GDP-32
II
INSTRUCTION MANUAL
May 2002 Section 16, Page 10
16.5 BD183 ANALOG BOARD
The small case GDP-32
II
T has an analog board capacity of 6 channels. The large case GDP-32
II
has an analog board capacity of 16 channels. The GDP-32
II
uses one board for each active
channel. Boards are installed (up to case capacity) into sequential slots beginning with slot 1, the
right-most slot of the analog card cage. .
Each analog board has a number of relay, gain, filter and DAC setting possibilities. Some
understanding of the signal path is necessary in order to control the board. The signal path goes
through a series of relays to the buffer/differential Amplifier Gain stage (G0), then to the Quad-
Notch Filter (BP), Amplifier Gain stage (G1), Antialias Filter, Amplifier Gain stage (G2),
Multiplexer, and ADC (see Figure 16.5 (a)).
RELAY FUNCTIONS
There are three input configurations:
1. The Calibrate and Ohmmeter relays are open, permitting measurement of the field
signal.
2. The Calibrate Relay is closed thus providing the Analog Board with a calibrated
signal from the Timing/Calibrate Board for self-calibration.
3. Both the Ohmmeter and Calibrate relays are closed allowing a constant current (100
µa generated on the calibrate board) to flow to the Analog I/O Panel and to the field
electrodes. This permits measurement of the electrode contact resistances for each
electrode pair.
The Attenuator relay inserts a resistor attenuator into the signal path to reduce the input
signal level by a factor of 0.125.
NOTE: Using the attenuator can cause phase shift at frequencies above 1 kHz due to an
increased RC time constant. Since this phase shift is a function of load resistance and
cannot be effectively removed, the attenuator is only used at high frequencies when
absolutely necessary.
DIFFERENTIAL AMPLIFIER
The input differential amplifier presents a high impedance to the input signal to minimize
signal loss and distortion. The differential amplifier combines signals to minimize
common mode voltages.
THE OFFSET DAC
The output of a Digital-To-Analog Converter (DAC) is supplied to each of the BD183
boards. This output offsets any DC signal level (such as SP) appearing on the signal
input.
The Digital to Analog Converter (DAC) accepts a digital word from the MPU and
produces an analog voltage. The maximum offset value is ±2.5 volts.
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