TC818 datasheet(20/24 Pages) MICROCHIP

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TC818A

DS21475B-page 20

FIGURE 3-15:

Ω CALCULATION (200Ω

MANUAL OPERATION)

Where:

VANCOM = Potential at Analog Common ≈ 2.7V

= 220

Ω

= 163.85

Ω

= 300

Ω

= Internal switch 33 resistance

≈ 600Ω

EQUATION 3-2:

For a 3.1V integrator swing, the value of R

ΩBUF is

easily calculated:

Where:

VINT

= Integrator swing = 3.1V

= Integration time = 100msec

= Integration capacitor = 0.1µF

RXMAX

=300

Ω

VXMAX

= 700mV

EQUATION 3-3:

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

be approximately 3.6V above the negative supply termi-

nal. With the integrator swinging down from analog com-

mon toward the negative supply, a 3.1V swing will set

the integrator output to 0.5V above the negative supply.

3.21

Capacitors - CINT,CAZ and CREF

The integration capacitor, CINT, must have low dielec-

tric absorption. A 0.1

µF 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.

3.22

Reference Voltage Adjustment

The TC818A 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

TC818A reference input voltage (REFHI, Pin 34) to

approximately 163.85mV.

With an input voltage near full scale on the 200mV

range, R19 is adjusted for the proper reading.

3.23

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.

3.24

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

3.25

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 3-6:

RESISTIVE LADDER

NETWORKS

163.85

Ω

220

Ω

300

Ω

RS ≈ 600Ω

SW33

Analog Common = VCC – 3V

VCC = 9V

ΩBUF =

(VCC –VANCOM)RX

(RX +RS +R1 +R8)

=0.63V

ΩBUF =

VXMAX(TI)

CI(VINT)

= 200k

Ω

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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