4 Theory of Operation
114 Keysight 34420A Service Guide
Function Switching
This discussion refers to the Chapter 8, "Function Switching Schematic". The
Function Switching allows one of three sources to be dynamically selected. The
three sources are: the “Measure Customer” input (MC), the “Measure Zero” input
(MZ), and “Precharge” (PRE). The “Measure Customer” input is the signal applied
to the input terminals. The “Measure Zero” input is the ground of the meter and
allows measurement of internal offset voltages which are also present in the
“Measure Customer” input. The displayed result is computed from MC-MZ. The
“Precharge” input is used to precharge internal capacitances to reduce charge
injection to the input terminal from dynamic switching of MC and MZ.
Q201 and Q202 form the “Measure Customer” switch. Closing these switches
applies the input signal to the DC Amplifier (INPUT AMP+). The state of the
“Measure Customer Switch” is controlled by U201A.
The “Measure Zero Switch” uses Q203, Q204, and Q205. During the measure zero
portion of the measurement cycle, Q203 is closed. The zero measurement for dc
Volts and 2-wire Ohms function is taken with Q204 and Q205 on. For 4-wire Ohms
function, Q204 and Q205 are off and U124 and Q134 are on.
U101B switches precharge, the high voltage divider, or ohms current to the DC
Amplifier. The high voltage divider is used only on the 100 V range. It is a 10 MΩ,
100:1 voltage divider that has a 1 Volt output for a 100 Volt input. The signal is
coupled by U126 and U101 to the input amplifier and then to the AID converter.
Additionally, the X1 amplifier signal is used by U206A to provide the BOOT signal.
This signal ensures the FET switches have the proper bias when turned on.
U203, U204, and U314 form a programmable gain amplifier. This circuit is used to
inject a signal via capacitors C201 and C202 that minimizes switching transients
at the input terminals when the “Measure Customer” switch is opened and closed.
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