Theory of Operation 5
Keysight 34970A/34972A Service Guide 129
The main controller, U205, is a 16-bit microcontroller incorporating many built-in
features:
– A 10-bit, successive approximation ADC with selectable inputs is used to
convert two signals: FLASH and FRQRNG. The FLASH signal is the residual
charge on the main integrating ADC output from the internal DMM assembly
(A4). The FRQRNG signal (also from the internal DMM) is used to make voltage
ranging decisions for frequency and period measurements.
– A pulse-width-modulation port, after filtering the 23 kHz output with
R221,C224, and R259, produces DC voltages between 0V and 5V. This
voltage, PREADJ, is used to adjust the precharge amplifier offset voltage in
U101 on the internal DMM assembly (A4).
– A full, UART controlled, serial port is used to communicate with the
I/O processor through opto-isolators U303 and U214. Data is sent in an 11-bit
frame at a rate of 187.5 kbits/second. The 11-bit frame is configured for one
start bit, nine data bits, and one stop bit.
– A timer is used to measure the power line frequency on LSENSE. Frequencies
between 55 Hz and 66 Hz result in the use of a 60 Hz standard for the DMM
integration period. All other frequencies will result in the use of a 50 Hz
standard.
– A 16-bit counter counts pulses on CNT to create, along with the 8-bit counter
in U209, a 24-bit counter for the internal DMM.
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