HP 3457A Service Manual page 196

SUL—SIL_ @ 10 sv_w 3v Rance al
TO 0.5V ON 300m RANGE eal
AC INPUT
c
5 Pw t0 mo |
n I commcer_ |
'
FROM 7 KH 100 Low
wicroprocessor ___| tt &
xX
1924
N= 0 To 1023
34520 F.12.4
Figure 8-4. Attenuator Compensation
There are two separate compensated input attenuators used in the AC section. One is used for the
30mV and 300mV ranges and the other is used for the 3V, 30V and 300V ranges. The attenuators are
each automatically compensated by the ACAL function.
During ACAL, a 0.5 volt and a 5.0 volt pulse signal are generated by the AC Autocal circuit A2U402
located on the A2 Input Signal Conditioning assembly. These values are determined by resistive divider
A2R301 and R302. Switch A2U402 is used to select the desired voltage level.
These pulse signals are applied to the input attenuators of the AC board (A3) and measurements are
made to determine the flatness of the signal. The first measurement is made shortly after the falling
edge of the pulse signal and a second is made at a later time. The first measurement will determine the
amount of the overshoot or undershoot (see Tl on Figure 8-4). The second measurement (made at T2
on Figure 8-4) will be the reference level to which the first measurement is adjusted. These measure-
ments are then nulled via the circuit composed of DAC U311 and the capacitors C304 or C308. These
capacitors shunt their respective attenuators. Their values are effectively varied from 0 to | times their
value in 1024 steps by the control lines to DAC U311.
As previously stated, the values of C304 and C308 are being changed, thus a variable capacitor is
electronically simulated. This is done by monitoring the output of A3U302 with A3U301 pin 5.
A3U301 is a buffer whose output goes to DAC U311. The DAC is not configured as typically seen but
is driven in a voltage mode and pin 15 of the DAC becomes the input signal to U301. The level of this
signal will vary and is dependent on the amount of compensation required. The gain of U301 is selec-
ted by the combination of U313, Q305 and Q306. For all ranges except the 300V range, Q305 is on and
this sets the gain of U301 to XI. On the 300V range. Q306 is on and this sets the gain of U301 to N10.
The gain is set to X10 for the 300V range because the input to amplifier Q307/U302 is taken from the
junction of R307/R308 and this signal level is a factor of 10 less than the signal at the junction of
R305/R307. The output of U301 to C304 and C308 is used to effectively vary the value of the
capacitance to provide frequency compensation.
DAC U311 controls the amount of feedback to capacitors C304 and C308. The amount of feedback is
calculated during Autocal, retained in memory and this correction value is used to control DAC U311
during each measurement.
The DAC receives it control instructions via data lines, DO -D9.
HP 3457A Multimeter
8-31