Datascope Accutorr Plus Service Manual page 85

stages. Wide gain range gives the board enough flexibility to acquire signals from fingers
spanning a wide range of thicknesses or alternatively from other sites of the body such as ears,
nose or toes. It also controls the remultiplexer (CHNLMUX) and provides calibration signal
CAL*, which is used by the sequencer to determine the operation mode, and test signal
DIAG*, which supplies a fixed voltage source at demultiplexer input for circuit diagnostics.
Again because the control registers can synchronously control the remultiplexer and gain values
for ac and dc gain stages, different gain settings can be selected for red and infrared signals.
An interference detector monitors the output of current to voltage preamp for voltages less than
negative 7.2 V in amplitude. This information is sent to the digital section through SAT* signal.
The patient probe signal enters the board as current pulses. After the signal goes through
the current to voltage preamp stage it is separated by the demultiplexer circuit, which
steers each voltage pulse to one of two signals, IR-CH or RD-CH. In addition, the circuit
sends a negatively amplified version of the signal level between LED pulses to both
channels. This residual signal is caused by ambient light on the photo-detector and offset
voltages from the preceding circuitry. The negative amplification sets-up cancellation of
the extraneous effect of the residual signal by the filter circuits that follow.
Signals IR-CH and RD-CH are then filtered identically by two parallel and matched
filters. The filters also reduce the effect of any noise source, which might interfere with
the measurement, such as an electro-surgical unit.
After passing through filter blocks red and infrared signals and are alternately selected
by the remultiplexer for further processing. Next, the signal is amplified by the dc gain
stage. Having the ability to apply a different gain to the two components, this block
functions as a coarse equalization of the multiplexed signal.
An offset voltage, determined by the DAC, is then subtracted by the subtraction circuit. The
plethysmographic waveform consists of a small component varying along with the physiological
pulse, sitting on top of a larger pedestal. The subtraction circuit pulls off most of this pedestal.
The subtraction circuit also helps to maintain the resultant signal in the amplifier linear region.
The residual multiplexed signal is once again processed through a microprocessor
controlled ac gain block. One of a few discrete gains is chosen for each of the two
components, such that the peak to peak size of the physiologically varying components
is large enough to be digitized with sufficient resolution.
After passing through the ac gain stage the signal is sampled by a sample and hold and
held for amplitude digitization. The digitization is performed under the microprocessor
control of the digital to analog converter. The DAC voltage is successively altered by the
microprocessor until it zones in on the signal being digitized. A comparator then
compares the signal and DAC voltages and sends COMP* signal to the digital section.
The DAC thus performs a dual function. It is used in both the subtraction and the comparator
blocks. Every 1/240 of a second, the circuit's control functions are flipped to process the alternate
component of the multiplexed signal. The multiplexer switches signals, the two microprocessor
controlled gains are changed, if necessary, and a new digital code is sent to the DAC for use in
the subtraction circuit. After settling to it's new value, the signal at the input of the comparator
is frozen by the sample and hold circuit. The DAC is now available to be used in the
digitization. At the next 1/240 second interval, all the control signals revert to the previous
values.
Accutorr Plus Service Manual 2-23
Revised 12/20/00
Chapter 2 - Theory of Operation