Theory of Operation 4
Keysight 34420A Service Guide 119
which will tum the current source transistor Q456 off. When a resistor value of less
than the range is connected to the input terminals, comparator U452A will reverse
state allowing the current source tum on. The meter can be programmed to 3
different levels of voltage clamping when operating in this mode. They are 20 mV,
100 mV, and 500 mV. Resistors R462, R463,and R474 along with programming via
register bits 47 and 48 set these levels. Capacitor C454 provides enough time
delay to minimize any voltage overshoot while comparator U452A is turning the
current source off. If the current source terminals are open circuited, R467
prevents the open circuit voltage from rising above the programmed “clamp
voltage”.
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