TC820CPL datasheet(18/22 Pages) TELCOM

18 / 22 page

3-166

TELCOM SEMICONDUCTOR, INC.

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

AND LOGIC PROBE

TC820

Figure 17. System Clock Generation

nominal

±2V integrator swing. An exact expression for CINT

is:

CINT =

where: fOSC = Clock frequency

VFS = Full-scale input voltage

RINT = Integrating resistor

VINT = Desired full-scale integrator output swing

CINT must have low dielectric absorption to minimize

roll-over error. A polypropylene capacitor is recommended.

Integrating Resistor — RINT

The input buffer amplifier and integrator are designed

with class A output stages. The integrator and buffer can

supply 40

µA drive currents with negligible linearity errors.

RINT is chosen to remain in the output stage linear drive

region but not so large that printed circuit board leakage

currents induce errors. For a 400mV full scale, RINT should

be about 100k

Ω.

Reference Voltage Selection

A full-scale reading (4000 counts for TC820) requires

the input signal be twice the reference voltage.

Table III. Reference Voltage Selection

Full-Scale

Input Voltage

(VFS) (Note 1)

VREF

Resolution

200mV

Note 2

–

400mV

200mV

µV

500mV

250

µV

500

µV

(Notes 3, 4)

NOTES: 1. TC820 in A/D converter mode, RANGE/FREQ = logic low.

2. Not recommended.

3. VFS > 2V may exceed the input common mode range.

See "10:1 Range Change" section.

4. Full-scale voltage values are not limited to the values

shown. For example, TC820 VFS can be any value from

400mV to 2V.

In some applications, a scale factor other than unity may

exist between a transducer output voltage and the required

digital reading. Assume, for example, that a pressure trans-

ducer output is 800mV for 4000 lb/in2. Rather than dividing

the input voltage by two, the reference voltage should be set

to 400mV. This permits the transducer input to be used

directly.

The internal voltage reference potential available at

analog common will normally be used to supply the

converter's reference voltage. This potential is stable when-

ever the supply potential is greater than approximately 7V.

The low-battery detection circuit and analog common oper-

ate from the same internal reference. This ensures that the

low-battery annunciator will turn on at the time the internal

reference begins to lose regulation.

The TC820 can also operate with an external reference.

Figure 18 shows internal and external reference applica-

tions.

Ratiometric Resistance Measurements

The TC820 true differential input and differential refer-

ence make ratiometric readings possible. In ratiometric

operation, an unknown resistance is measured with respect

to a known standard resistance. No accurately defined

reference voltage is needed.

The unknown resistance is put in series with a known

standard and a current is passed through the pair (Fig-

ure 19). The voltage developed across the unknown is

applied to the input and voltages across the known resistor

A/D

COUNTER

TC820

÷40,000

COUNTER

TIME BASE

÷240

LCD

BACKPLANE

DRIVER

÷8

BUZZER

÷2

OSC1

OSC2

OSC3

XTAL

OSCILLATOR

COMPONENTS

R/C

OSCILLATOR

COMPONENTS

4000 VFS

VINT RINT fOSC

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