8

TC811

© 2001 Microchip Technology Inc.

DS21472A

3-1/2 Digit Analog-To-Digital Converter with

Hold and Differential Reference Inputs

TC811-7 11/5/96

COMPONENT VALUE SELECTION

influence on system noise. A 0.47

µF capacitor is recom-

mended for 200mV full-scale applications where 1LSB is

100

µV. A 0.10µF capacitor should be used for 2.0V full-

scale applications. A capacitor with low dielectric absorption

(Mylar) is required.

The reference voltage used to ramp the integrator

output voltage back to zero during the reference integrate

application requires a sensitivity of 200mV full-scale, in-

1/2 count. A good quality, low leakage capacitor, such as

Mylar, should be used.

CINT should be selected to maximize integrator output

voltage swing without causing output saturation. Analog

common will normally supply the differential voltage refer-

ence. For this case a

±2V integrator output swing is optimum

when the analog input is near full-scale. For 2 or 2.5 reading/

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

nominal

is :

where:

roll-over error. A polypropylene capacitor is recommended.

The input buffer amplifier and integrator are designed

with class A output stages which have idling currents of 6

µA.

The integrator and buffer can supply 1

µA drive currents with

output stage linear drive region but not so large that printed

circuit board leakage currents induce errors. For a 200mV

requires abut 1.8M

Ω.

Oscillator Components

The internal oscillator has been designed to operate

with a quartz crystal, such as the Statek CX-1V series. Such

crystals are very small and are available in a variety of

generate the TC811 internal control clock. The backplane

To achieve maximum rejection of ac-line noise pickup,

a 40kHz crystal should be used. This frequency will yield an

integration period of 100msec and will reject both 50Hz and

60Hz noise. For prototyping or cost-sensitive applications a

32.768kHz watch crystal can be used, and will produce

about 25dB of line-noise rejection. Other crystal frequen-

cies, from 16kHz to 48kHz, can also be used.

Pins 39 and 40 make up the oscillator section of the

TC811. Figures 6a and 6b show some typical conversion

rate component values.

The LCD backplane frequency is derived by dividing the

oscillator frequency by 800. Capacitive loading of the LCD

may compromise display performance if the oscillator is run

much over 48kHz.

A full-scale reading (2000 counts) requires the input

signal be twice the reference voltage.

In some applications a scale factor other than unity may

exist, such as between a transducer output voltage and the

required digital reading. Assume, for example, a pressure

dividing the input voltage by two, the reference voltage

should be set to 200mV. This permits the transducer input to

be used directly.

9V

+

470k

22M

Ω

40.0 kHz

10pF

20pF

OSC2

V+

38

39

40

OSC1

TC811

Figure 6a. TC811 Oscillator