Keysight Technologies E4417A Service Manual page 82

Theory of Operation 4
The recorder is a 12 bit DAC driven by the microcontroller. The DAC output is
buffered, filtered and scaled to give a 1 V full scale output with a nominal 1 kΩ
output impedance.
Circuitry for the keyboard driver includes some damage protection, but it is
basically a direct connection from the keypad row and column matrix to the
microcontroller's control lines.
The LCD controller on the A3 front panel assembly is configured as a memory
mapped peripheral, and as such requires only to be fed with the appropriate
address, data and control lines from the microcontroller circuits. The bias voltage
for the LCD is produced by a DC to DC converter that takes the +5 V (DIST)
voltage and converts that to a nominal +21 V. The DC to DC converter is adjusted
by a combination of the contrast
control signal from the microcontroller and the temperature sense voltage that is
generated on the A3 front panel assembly. The temperature sense voltage helps
to compensate for the normal variation of LCD contrast with temperature. Current
to the LCD's LED backlight array is sourced from a constant current circuit that
compensates for variation in LED knee voltage. A control input to this circuit from
the front panel temperature sense voltage allows for de-rating of the maximum
LED current at high operating temperatures. A control input from the
microcontroller allows the optimum backlight brightness to be factory set. The
temperature sense voltage, LED current and LCD bias voltage are all measured at
self test.
An analog multiplexer provides means by which several diagnostic points can be
switched to the A6 measurement assembly for analogue to digital conversion. The
microcontroller uses this function to perform a self test.
A6 measurement assembly to microcontroller communication is conducted on a
parallel expansion bus. This bus is connected to the bi-directional serial port on
the microcontroller via buffers. The 20 MHz system clock
and some control signals are connected to the measurement modules. The
measurement module interface is completed with the distribution of switched and
direct power (+12 V, −12 V and +5 V).
Keysight E4416A/E4417A Power Meters Service Guide 89