HP 3457A Service Manual page 177

could be produced with passive components. This circuit is called a Current Mirror because its
eS operating characteristics force the same currents to flow in RII1 and R112.
Since the low voltage range for the voltmeter is 30mV, the input amplifier offset voltage could be a
large fraction of the total scaled input to the A/D converter. Adjustable resistor R113 is provided as a
means of reducing the offset voltage and the potential errors it may generate. There is a diagnostic test,
called Input Amplifier Offset Adjustment, that forces this offset to be adjusted to less than 500uV
while Self Test checks it to limits of +1.5mV.
Output Gain Stage
The final stage of the input amplifier is output gain stage ULI1b. UI! 1b is configured an an integrator
with C111 to provide additional gain and output drive to the A/D converter.
8-26. Ohms Current Source
The purpose of the Ohms Current Source is to provide a stable current for resistance measurements.
The current is applied to the unknown resistance and the resultant voltage drop across the resistance is
measured, All ohms ranges except the 30 ohm and the 300 ohm range generate a full scale of 3 volts.
These ranges have 30mV and 300mV full scale voltages. To achieve this, precision current sources rang-
ing from ImA down to 100nA are needed. The ohms current source can be divided into 5 sections as
follows:
Ohms Voltage Reference
Switch Amplifier
Output FET Gate Bias Amplifier
Offset Compensation Switching
es Ohms Overload Protection
The following discusses each section except for the Ohms Overload Protection circuitry which was
described earlier in the section titled Over Voltage Protection.
8-27. Ohms Voltage Reference
In order to achieve a precise and stable ohms function, the ohms current source is referenced to a very
stable +10 volts, This +10 volt reference at UI01 pin 4 is divided to values of 8 volts and 5.6 volts
within input hybrid ULOI and switched to ohms voltage reference amplifier U211 by U101 MOSFET
switches 11 and 77. Eight volts is selected for all ranges below 30M ohm and 5. volts is selected for
30M ohm and above. The gain of U211 is set at X1.5 by the 17.1K and 2.9K ohm resistors within U101
and the 40K ohm resistor, R211.
8-28. Switch Amplifier
The switch amplifier (U212) receives an input voltage of either 12 volts or 8.4 volts from U2I1. U212
is a unity gain follower used to enable one of the four range resistors (4K, 40K, 400K or 4M ohm) in
UIOL that are used to set the output current value. When a range below 30M ohm is selected, the input
to U212 will be 12 volts. This voltage will be applied to one of the four resistors by a pair of UI0I
MOSFET switches (12, 13, 14 or 15). The other side of the range resistors will always have 8 volts ap
plied (refer to the following section entitled Output FET Gate Bias Amplifier); therefore, the current
selected will be 4 volts/selected R. When a range of 30M ohm and above is selected the input to U212
will be 8.4 volts (1.5 x 5.6). This voltage will be applied to one of the four range resistors by a pair of
UL0I MOSFET switches discussed previously. The other side of the resistors will still have 8 volts ap-
e plied; therefore, the current selected will be 0.4 volts/selected R.
HP 3457A Multimeter
8-11
Sn! Ele