ICL7129_00 Datasheet, PDF (8/10 Pages) Intersil Corporation – 4½ Digit LCD, Single-Chip A/D Converter

ICL7129
BP1
BP2
BP3
ON SEG.
FIGURE 9. TYPICAL BACKPLANE AND ANNUNCIATOR
DRIVE WAVEFORM
LOW BATTERY CONTINUITY
ANNUNCIATOR
µ
m
K
M
BACKPLANE
ANNUNCIATOR
AMPS
VOLTS
Ω
BACKPLANE
FIGURE 10. MULTIMETER EXAMPLE SHOWING USE OF
ANNUNCIATOR DRIVE OUTPUT
Display Temperature Compensation
For most applications an adequate display can be obtained by
connecting V_DlSP (pin 19) to DGND (pin 36). In applications
where a wide temperature range is encountered, the voltage
drive levels for some triplexed liquid crystal displays may need
to vary with temperature in order to maintain good display
contrast and viewing angle. The amount of temperature
compensation will depend upon the type of liquid crystal used.
Display manufacturers can supply the temperature compen-
sation requirements for their displays. Figure 11 shows two
circuits that can be adjusted to give a temperature compensa-
tion of ≈ +10mV/^oC between V+ and V_DISP . The diode
between DGND and V_DISP should have a low turn-on voltage
to assure that no forward current is injected into the chip if
V_DISP is more negative than DGND.
Component Selection
There are only three passive components around the
ICL7129 that need special consideration in selection. They
are the reference capacitor, integrator resistor, and integra-
tor capacitor. There is no auto-zero capacitor like that found
in earlier integrating A/D converter designs.
The integrating resistor is selected to be high enough to
assure good current Iinearity from the buffer amplifier and
integrator and low enough that PC board leakage is not a
problem. A value of 150kΩ should be optimum for most appli-
cations. The integrator capacitor is selected to give an opti-
mum integrator swing at full-scale. A large integrator swing
will reduce the effect of noise sources in the comparator but
will affect rollover error if the swing gets too close to the posi-
tive rail (≈0.7V). This gives an optimum swing of ≈2.5V at full-
scale. For a 150kΩ integrating resistor and 2 conversions per
second the value is 0.1µF. For different conversion rates, the
value will change in inverse proportion. A second requirement
for good linearity is that the capacitor have low dielectric
absorption. Polypropylene caps give good performance at a
reasonable price. Finally the foil side of the cap should be
connected to the integrator output to shield against pickup.
The only requirement for the reference cap is that it be low
leakage. In order to reduce the effects of stray capacitance,
a 1µF value is recommended.
Clock Oscillator
The ICL7129 achieves its digital range changing by integrat-
ing the input signal for 1000 clock pulses (2,000 oscillator
cycles) on the 2V scale and 10,000 clock pulses on the
200mV scale. To achieve complete rejection of 60Hz on
both scales, an oscillator frequency of 120kHz is required,
giving two conversions per second.
V+
V+
1N4148
5K
75K
39K
200K
24
ICL7611
-
+
19
V_DISP
ICL7129
36
DGND
39K
24
20K
2N2222
19
V_DISP
ICL7129
36
DGND
18K
23
23
V-
V-
FIGURE 11. TWO METHODS FOR TEMPERATURE COMPENSATING THE LIQUID CRYSTAL DISPLAY
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