TC7129CPL datasheet(8/24 Pages) MICROCHIP

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TC7129

DS21459B-page 8

FIGURE 3-1:

STANDARD CIRCUIT

3.3

Integrating Capacitor (CINT)

The charge stored in the integrating capacitor during

the integrate phase is directly proportional to the input

voltage. The primary selection criterion for CINT 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 3-3:

Using the values derived above (assuming 60Hz

operation), the equation becomes:

EQUATION 3-4:

The capacitor should have low dielectric absorption to

ensure good integration linearity. Polypropylene and

Teflon capacitors are usually suitable. A good mea-

surement of the dielectric absorption is to connect the

reference capacitor across the inputs by connecting:

Pin to Pin:

→ 33 (CREF+toIN HI)

→ 32 (CREF-to IN LO)

A reading between 10,000 and 9998 is acceptable;

anything lower indicates unacceptably high dielectric

absorption.

3.4

Reference Capacitor (CREF)

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 ofatleast 1

µF is recommended.

Low Battery

Continuity

V+

5pF

120

kHz

10pF

0.1

µF

Ω

0.1

µF

100k

Ω

CINT

0.1

µF

V+

VIN

–

330k

Ω

Crystal

CO2

CRF

DREF

RREF

CIF

RIF

CREF+

µF

10k

Ω

RBIAS

150k

Ω

RINT

Display Drive Outputs

TC7129

CO1

CINT =

tINT xIINT

VSWING

Where tINT is the integration time.

CINT == 0.1µA

16.7msec x 13.3

µA

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