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16.7 Preamp
16.8 Programmable Gain Amplifier (PGA), Demodulator and Demultiplexer
16.9 Filters and Level Shifter
The current-to-voltage (I-to-V) converter has a gain of -249 K V/A and a
low-pass corner frequency of 30 kHz. The voltage amplifier has a gain of
-2 V/V and a low-pass corner frequency of 20 kHz. The voltage amplifier is
disconnected from the I-to-V converter during LED switching transients to
prevent transmission of the switching spikes into the programmable gain
amplifier (PGA).
The ambient light canceller (ALC), which consists of U1A3, R6, and C6,
generates a current opposing the DC current coming from the photodetector,
which is caused by ambient light. The ALC switches on during the Red cycle
and has a closed-loop frequency response of 240 Hz. It can cancel up to 36
microamps of DC photocurrent. The response time to a change in LED drive is
less than 2 milliseconds.
The RC filter that removes sensor cable noise is comprised of R1 and C49.
Component CR4 serves as an ESD-protection diode.
The PGA U1A4 provides a variable gain to accommodate a wide range of signal
strengths. As the PGA amplifies the signal from the preamp by a programmable
gain, the demodulator shifts the frequency down to baseband, while the
demultiplexer separates the IR and Red components of the signal. The PGA has
a programmable gain from 1 to 128 in powers of 2.
The PGA output goes to the peak detectors for status monitoring and to the
demodulator/demultiplexer (U1A5, which is internal to the Oxichip circuit) for
signal processing.
There are two peak detectors: one detects positive peaks (above the 2.2-volts
reference) and one detects negative peaks, or valleys (below the 2.2-volts
reference). This measurement point is referred to as COMPARE2, or C2. The
peak and valley detector circuitry consists of CR3, R26, R41, R42, R43, C42,
and C41.
The response of the peak detectors is nonlinear since the diode CR2 impedance
changes as the voltage on the diode changes. The charging time constant is
limited to no faster than 20.8 microseconds. The decay time constant is 19.8
milliseconds.
The filters and level shifter circuitry are shown in Figure 17-2.
The 10 Hz filters eliminate the high frequency components of the optical signal
and get it ready for conversion to a digital signal, thereby smoothing out the Red
and IR signal from the demultiplexer. The gain of the Red filter circuit is eight,
while the gain of the IR filter circuit is five.
Section 16: MP-205 Service Manual; 044540A-0296
16-3
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Troubleshooting
