5 Theory of Operation144 Keysight 34970A/34972A Service GuideA-to-D ConverterUnless otherwise noted, components in this discussion are located on the A4circuit assembly (34970-66504).The analog-to-digital converter (ADC) is used to change DC voltages into digitalinformation (schematic shown on page 16-20). The circuitry consists of anintegrator amplifier (U402 and U420), current steering switch U411, resistornetwork U102E, voltage reference U403, ADC controller U209, and residue ADCU205.The ADC method used is called multislope III. It is based on patented KeysightADC technology. Multislope III is a charge balancing continuously integratinganalog-to-digital converter. The ADC charge balancing algorithm is alwaysrunning, even when the multimeter is not triggered. The input voltagecontinuously forces charge onto the integrator capacitors C400 and C401 throughU102E–R16.Switches U411A and U411B steer fixed positive or negative reference currentsonto the integrator capacitor to cancel, or balance, the accumulated inputcharge. The level shifted (R403 and R406) output of the integrator is checkedevery 2.66 μs by the A1U209 COMP input. Logic state machines in A1U209control the U411 current steering to continuously seek an approximate 2.5 V levelon the integrator amplifier output, FLASH. If the ADC input voltage ADIN isbetween ±15 V, the integrator output (FLASH) will remain within the 0 V to 5 Vrange of the A1U205 on-chip ADC. An input greater than +15 V may cause theintegrator output (U402–6) to saturate at about –18 V. An input less than –15 Vmay cause U402 to saturate with an output of about +18 V. The A1U205 ADCinput (FLASH) is clamped to 0 V or 5 V by R405 and CR403 to protect A1U205.The integrator amplifier is formed by U402 and U420. Resistors R420 and R421affect the amplifier stability. Amplifier oscillation may occur if their values areincorrect. Amplifier U420 improves the offset voltage characteristics of integratoramplifier U402.
