TC7106CLW datasheet(14/34 Pages) MICROCHIP

14 / 34 page

TC7106/A/TC7107/A

DS21455D-page 14

7.0

COMPONENT VALUE

SELECTION

7.1

Auto-Zero Capacitor (CAZ)

The CAZ capacitor size has some influence on system

noise. A 0.47 µF capacitor is recommended for 200 mV

full scale applications where 1LSB is 100 µV. A

0.047 µF capacitor is adequate for 2.0V full scale

applications. A mylar type dielectric capacitor is

adequate.

7.2

Reference Voltage Capacitor

(CREF)

The reference voltage used to ramp the integrator out-

put voltage back to zero during the reference integrate

cycle is stored on CREF. A 0.1 µF capacitor is

acceptable when VIN- is tied to analog common. If a

large Common mode voltage exists (VREF- – analog

common) and the application requires 200 mV full

scale, increase CREF to 1.0 µF. Rollover error will be

held to less than 1/2 count. A mylar dielectric capacitor

is adequate.

7.3

Integrating Capacitor (CINT)

CINT should be selected to maximize the integrator

output voltage swing without causing output saturation.

Due to the TC7106A/TC7107A superior temperature

coefficient specification, analog common will normally

supply the differential voltage reference. For this case,

a ±2V full scale integrator output swing is satisfactory.

For 3 readings/second (FOSC = 48 kHz), a 0.22 µF

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:

EQUATION 7-1:

CINT must have low dielectric absorption to minimize

rollover

error.

polypropylene

capacitor

recommended.

7.4

Integrating Resistor (RINT)

The input buffer amplifier and integrator are designed

with class A output stages. The output stage idling

current is 100 µA. The integrator and buffer can supply

20 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 200 mV full scale,

RINT is 47 kΩ. 2.0V full scale requires 470 kΩ.

TABLE 7-1:

COMPONENT VALUES AND

NOMINAL FULL SCALE

VOLTAGE

7.5

Oscillator Components

ROSC (Pin 40 to Pin 39) should be 100 kΩ. COSC is

selected using the equation:

EQUATION 7-2:

Note that FOSC is divided by four to generate the

TC7106A internal control clock. The backplane drive

signal is derived by dividing FOSC by 800.

To achieve maximum rejection of 60 Hz noise pickup,

the signal integrate period should be a multiple of

60 Hz. Oscillator frequencies of 240 kHz, 120 kHz,

80 kHz, 60 kHz, 48 kHz, 40 kHz, etc. should be

selected. For 50 Hz rejection, oscillator frequencies of

200 kHz, 100 kHz, 66-2/3 kHz, 50 kHz, 40 kHz, etc.

would be suitable. Note that 40 kHz (2.5 readings/

second) will reject both 50 Hz and 60 Hz.

7.6

Reference Voltage Selection

A full scale reading (2000 counts) requires the input

signal be twice the reference voltage.

Where:

FOSC

Clock Frequency at Pin 38

VFS

Full Scale Input Voltage

RINT

Integrating Resistor

VINT

Desired Full Scale Integrator Output

Swing

INT

4000

()

OSC

-------------

⎝⎠

⎛⎞ VFS

INT

-----------

⎝⎠

⎛⎞

INT

------------------------------------------------------

Component

Value

Nominal Full Scale Voltage

200.0 mV

2.000V

CAZ

0.47 µF

0.047 µF

RINT

47 k

470 k

CINT

0.22 µF

Note:

FOSC = 48 kHz (3 readings per sec).

Required Full Scale Voltage*

VREF

200.0 mV

100.0 mV

2.000V

1.000V

*VFS = 2VREF

OSC

0.45

----------

Where:

FOSC

=48 kHz

COSC

100 pF

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