Zonge GDP-32 - The Calibration and Timing Board; The Calibrate Section and Multiplexer; Ohmmeter

GDP-32
II
INSTRUCTION MANUAL
May 2002 Section 16, Page 18
16.6 THE CALIBRATION AND TIMING BOARD
Each GDP-32
II
receiver contains a Calibration and Timing Board (BD244 or BD288) located in
the analog card cage. Figure 16.6 (a) is a simplified block diagram that shows the major
functional elements of the board. The three main sections are: Calibrate, Ohmmeter (for
checking contact resistance), and Timing.
THE CALIBRATE SECTION AND MULTIPLEXER
The calibration section consists of an AC signal and a DC voltage level from a 16-bit
DAC, and temperature and humidity sensors. All of these components are routed to each
of the analog channels. Under the control of the GDP-32
II
software the calibration
section supports the following tasks:
Calibration and/or diagnosis of the analog channels
Measurement of internal temperature and humidity of the GDP-32
II
.
Measurement of electrode resistance
The multiplexer circuit consists of a power supply, a high linearity 16 bit DAC and a four
port multiplexer. The power supply on this card and an analog channel must be turned on
in order for this circuit to work and the multiplexed output to be read. The multiplexer
selects one of four analog signals:
a) Digital to Analog Converter (DAC) - The 16 bit DAC on the Calibrate and Timing
board generates a DC voltage between -8.35 and +8.35 volts as requested by the
digital control signals. Selecting the DAC port provides this level to the system
calibration bus. This makes it available to all analog input cards. This DC voltage is
used for an accurate system gain calibration.
b) Calibration - Selecting this input allows the DAC voltage generated above to be
switched at the frequency selected by the timing board, thus generating both a
positive and negative signal. The signal generated is either a frequency or time-
domain signal as determined by the duty cycle setting on the timing card.
c) Temperature - This input is a voltage proportional to internal instrument temperature
in °C.
d) Humidity - This input is the voltage proportional to internal relative humidity in
percent.
OHMMETER
The ohmmeter circuit provides a constant current level to the calibrate bus. An isolated
power supply provides power to the ohmmeter. When this relay is turned on, the
Calibrate power, if on, is turned off and the Output relay is switched to OHM. The
constant current level is selectable at 1.0 ma or 0.1 ma.