TC7129
Low Battery
Continuity
V+
5 pF
C
O1
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
DP
4
/OR
21
V
DISP
CONTINUITY
INT OUT
INT IN
V+
22
23
24
25
26
27
ANNUNC
OSC3
OSC1
Display Drive Outputs
TC7129
COMMON
REF LO
RANGE
REF HI
C
REF
+
DGND
C
REF
–
BUFF
IN LO
IN HI
120
kHz
OSC2
DP
1
Crystal
V–
LATCH/
HOLD
DP
3
/UR
DP
2
28
29
30
31
32
33
34
35
36
37
38
39
40
330 kΩ
C
INT
0.1 µF
C
REF
+
1 µF
150 kΩ
R
INT
+
9V
R
O
10 pF
0.1
µF
C
IF
R
REF
20
kΩ
D
REF
C
RF
0.1 µF
C
O2
V+
10 kΩ
R
BIAS
–
V
IN
R
IF
100 kΩ
+
Figure 3-1:
Standard Circuit.
The capacitor should have low dielectric absorption to
ensure good integration linearity. Polypropylene and
Teflon
®
capacitors are usually suitable. A good
measurement of the dielectric absorption is to connect
the reference capacitor across the inputs by
connecting:
Pin-to-Pin:
20
→
33 (C
REF
+ to IN HI)
30
→
32 (C
REF
– to IN LO)
A reading between 10,000 and 9998 is acceptable;
anything lower indicates unacceptably high dielectric
absorption.
3.3
Integrating Capacitor (C
INT
)
The charge stored in the integrating capacitor during
the integrate phase is directly proportional to the input
voltage. The primary selection criterion for C
INT
is to
choose a value that gives the highest voltage swing
while remaining within the high-linearity portion of the
integrator output range. An integrator swing of 2V is the
recommended value. The capacitor value can be
calculated using the following equation:
EQUATION 3-1:
C
INT
=
t
INT
x I
INT
V
SWING
Where t
INT
is the integration time.
Using the values derived above (assuming 60 Hz
operation), the equation becomes:
3.4
Reference Capacitor (C
REF
)
EQUATION 3-2:
C
INT
= 16.7 msec x 13.3
μA
= 0.1
μA
2V
The reference capacitor stores the reference voltage
during several phases of the measurement cycle. Low
leakage is the primary selection criterion for this com-
ponent. The value must be high enough to offset the
effect of stray capacitance at the capacitor terminals. A
value of at least 1
μF
is recommended.
DS21459D-page 8
©
2006 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
