Keithley 2182 Service Manual page 56

Input signal conditioning
Protection circuits
The CH1 HI, CH2 HI, and CH2 LO inputs have protection circuits designed to prevent circuit
damage from over-voltage signal conditions. Q666 and Q667 provide protection for CH1 HI,
while Q661 and Q662 provide similar protection for CH2 HI.
Signal paths
There are two basic signal paths: DIV_COM (divider common) and LN_COM (low noise
common). The divider common path has an attenuation factor of 100:1, and is used for the 100V
range, while the low-noise preamplifier is used for the 10mV-10V ranges.
Buffering
U601 is a unity-gain amplifier that provides signal buffering for the divider common signal
path. A 100:1 resistive divider is switched in to attenuate the signal for the 100V range.
Low-noise preamplifier
The low-noise preamplifier is made up of Q624, U614, and associated components. Q624 and
U614 form a composite op amp with switchable gain depending on the selected measurement
function. Q624 forms the differential amplifier input, while U614 provides the required high
open-loop gain. Circuit gain is controlled through feedback elements and signal paths by
switches Q611-Q614, Q622, Q623, and U645.
Multiplexer and gain amplifier
Multiplexer
U626 is the multiplexer that switches among a number of signals during the various phases
of the measurement cycle. In addition to the two main signal paths, DIV_COM and LN_COM,
the mux also switches input LO, CH2 LO, the front panel temperature sensor, as well as the 7V,
1V, and 0.1V reference signals. The two LO signals are measured to null offsets, the temperature
sensor is measured for temperature measurements and TCAL, and the reference voltages
provide stable comparison voltages for accurate measurements.
Gain amplifier
U630, Q651, and associated components are gain amplifiers that provide X1, X10, or X100
gain factors, depending on the selected range. Gain is controlled by switching in various
resistors into the feedback loop. Note that the output of U630 is the signal that is applied to the
A/D converter for conversion into digital data that can be read my the microprocessor.
Troubleshooting 4-9