Chopper; Ohmsconversion; Analogoutput; Clock - Keithley 177 Instruction Manual

demodulator JFET Q105. Resistors R118, R119 and R142
and capacitors Cl31 and Cl32 affect the rise and fall times
of drive signals. R142 adjusts the rise and fall time of the
drive signal to Q104A only, so that the total charge, both
positive and negative, transfered to the input can be
minimized. The frequency of the drive signal is determined
by the clock circuit. Circuitry in UllO divides down a
nominal clock frequency of 100kHz to produce 390Hz.
6. Adjustments and Input Protection-The front panel zero
control CR2031 permits adjustment of the chopper feed-
back to compensate for input thermal EMFs. It functions
only on ranges where chopper gain is greater than one. A
span of f75V is considered adequate; however, it is
typical for R203 to provide a greater than 400V change as
it is turned end-to-end. The span is skewed by R128 to
give best resolution in both DCV and n. Potentiometer
R146 provides a calibration adjustment to compensate
200mV range (X10 gain configuration) and R147 adjusts
the Xl00 gain used on the 20mV range. Chopper input
resistor R143 limits the input current during momentan/ in-
put overloads to 12mA. which is then absorbed by the
chopper load resistance (R145 and/or R149) or the power
supplies through CR116 and CR117.
7. DC Current Measurements-As previously described, DC
current measurements are made by placing the ap-
propriate buffer/amplifier shunt resistor on the chopper in-
put. A chopper gain of X10 is used for all current ranges
except 20A. which uses X100. The front panel zero control
(R203) is functional for all DC current ranges.
4.5 OHMS CONVERSION
The ohms converter contains two sections: a floating current
source and a high input impedance buffer/amplifier. The
chopper employed as a buffer/amplifier in DCV is also used
as the high impedance buffer/amplifier for n. Refer to
paragraph 4.4 for addition1 chopper theory.
4.5.1 Floatig Current Source
The floating current source involves a differential amplifier
W107) driven by the AID reference voltage and the chopper
feedback voltage. This results in a constant voltage across
the source resistance I = -V reference) which results in a con-
stant current in the unknown resistance. One or more range
resistors are always connected as the ohms source resistor
IRS) when ohms function is selected. Five different constant
current levels are supplied by changing the value of R, with
the range pushbuttons. These levels are given in Table 4-4.
For n, INPUT HI terminal is negative with respect to INPUT
LO.
4.5.2 Chopper
For ohms, the full scale chopper output is -2V. Gain is
employed on the 200 and 2OOQ ranges to give -2VFS out with
the nominal 1mA source current. The front panel zero control
functions on these two ranges, allowing compensation for up
4-4
to 200m0 of lead resistance. Settling time is determined
primarily by chopper input (T = R C) which results in two
seconds to full scale
on the 200 '6 kQ and 20MO
ranges.
(.047@F X 2Mfl and .0047wF X 20MW.
Table 4-4. Ohms Conversion
4.5.3 Adjustments and Input Overload Protection
Two 0 source calibration adjustments are required. Poten-
tiometer R162 compensates primarily for the tolerance of R,
(R158). while R159 compensates for ratio variations in the
four gain resistors (R150 through R153). The protection net-
work at the source output isolates the amplifier from positive
voltage overloads (Q103) and negative voltage overloads
lCR120) applied to INPUT HI. Transistor Q107 clamps the
source input during overload. The ratio of the base to emitter
resistors on 0103 sets the open circuit voltage.
4.6 ANALOG OUTPUT
The rear panel ANALOG OUTPUT provides a -t2V full scale
DC output signal (10K counts = 1V). The analog output
signal is available on all instrument functions and ranges, with
a negative polarity on ohms W and a positive polarity on AC.
The output is unbuffered, supplied by either the chopper or
the TRMS converter through 5kt7 effective resistance, with
1kR placed in the LO leg to prevent ground loops through IN-
PUT LO.
4.7
CLOCK
A single chip CMOS oscillator/ +2N package is used to
generate the 100kHz A/D clock and the 390Hz chopper drive
signals. The 1OOkHz OV to 5V signal drives the A/D and bat-
tery pack directly. The 390Hz signal is level shifted to OV to
-15V and split into two phases for driving the chopper FETs.
An RC adjustment (R142) provides chopper input bias cur-
rent compensation. Balancing and guarding is employed to
keep the drive signal out of the chopper AC amp stage.
4.8 A/D CONVERTER
The AID converter operates on the dual slope principle. The
timing is divided into three periods as described below.