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TC811

3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS

TelCom Semiconductor Inc => Microchip 

3-1/2 DIGIT A/D CONVERTER WITH HOLD AND

DIFFERENTIAL REFERENCE INPUTS

TC811

COMPONENT VALUE SELECTION

Auto Zero Capacitor - CAZ

The value of the Auto Zero capacitor (CAZ) has some

influence on system noise. A 0.47µF capacitor is recom-

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

100µV. A 0.10µF capacitor should be used for 2.0V full-

scale applications. A capacitor with low dielectric absorption

(Mylar) is required.

Reference Voltage Capacitor -CREF

The reference voltage used to ramp the integrator

output voltage back to zero during the reference integrate

cycle is stored on CREF. A 0.1µF capacitor is typical. If the

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

crease CREF to 1.0µF. Rollover error will be held to less than

1/2 count. 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 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/

second (fOSC = 32kHz or 40kHz) and VFS = 200mV, a .068µF

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

CINT must be changed in inverse proportion to maintain the

nominal ±2V integrator swing. An exact expression for CINT

is :

CINT =

4000 VFS

VINT RINT fOSC

where:

fOSC = Clock frequency at Pin 39

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 which have idling currents of 6µA.

The integrator and buffer can supply 1µA 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 200mV

full-scale, RINT should be about 180kΩ. A 2.0V full-scale

requires abut 1.8MΩ.

3-144

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

standard frequencies. Note that fOSC is divided by four to

generate the TC811 internal control clock. The backplane

drive signal is derived by dividing fOSC by 800.

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.

Reference Voltage (VREF)

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

transducer output is 400mV for 2000lb/in2. Rather than

dividing the input voltage by two, the reference voltage

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

be used directly.

TC811

OSC1

40

22MΩ

40.0 kHz

OSC2 V+

39

38

470k

10pF

20pF

9V

+

Figure 6a. TC811 Oscillator

TELCOM SEMICONDUCTOR, INC.

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