TC815 datasheet(22/26 Pages) MICROCHIP

22 / 26 page

TC815

DS21474B-page 22

For a 3.1V integrator swing, the value of R

ΩBUF is

easily calculated:

VINT

= Integrator swing = 3.1V

= Integration time = 100msec

= Integration capacitor = 0.1µF

RXMAX

=300

Ω

VX MAX = 700mV

EQUATION 11-3:

FIGURE 11-2:

Ω CALCULATION (200Ω

MANUAL OPERATION)

With a low battery voltage of 6.6V, analog common will

be approximately 3.6V above the negative supply ter-

minal. With the integrator swinging down from analog

common toward the negative supply, a 3.1V swing will

set the integrator output to 0.5V above the negative

supply.

11.4

Capacitors - CINT,CAZ and CREF

The integration capacitor, CINT, must have low dielec-

tric absorption. A 0.1µF polypropylene capacitor is sug-

gested. The auto-zero capacitor, CAZ, and reference

capacitor, CREF, should be selected for low leakage

and dielectric absorption. Polystyrene capacitors are

good choices.

11.5

Reference Voltage Adjustment

The TC815 contains a low temperature drift internal

voltage reference. The analog common potential

(Pin 29) is established by this reference. Maximum drift

is a low 75ppm/°C. Analog common is designed to be

approximately 2.6V below VCC (Pin 28). A resistive

divider (R18/R19, Functional Diagram) sets the TC815

reference input voltage (REFHI, Pin 34) to approxi-

mately 163.85mV.

With an input voltage near full scale on the 200mV

range, R19 is adjusted for the proper reading.

11.6

Display Hold Feature

The LCD will not be updated when HOLD (Pin 60) is

connected to GND (Pin 58). Conversions are made, but

the display is not updated. A HOLD mode LCD annun-

ciator is activated when HOLD is low.

The LCD HOLD annunciator is activated through the

triplex LCD driver signal at Pin 13.

11.7

Flat Package Socket

Sockets suitable for prototype work are available. A

USA source is:

• Nepenthe Distribution

2471 East Bayshore, Suite 520

Palo Alto, CA 94303

(415) 856-9332

TWX: 910-373-2060

“CBQ” Socket, Part No. IC51-064-042

11.8

Resistive Ladder Networks

Resistor attenuator networks for voltage and resistance

measurements are available from:

• Caddock Electronics

1717 Chicago Avenue

Riverside, CA 92507

Tel: (714) 788-1700

TWX: 910-332-6108

TABLE 11-2:

RESISTIVE LADDER

NETWORKS

ΩBUF =

VXMAX(TI)

CI(VINT)

= 200k

Ω

163.85

Ω

220

Ω

300

Ω

RS ≈ 600Ω

SW33

Analog Common = VCC – 3V

VCC = 9V

Attenuator

Accuracy

Attenuator Type

Caddock

Part Number

0.1%

Voltage

1776-C441

0.25%

Voltage

1776-C44

0.25%

Resistance

T1794-204-1

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