Analog Circuitry; Direct I/O Signals; Gain-Controlled Amplifier; Dark Correction D/A Converter - Thermo Scientific GENESYS 20 Service Manual

Electrical Circuit Description and Adjustment
8.3.2

Analog Circuitry

Direct I/O Signals

A number of direct I/O signals, located on the Microcontroller ports 1 and 3, are
used for timing-critical controls. Devices controlled include the serial port,
display, printer, write control, address bits, and the grating motor limit switch.

Gain-Controlled Amplifier

The photometric signal arrives on the board at J10 pin 3 already amplified by the
preamplifier on the Detector Board (4001-6034). The typical voltage range is 0
to -4 volts, but voltages as low as -8 volts can occur when the detector is
saturated with light. GAIN0, GAIN1, and GAIN2 signals control the
preamplifier gain and allow 8 different gain steps. The GAIN3 control signal to
the U43 bilateral switch allows for an additional 9th amplification step if
necessary. The signal is then added to the output of the 8-bit D/A converter
(U44). Op-amp U40 adds the two signals, low-pass filters the sum (R17, C28),
and reverses the polarity so a positive voltage is presented to the logarithmic
A/D converter.

Dark Correction D/A Converter

The output of U44, an 8-bit D/A converter, sinks 2mA full scale and creates a
voltage range of +5.00 to -5.00 volts at R33. This voltage divided down by the
U40, R32, R23 amplifier to provide a ± 16.6 mV offset adjustment to the
photometric signal. The voltage adjustment is chosen so zero volts is presented
to the A/D converter within ½ lsb (65
sample compartment. This provides a true dark or 0%T reading by correcting
for leakage currents and op-amp offset voltages and bias currents. Any dark
voltage outside this range cannot be zeroed out and will cause an instrument
failure (hardware error #13 or 14).
A/D Converter
The A/D converter multiplexes its inputs through U31 and operates on a
logarithmic principle with an equivalent resolution of roughly 22 bits. A
capacitor (C37) is charged to a precision 5.00 Volt reference supplied by U42.
The capacitor is then discharged through a fixed resistor (R26) to ground until its
voltage is equal to the input signal being measured. At this point, a comparator
(U41) trips and sends a signal through the S-R latch (U32) to the
microprocessor. The latch is reset and the capacitor recharged by switching U43
with the MEASURE control signal set LOW. The U41 comparator then has
feedback (R25, R28) switched in by U43 with the /ZERO signal set LOW. With
U41 now in a linear operation, the input signal is changed to ground by the
multiplexer. In this "zero" mode, C35 stores the offset voltage of U41. The
voltage on C35 will be added to the signal being measured next to cancel the
8 - 4
v olts) when no light passes through the
F