Agilent 34970A/34972A Data Acquisition / Switch Unit

Chapter 5 Theory of Operation
Floating Logic
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.
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• 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.
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• 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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