TC820CLW datasheet(17/22 Pages) TELCOM

17 / 22 page

3-165

TELCOM SEMICONDUCTOR, INC.

TC820

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

AND LOGIC PROBE

Figure 15. Suggested Crystal Oscillator Circuit

5pF

110k

Ω

10pF

75pF

TC820

Figure 14. DGND and COM Outputs

–

µA

TC820

LOGIC

SECTION

3.2V

VDD

VSS

COM

DGND

Figure 16. R-C Oscillator Circuit

Typical values are R = 10k

Ω and C = 68pF. The resistor

value should be

≥100kΩ. For accurate frequency measure-

ment, an R-C oscillator frequency of 40kHz is required.

System Timing

All system timing is derived from the clock oscillator. The

clock oscillator is divided by 2 prior to clocking the A/D

counters. The clock is also divided by 8 to drive the buzzer,

by 240 to generate the LCD backplane frequency, and by

40,000 for the frequency counter time base. A simplified

diagram of the system clock is shown in Figure 17.

Component Value Selection

Auto Zero Capacitor — CAZ

The value of the auto-zero capacitor (CAZ) has some

influence on system noise. A 0.47

µF capacitor is recom-

mended; a low dielectric absorption capacitor (Mylar) is

required.

Reference Voltage Capacitor — CREF

The reference voltage capacitor used to ramp the inte-

grator output voltage back to zero during the reference

integrate cycle is stored on CREF. A 0.1µF capacitor is

typical. A good quality, low leakage capacitor (such as

Mylar) should be used.

Integrating Capacitor — CINT

CINT should be selected to maximize integrator output

voltage swing without causing output saturation. Analog

common will normally supply the differential voltage

reference. For this case, a

±2V integrator output swing is

optimum when the analog input is near full scale. For 2.5

readings/second (fOSC = 40kHz) and VFS = 400mV, a 0.22µF

value is suggested. If a different oscillator frequency is used,

CINT must be changed in inverse proportion to maintain the

0.3

Clock Oscillator

The TC820 oscillator can be controlled with either a

crystal or with an inexpensive resistor-capacitor combina-

tion. The crystal circuit, shown in Figure 15, is recommended

when high accuracy is required in the frequency counter

mode. The 40kHz crystal is a standard frequency for ultra-

sonic alarms, and will provide a 1-second time base for the

counter or 2.5 analog-to-digital conversions per second.

Consult the crystal manufacturer for detailed applications

information.

Where low cost is important, the R-C circuit of Figure 16

can be used. The frequency of this circuit will be approxi-

mately:

fOSC =

TC820

40kHz

5 pF

10pF

22 M

Ω

470k

Ω

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