铮?/div>
Differential Input
The input can accept differential voltages anywhere within the
common mode range of the input amplifier, or specifically from
0.5V below the positive supply to 1V above the negative
supply. In this range, the system has a CMRR of 86dB typical.
However, care must be exercised to assure the integrator
output does not saturate. A worst case condition would be a
large positive common mode voltage with a near full scale
negative differential input voltage. The negative input signal
drives the integrator positive when most of its swing has been
used up by the positive common mode voltage. For these
critical applications the integrator output swing can be
reduced to less than the recommended 2V full scale swing
with little loss of accuracy. The integrator output can swing to
within 0.3V of either supply without loss of linearity.
Auto-Zero Phase
During auto-zero three things happen. First, input high and low
are disconnected from the pins and internally shorted to analog
COMMON. Second, the reference capacitor is charged to the
reference voltage. Third, a feedback loop is closed around the
system to charge the auto-zero capacitor C
AZ
to compensate
for offset voltages in the buffer amplifier, integrator, and
comparator. Since the comparator is included in the loop, the A-
Z accuracy is limited only by the noise of the system. In any
case, the offset referred to the input is less than 10碌V.
Signal Integrate Phase
During signal integrate, the auto-zero loop is opened, the
internal short is removed, and the internal input high and low
are connected to the external pins. The converter then
integrates the differential voltage between IN HI and IN LO for a
fixed time. This differential voltage can be within a wide
common mode range: up to 1V from either supply. If, on the
other hand, the input signal has no return with respect to the
converter power supply, IN LO can be tied to analog COMMON
to establish the correct common mode voltage. At the end of
this phase, the polarity of the integrated signal is determined.
Differential Reference
The reference voltage can be generated anywhere within the
power supply voltage of the converter. The main source of
common mode error is a roll-over voltage caused by the
reference capacitor losing or gaining charge to stray capacity
on its nodes. If there is a large common mode voltage, the
reference capacitor can gain charge (increase voltage) when
called up to de-integrate a positive signal but lose charge
(decrease voltage) when called up to de-integrate a negative
input signal. This difference in reference for positive or negative
input voltage will give a roll-over error. However, by selecting the
reference capacitor such that it is large enough in comparison
to the stray capacitance, this error can be held to less than 0.5
count worst case. (See Component Value Selection.)
De-Integrate Phase
The final phase is de-integrate, or reference integrate. Input
low is internally connected to analog COMMON and input
high is connected across the previously charged reference
capacitor. Circuitry within the chip ensures that the capacitor
will be connected with the correct polarity to cause the
integrator output to return to zero. The time required for the
STRAY
C
REF
+
V+
10碌A
31
IN HI
INT
DE-
DE+
INPUT
HIGH
34
REF HI
36
A-Z
C
REF
REF LO
35
A-Z
STRAY
R
INT
C
REF
-
33
BUFFER V+
28
1
29
C
AZ
A-Z
INTEGRATOR
C
INT
INT
27
-
+
+
-
+
-
2.8V
6.2V
A-Z
TO
DIGITAL
SECTION
A-Z
N
32
COMMON
INT
30
IN LO
V-
A-Z AND DE(卤)
INPUT
LOW
DE+
DE
-
+
-
COMPARATOR
FIGURE 3. ANALOG SECTION OF ICL7106 AND ICL7107
6
FN3082.7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
