Specifications

MODEL 370 REV: D Page 12 of 26
overrange. Push “auto/ manual”, then the up/down keys to locate the best
range, start operation by pressing “start”.
3.4.5.2 Zero
Zero stores the present reading for subtraction from the next readings. It is
useful when measuring LED or laser power in the presence of ambient light.
Turn off, or block the source to be measured while the instrument is
operating, then perform the “zero” operation.
4.0 THEORY OF OPERATION
4.1 The Model 370 contains an analog current to voltage converter, digital interface,
processor, IEEE interface, and power supply sections. Normally the instrument is
used with photodiodes, which generates photocurrent proportional to the power of
incident light. Photodiodes can be silicon, germanium, or InGaAs or other junction
or inversion layer types, vacuum or photomultiplier tubes; photoconductive devices
may also be used.
4.1.1 Preamplifier
Referring to the schematic, the sensor diode is connected to U1, which is an op amp
functioning in the transimpedance mode. This stage converts the current from the
sensor to signal voltage. The gain of the amplifier (actually the transimpedance
gain) is determined by the resistor in the feedback network. Relays K1 through K3
select gain resistors R1 through R3. Resistor R4 is always in the circuit. Actual gain
of the front end, the parallel resistance of R4 and R1 through R3 (if selected) is the
gain in volts “out” per amps “in”. The second stage, U2 provides voltage gain of 10
to fill in decade gains from 10 V/A through 10 V/A. Analog output voltages from
both channels are available from rear panel BNC connectors.
4.1.2 Digital Electronics
The A/D converter U9 is a 15 bit plus sign intergrating converter. The integration
period is 100mS which rejects 50 pr 60Hz powerline ripple. A 1.235V band gap
reference, CR2 provides for radiometric signal measurement. The precise reference
voltage is recorded during test and stored with calibration data.
The microprocessor is a CMOS version of the Z80 operating at a clock frequency
of 6MHz. The processor is responsible for system control and data processing.
Digital input and output are handled by U4, a programmable I/O controller. The
controller interfaces the processor to the keyboard, display, range select switches,
and power supply.
The display is a two line by 16 segment alphanumeric liquid crystal display which
uses the ASCII character set in communication with the processor.
The display may be illuminated with the integral electroluminescent panel. A
viewing angle adjustment potentiometer is provided on the rear panel to adjust for
best contrast. The Texas Instrument TMS9914A IEEE controller, 75160 and 75161
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