum filtering is sufficient to measure small resistance values with large
excitation, but that is not the true purpose of the AC bridge. As resistance increases
and excitation decreases, additional filtering and longer time constants are necessary
to maintain good resolution.
Additional filtering is done in the instrument firmware as a linear average. Linear
averaging requires more memory than exponential filter algorithms, but it gives the
best possible settling time. Exponential filters implemented in firmware can take sev-
eral times as long to settle for the same number of filter points. Unfortunately, linear
filters do not have a well behaved frequency response. The frequency response of a
linear filter has the shape of a damped sine wave rather than the sloped roll-off of a
first order low pass. This can create problems when trying to filter periodic noise that
has a frequency lower than the excitation frequency.
The input hardware settles about 1 s after a large resistance change if no range
change is required. When the firmware filter is turned on, that filter settling time
must be added to the hardware settling time to approximate overall settling. Firm-
ware filter settling time settings are available 1 s to 200 s. Please remember that fil-
tering does not reduce the electrical noise that reaches the resistor and therefore will
not reduce self-heating from noise.