Keithley 2520, Service Manual, Page: 81 / 137

Model 2520 Pulsed Laser Diode Test System Service Manual Troubleshooting 4-9
Source circuits
Voltage bias source circuits
Each Model 2520 voltage bias source is a digitally controlled source that can source up to
±20V @ 100mA. Digital control information from the MPU is converted by a DAC into
an equivalent analog signal. U1 converts the detector 1 source data, while U2 converts
detector 2 source data. Gain and 100mA drive capability for detector 1 and detector 2
sources, respectively, are provided by U20 and U3 to provide the full ±20V, 100mA out-
put capability for each source.
Current pulse source circuits
The Model 2520 current source can output a maximum current of 1A DC @10V (10W) or
5A @ 9V (45W) current pulses.
DC and pulse high current source data from the MPU is converted into an analog signal by
a 16-bit DAC made up of U54 and associated components. Pulse low data is converted by
DAC U1, and U5 performs pulse high/low switching. The signal from U5 is amplified by
the current amplifiers, U8 and U18, and applied to the output stage (Q10 - Q12), which
provides the current drive capability for the current source.
The voltage clamp circuit made up of U4, U6, U16, and Q9 maintains the output voltage at
the programmed compliance value regardless of load, while the compliance detection cir-
cuit made up of U9 and U19 provides feedback to the MPU for an over-compliance condi-
tion. Over temperature detection for the current source is performed by RT1, R46, U15,
and associated components.