Keysight Technologies 34401A - Page 111

Each analog-to-digital conversion begins when the multimeter is
triggered. The ADC starts by clearing the integrator slope count in U501.
At the end of the integration period, the slope count is latched.
The slope count provides the most significant bits of the input voltage
conversion. The least significant bits are converted by the on chip
ADC
of CPU U500.
The instrument precision voltage reference is
U403. Resistor R409
provides a stable bias current for the reference zener diode. R408 and
CR404 provide a bias to assure that the reference zener biases to +7 V
during power up. IC U400A amplifies the voltage reference to +10 V
while amplifier
U401A inverts the +10 V reference to –10 V. The reference
voltages force precision slope currents for the integrating ADC through
U102E–R17, R18. Amplifier U401B provides a precise +5 V reference for
the U500 on chip ADC.
Chapter 5 Theory of Operation
A-to-D Converter
102