Agilent Technologies 34970A Service Manual page 139

Chapter 5 Theory of Operation
Internal DMM
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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