ASL INTERCOM CTR5000, Руководство пользователя

Страница: 116 / 124

CTR5000 Operating Instructions
116 ASL operating instructions, model CTR5000
18. Appendix 6– AC Bridge operation
18.1 AC Bridge basics
The CTR5000 Bridge measures the voltage (V
t
) developed across the unknown sensor
resistance (R
t
) and the voltage (V
s
) across a stable internal reference resistance (R
s
). The
voltages are proportional to the resistances so the thermometer resistance is derived from –
s
t
s t
V
V
R R
 
This technique achieves immunity from slow moving time and temperature drifts in the
electronics as it is not affected by voltage measurement gain variations or current source
fluctuations. In addition, the AC Bridge technique used achieves superior stability
performance (over DC Bridges) by eliminating all other sources of thermally generated EMF’s
(voltages) and sources of drift
The internal reference resistor must have a known (calibrated) value for an accurate
determination of the thermometer resistance R
t
.
Although the principle of measuring the ratio of two resistors (see introduction) is identical
between AC and DC bridges, the bridges operate very differently in the way they perform this
ratio measurement.
DC bridges require very stable and accurate electronics to achieve their performance; AC
bridges do not require such high-stability. Since they are based on an AC measurement,
these bridges are also immune to all thermal problems associated with DC bridges.