Keithley 172A Instruction Manual page 67

INSTRUCTION MANUAL
THEORY OF OPERATION
Digital Multimeter
Models 172A. 172A
+v
AUTOZERO
QZ
STORAGE
RZ
*
Qv"
4- 'z
C
a
'i
0
I I
I
b
ACTIVE
INTEGRATOR
v.
Ri
I"- :;
COMPARATOR
'in"-,
Rf
2hh
Dl
CURRENT SWITCH
DRIVER
Cf
REFERENCE CURRENT
SWITCH
'r
i
REFERENCE CURRENT
SOURCE
DIGITAL PULSE TRAIN (QR)
D TO U9Dl
AUTO CnL STORAGE
e.
A/D Converter Simplified Schematic Description (Figure 5-14).
I) The active integrator, reference current switch and driver, comparator, AZ and
ACAL storage circuits are shown on the simplified schematic diagram.
After the buffer,
the A/D converter becomes unipolar, using only one reference current.
Since bipolar
inputs are applied to Ri, the nominal value of lz
is such that if Vin = -3 volts,
1~ - Iin 50.
Thus, -3 volts is zero cOUnts, 0 volts is 30,000 counts and +3 volts is
60,000
counts. The LSI circuit (U9Ol) counts in this way, providing digital polarity
sensing.
It should be noted that a 60,000 count span is used as described above, with
,
the exception that it is shifted up from zero count to center on the approximate 66,900
counts produced by the clock during signal integrate.
This is done so that LSI circuit
can detect overrange conditions in both polarities and to permit a signal settling time,
J after connecting it to the buffer,
before beginning the conversion.
5-17