Theory of Operation 5
Keysight 34970A/34972A Service Guide 145
Each analog-to-digital conversion begins when the multimeter is triggered. The
ADC starts by clearing the integrator slope count in A1U209. 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 A1U205.
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.
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