Keithley 2015, Service Manual

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Troubleshooting 4-15
Distortion analog circuitry
Refer to Figure 4-5 for the following discussion on distortion analog circuitry.
Figure 4-5
Distortion analog circuitry block diagram
Amplifier filter
The buffered and scaled AC waveform from the analog circuitry (Figure 4-3) is fed to U309
and U310. U309 and U310 form an amplifier, antialiasing filter, and DC shifting circuit to con-
dition the AC waveform for the distortion measurement ADC converter.
ADC converter
The distortion measurement circuitry uses a separate ADC converter from the rest of the
DMM measurements (see U165 in Figure 4-3). This ADC is a high-speed, high-resolution, low-
distortion sigma delta type. The ADC digital output is sent to the DSP through opto-isolators
shown in Figure 4-5.
ADC clock
Y301 is a fixed frequency clock that forms the input to U325, an adjustable frequency clock
generator. The output of this generator clocks the ADC, setting the acquisition rate. The clock
generator frequency is set by the microprocessor and is communicated through U314, the
FPGA. The FPGA, U314, is configured by U315, an EEPROM, upon power-up.
Analog
Circuitry
(See Figure 4-3)
Amplifier Filter
U309, U310
ADC Converter
U311
OSC
Y301
ADC Clock
Generator
U325
FPGA U314
EEPROM
U315
Distortion
Digital
Circuitry
(See Figure 4-4)