Keithley 2520, Service Manual, Page: 79 / 137

Model 2520 Pulsed Laser Diode Test System Service Manual Troubleshooting 4-7
Analog circuits
Figure 4-2 shows a simplified block diagram of the analog circuits.
Measurement circuits
Voltage measurement circuits
The laser diode voltage measurement circuits consist of U12, U13, and associated cir-
cuitry. The two sections of U12 provide buffering for the HI and LO sense lines, while
U13 controls ranging with appropriate feedback elements.
Current measurement circuits
Signal conditioning for the detector input currents are provided by I-V converters, which
convert the input signal currents to voltages that can be used by the A/D converters. U7,
U8, Q10, and Q11 form a compound op amp for detector #1 I-V conversion, while U10,
U17, Q15, and Q16 make up an identical op amp for detector #2 I-V conversion. Current
ranging is provided by selecting various feedback resistors for the corresponding current
ranges (10mA, 20mA, 50mA, and 100mA).
A/D converters
Each of the three measurement channels has its own 10MHz, 14-bit A/D converter. U31
provides A/D conversion for laser diode voltage measurements, while U43 and U53
perform A/D conversion for detector 1 and detector 2 current measurements, respectively.
A/D digital data is stored in high-speed FIFO (first-in, first-out) SRAM circuits (U27,
U29, U38-U42, and U49) for use by the DSP section (described below).