Page 114 - Mechanical Engineers' Handbook (Volume 2)

7 Resistance Bridge Transducer Measurement System Considerations 103

8. Temperature Coefficient. The change in output voltage per degree change in ambient
temperature.

Reference 10 defines test procedures for these specifications.

7.2 Signal Amplification
In addition to providing a precision power source to bridge transducers, the resultant millivolt
signals from these transducers often require amplification. This amplification is usually per-
formed by a differential dc amplifier. A differential dc amplifier is an electronic circuit whose
input lines are conductively isolated from the output lines, power, and chassis ground and
whose output voltage is proportional to the differential input signal voltage. Ideally, both
input lines have equal impedance and transfer characteristics with respect to the amplifier
ground structure. The amplifier has a frequency response from 0 Hz to a value determined
by the bandwidth of the amplifier.
Selecting amplifiers can be difficult because specification terminology is not universally
standardized. Amplifier specifications are either referred to input (RTI) or referred to output
(RTO). Discussing these specifications can lead to an understanding of the amplifiers them-
selves.

1. Input Impedance. The minimum impedance the amplifier will present when operated
within its specification. It is the impedance seen between the two ungrounded input
lines of the amplifier.
2. Source Current. The bias current flowing through the circuit comprised of the am-
plifier input terminals closed through the source resistance. The amplifier input
transistors act as constant-current generators in series with the input terminals. This
current can result in both offset voltage and common-mode voltage.
3. Common Mode Rejection. The measure of the conversion of common-mode voltage
to normal differential signal. The common-mode input voltage is the voltage com-
mon with both inputs to the amplifier. A common-mode rejection of 60 dB implies
that a 10-V signal applied simultaneously to both inputs produces an error signal
RTIof10 mV.
4. Linearity. The maximum deviation from the least-squares straight line established
through the output voltage versus differential input voltage characteristic. In eval-
uating linearity, it is usually sufficient to test at the highest and lowest gains, since
linearity will be worst at these settings.
5. Gain Range. The slope of the least-squares straight line established through the
output voltage versus the differential input voltage characteristic of the amplifier.
The gain range is the maximum and minimum values of gain available from the
amplifier without causing any degradation in performance beyond the limits of the
specification.
6. Gain Stability with Temperature. The change in amplifier gain as a function of
ambient temperature for any gain in the specified gain range.
7. Zero Stability with Temperature. The change in output voltage with temperature. It
must be specified as RTI or RTO, and this test is typically performed with the
amplifier input leads terminated with the maximum source impedance and no signal
applied. A warm-up period is usually specified for both this test and gain stability
with temperature.