Keithley 2500, Service Manual

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4-6 Troubleshooting Model 2500 Service Manual
Analog circuits
Figure 4-2 shows a simplified block diagram of the analog circuits.
Current measurement circuits
Signal conditioning for the input current is provided by an I/V converter, which converts
the input signal current to voltage that can be used by the A/D converter. Current ranging
is provided by selecting various feedback resistors and switching in an X10 gain amplifier
at appropriate times. The multiplexer (not shown) switches among different signals during
the various phases of the measurement cycle, and passes the signal along to the A/D
converter.
A/D converter
Each Model 2500 channel uses a multi-slope charge balance A/D converter with a single-
slope run-down. The converter is controlled by a gate array. Commands are issued by the
MPU to the gate array, and the gate array IC sends A/D reading data back to the MPU for
calibration and processing.
Bias voltage circuits
Each Model 2500 voltage bias source is a digitally controlled source that can source up to
±100V @ 20mA. Digital control information from the MPU is converted into an equiva-
lent analog signal, which is amplified to provide the full ±100V, 20mA output capability.
Source ranging (10V or 100V) is performed by controlling the overall gain of the amplifier
stage. Compliance circuitry detects when the output current exceeds a fixed 20mA limit
and sends an over-compliance flag bit back to the MPU.