Photodiode; Detector Board - Thermo Scientific GENESYS 20 Service Manual

Electrical Circuit Description and Adjustment
8.5

Detector Board

4001-6034,
Revision A
THE DETECTOR BOARD IS EXTREMELY SENSITIVE TO SURFACE
CONTAMINATION AND SHOULD ONLY BE HANDLED BY THE
EDGES.
The Detector Board contains a transimpedance (I to v) preamplifier with a "T-
gain" stage that allows for 8 gain steps by adjusting the "T-gain" resistor value
with an 8-channel multiplexer (U2). Multiple gain steps allow the output voltage
to be presented to the A/D converter in a usable range of roughly 1.5 to 3.5 volts
for 100%T. The gain values ranges from 2e+6 to 1.6e+9 including the 9th gain
step on the Main Board. It is possible to set 100%T with less than 1nA of
photocurrent or as much as 1.75
The photodiode is operated in the photovoltaic or zero-bias mode which
produces a linear relationship between the incident light power and the generated
photocurrent. The op-amp used is chopper stabilized which reduces its own
offset voltage to a few
F
capacitors (C5 and C6) during one cycle and using the caps to cancel the offset
voltage during the next cycle. This chopping action occurs at a rate of roughly
450 Hz. The op-amp employs a JFET input stage to provide low input bias
currents in the picoamp range. Filtering caps C3 and C4 are used to stabilize the
circuit and filter out electronic noise. Strapping the diode across the op-amp
inputs takes advantage of the common mode noise rejection of the op-amp to
reduce pickup of electrostatic fields.

Photodiode

A single silicon blue-enhanced photodiode with a 13mmx15mm active area is
used for the entire 325-1100nm wavelength range. The photodiode is cemented
with conductive epoxy to a FR-4 printed circuit board. Two gold-plated pins
protruding from the board are inserted through a light shield into pin sockets on
the Detector Board. One pin is soldered to maintain contact during product
shipment.
8 - 8
CAUTION
A .
F
v olts by storing the actual op-amp offset voltage on