HP 3457A Service Manual page 175

8-23. Offset DAC
The offset DAC section (U217, U122, U1L23 and U218) is used by both the A2 and A3 assemblies. The
DAC provides a bipolar
+10V output
in 1024 steps of approximately 2mV
step size. A code of 0
programmed
to the DAC yields an output
voltage at JM102
of -10 volts while a code of S12 gives 0
volts and 1023 provides a full range output of +10 volts.
Shift register U123 also provides miscellaneous control bits to the DCI switch U403 and AC ACAI
source U402. The HTSO bit is the actual step used as the flatness ACAL source.
8-24, Pre-charge Stage and Pre-charge Offset Adjust
Pre-charge Stage
The pre-charge amplifier in UI0I is used to pre-charge the stray capacitance at the input node of the
input amplifier section. During a zero measurement, when Autozero is On, the input to the input
amplifier is continuously switched between the input terminal through U101 switch MC and the analog
ground through UI01 switch MZ.
The parasitic capacitance (approximately
10pf) at the amplifier in
put has to be charged and discharged. The charging current would come through switches MC and MZ
if the pre-charge circuit were not present.
This could load the input voltage and cause reading errors.
To prevent this, the input of the input amplifier is pre-charged to the input voltage before the input
measurement
is made.
This is done by the pre-charge amplifier and MOSFET switch PRE
which are
both located within U101. The operation is as follows:
A, After the zero measurement is made, switch PRE closes (switch MC is still open).
B. The input voltage is connected to the pre-charge amplifier using the input paths.
C. Since the pre-charge amplifier is a X1 gain amplifier, a voltage with the same polarity and value
as the input voltage is applied through PRE to the input amplifier. This pre-charges the stray
capacitance to the input voltage.
HP 3457A Multimeter
8-9