4 Theory of Operation
116 Keysight 34420A Service Guide
Operational amplifiers U312A and U312B and DAC U313 provide programmable
offset adjustments to null the input FET U315 offset voltage. The DAC is
programmed “on the fly” during measure zero time. The offset voltage is checked
after each DAC adjustment and further adjustments are made as necessary to
obtain a voltage that is less than 10% of full scale during autozero.
The dc amplifier has a programmable 2-pole analog filter. C306 is switched with
Q311. C303 through C305 are switched in with JFET Q310. Both Q310 and Q311
are switched in or out depending upon the state of switch U304C. Both pole
frequencies are approximately 11 Hz.
U311A and U311B are overload comparators that sense the output of the DC
Amplifier. If the peak level at the output of the dc amplifier exceeds approximately
± 14 volts, the comparator output U311A or U311B will change state. This
information is sent to the CPU and will cause the reading to output as an OVLD.
To Next Page
Loading...
Need help?
General
