Theory of Operation 5
Keysight 34970A/34972A Service Guide 143
The programmable capacitance is implemented by varying the signal level across
a compensating capacitor. In the x0.2 configuration, low frequency gain is set by
R301, R302, and R304. The variable gain element U302/U303 essentially varies
the value of C306 from 0 to 1 times its value in 256 steps. The exact gain constant
is determined during the 50 kHz AC voltage range calibration procedure. In the
x0.002 configuration, low frequency gain is set by R301, R302, and R303. The
variable gain element U302/U303 essentially varies the value of C305 plus C306
from 0 to 1 times their value in 256 steps. The exact gain constant is determined
during the 50 kHz AC voltage range calibration procedure.
The second stage is made up of two amplifiers (U305 and U312) each configured
for a fixed gain of x10. Overall 2nd stage gains of x1, x10, and x100 are produced
by routing the 1st stage output either around, or through one or both amplifiers as
shown in the table below.
The output of the 2nd stage is connected to the rms-to-dc converter stage. Any
residual DC offset from the amplifier stages is blocked by capacitor C316. Buffer
U307 drives the input to the rms-to-dc converter as well as the frequency
comparator (U310A) input. The rms-to-dc converter has two selectable averaging
filters (C318 and C318 plus C321) for the analog computer circuit of U308. The
two analog averaging filters together with digital filters running in the main CPU
implement the three selectable AC filters: slow, medium, and fast. The faster
analog filter (using C318) is used for all AC V, AC I, and frequency or period
autoranging. The slower analog filter is used only with the slow and medium AC
filter choices.
In frequency or period measurements, U310A generates a logic signal (FREQIN)
for every input zero crossing. The AC sections FREQRNG DC output is measured
directly by the main CPU’s 10-bit ADC during frequency or period measurements.
This lower resolution measurement is sufficient to perform voltage ranging
decisions for these functions. The frequency comparator output is disabled during
AC voltage and current measurements by U310B forcing U310A’s input to –15
volts.
2nd State Gain U306A U306B U306C U306D U304C
x1
x10
x100
ON
OFF
OFF
OFF
ON
ON
OFF
OFF
ON
OFF
ON
OFF
OFF
OFF
ON
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