Keithley 6512 Instruction Manual page 106

Theory of Operation
U138A
BUFFER
INTEGRATOR OUTPUT WAVEFORM
DELAY DELAY
CHARGE SINGLE
BALANCE SLOPE
PHASE PHASE
Figure 6-15
A/D converter
The integrator is made up of U138B and C128. When the in-
put to the integrator is applied, the integrator output ramps up
until its voltage is slightly higher than the voltage applied to
the inverting input of the charge balance comparator
(U137B). When the Q4 output of the clock generator (U127)
goes high, the output of U135B is low, and the Q1 output of
U126A will go high. This action injects the charge balance
current into the integrator input. Since the charge balance
current is much larger than the sum of the input and level-
shift currents, the integrator output now ramps in the nega-
tive direction. The integrator output will continue to ramp in
the negative direction until the output of U135A goes low.
Note that the Q3, Q4, and Q5 outputs of U127 must all be
high for the output of U135A to go low.
6-8
I SS
Q108
R153G
U138B
R153H
-7V
LEVEL SHIFT
INTEGRATOR
CURRENT
R149A
I CB
INPUT
ENABLE/DISABLE
1.2288MHz
CLOCK
U128D
NEXT
MEASUREMENT
NC
U136B
R149H
C128
SINGLE SLOPE
COMPARATOR
U137A
CHARGE BALANCE
COMPARATOR
+1.5V
U137B
U135B
CR113
Q5 Q4 Q3
CLOCK
GENERATOR
U127
CHARGE BALANCE CURRENT
The output of U136C is gated with the Q2 output of the clock
generator by U135C. Each time Q2 goes high while the
U136C output is high, a pulse is fed to the microprocessor.
The MPU then counts the total number of pulses that occur
during the charge balance phase.
The charge balance phase lasts for 100msec. At the end of
this period, the output of the integrator is resting at some pos-
itive voltage. Since the integrator output is connected to the
non-inverting input of the single-slope comparator (U137A),
the single-slope comparator output remains high until the in-
tegrator output ramps in the negative direction.
During the single-slope phase, Q108 is turned off to discon-
nect the input and charge balance currents from the integra-
tor input. In place of these two currents, the single-slope
current (I ) is injected into the integrator input. This current
SS
SINGLE SLOPE
ENABLE/DISABLE
U136C
U211C
R138
U126A
U135C
U210B
D1 Q1
CLK
S1
U135A
Q2
TO MPU