HP 3457A Service Manual page 178

Transistor Q211 allows a more linear operation of U212 (increased loop gain since it's not required to
output a voltage so close to its limits). Without it, U212 would be required to output +12 volts from a
supply of only +15 volts and linearity would not be as good.
8-29. Output FET Gate Bias Amplifier
Amplifier U213 in combination with the output MOSFET in U101 and switch amplifier U212 comprise
a voltage-to-current Converter which is the Ohms Current Source. The noninverting input of U213 is
driven by a voltage derived from the Ohms Voltage Reference Amplifier, U2I1. This input is main-
tained at +8 volts independent of the output of U2I11 by selecting one of two sense points in the gain
network consisting of the 17.1K and 2.9K ohm resistors in Ul0!. UI0! MOSFET switch 11 will be on
when the input to U211 is +8 volts and this voltage is switched to the input of U213. When the input
to U2I1 is +5.6 volt, U10l MOSFET switch 77 will be on. This will select the +8 volts at the junction
of the 17.1K and 2.9K ohm resistors in U101 and apply it to the input of U213. The operation of U213
then forces the inverting input of U213 to +8 volts as long as current can be made to flow through one
of the range resistors (ULO1, 4K, 40K, 400K or 4M ohm). With a path for current to flow, the range
resistor will have either 4 volts or 0.4 volts across it, thus generating a precise output current.
8-30. Offset Compensation Switching (OCOMP command)
The ohms function can be thought of as simply measuring the voltage across a resistor that has a preci-
sion current passed through it. However, if there are unwanted voltages in the path, errors will be in-
troduced. For example, the error voltage may be a thermally generated offset in the resistor under test.
If this unwanted voltage could be eliminated from the desired voltage, then potentially large sources of
error in the measurement could be removed.
Offset compensation is a technique which removes sources of errors. While in the ohms mode. a
measurement is periodically made with the ohms current source off. This result is the error voltage we
want to remove. This value is subsequently subtracted from normal ohms readings to yield an error
free result.
Analog switch U214 is used to switch the ohms current source on and off. The function of U213 is to
control the gate voltage and thus the current flowing to UIOI pin 15 from the output MOSFET in
U101. When the gate voltage is positive the current source is off. The purpose of U214 is to toggle be-
tween current source on (switch from pins I-16 on) and current source off (switch from pins 3-4 on),
Independent of offset compensation, the network composed of R218, Q217 and CR202 provides an al-
ternate current path when the output MOSFET in ULOI is of f. Without this network, the combination
of the range resistors (4K, 40K, 400k and 4M ohm) in ULOI and stray capacitance at the inverting input
of U213 can cause stability problems on the higher ohms ranges. With this network, when the output
of U213 approaches -7.5 volts, the diode connected low leakage JFET Q217 forward biases and output
current is diverted away from the ohms output MOSFET.
HP 3457A Multimeter
8-12