3-38

Display Temperature Compensation

For most applications an adequate display can be obtained by

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

to assure that no forward current is injected into the chip if

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 integrator

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 150k

Ω 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 positive

rail (

≈0.7V). This gives an optimum swing of ≈2.5V at full-

scale. For a 150k

Ω integrating resistor and 2 conversions per

second the value is 0.1

µF. 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 1

µF 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.

ON SEG.

BP1

BP2

BP3

FIGURE 9. TYPICAL BACKPLANE AND ANNUNCIATOR

DRIVE WAVEFORM

FIGURE 10. MULTIMETER EXAMPLE SHOWING USE OF

ANNUNCIATOR DRIVE OUTPUT

LOW BATTERY CONTINUITY

µ

m

K

M

BACKPLANE

ANNUNCIATOR

AMPS

VOLTS

Ω

BACKPLANE

ANNUNCIATOR

ICL7129

DGND

+

-

19

36

23

24

1N4148

5K

75K

39K

200K

ICL7611

V+

V-

ICL7129

DGND

19

36

23

24

V+

V-

18K

39K

20K

2N2222

FIGURE 11. TWO METHODS FOR TEMPERATURE COMPENSATING THE LIQUID CRYSTAL DISPLAY

ICL7129