3-237

TELCOM SEMICONDUCTOR, INC.

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TC7129

4-1/2 DIGIT ANALOG-TO-DIGITAL

CONVERTER WITH ON-CHIP LCD DRIVERS

The charge stored in the integrating capacitor during

the integrate phase is directly proportional to the input

a value that gives the highest voltage swing while remain-

ing 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 from the

equation:

,

Using the values derived above (assuming 60 Hz

operation), the equation becomes:

= 0.1

µF.

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

30

→ 32

A reading between 10,000 and 9998 is acceptable;

anything lower indicates unacceptably high dielectric ab-

sorption.

The reference capacitor stores the reference voltage

during several phases of the measurement cycle. Low

leakage is the primary selection criterion for this component.

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.

A TC04 band-gap reference provides a high-stability

voltage reference of 1.25V. The reference potentiometer

voltage; any value above 20 k

Ω is adequate. The bias

150

For added stability, an RC input noise filter is usually

included in the circuit. The input filter resistor value should

not exceed 100 k

Ω. A typical RC time constant value is

16.7msec to help reject line-frequency noise. The input filter

capacitor should have low leakage for a high-impedance

input.

Battery

The typical circuit uses a 9V battery as a power source.

Any value between 6V and 12V can be used. For operation

from batteries with voltages lower than 6V and for operation

from power supplies, see "Powering the TC7129."

SPECIAL APPLICATIONS

The TC7129 as a Replacement Part

The TC7129 is a direct pin-for-pin replacement part for

the ICL7129. Note, however, that part requires a capacitor

and resistor between pins 26 and 28 for phase compensa-

tion. Since the TC7129 uses internal phase compensation,

these parts are not required and, in fact, must be removed

from the circuit for stable operation.

Powering the TC7129

While the most common power source for the TC7129

is a 9V battery, there are other possibilities. Some of the

more common ones are explained below.

16.7msec x 13.3

µA

2V