TC7126ACKW datasheet(11/22 Pages) MICROCHIP

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DS21458B-page 11

TC7126/A

5.1

System Timing

The oscillator frequency is divided by four prior to

clocking the internal decade counters. The four-phase

measurement cycle takes a total of 4000 counts

(16,000 clock pulses). The 4000-count cycle is inde-

pendent of input signal magnitude.

Each phase of the measurement cycle has the following

length:

Auto-Zero Phase: 1000 to 3000 counts

(4000 to 12,000 clock pulses).

For signals less than full scale, the auto-zero

phase is assigned the unused reference integrate

time period.

Signal Integrate: 1000 counts

(4000 clock pulses).

This time period is fixed. The integration period is:

EQUATION 5-1:

Reference Integrate: 0 to 2000 counts

(0 to 8000 clock pulses).

The TC7126A is a drop-in replacement for the TC7126

and ICL7126, which offer a greatly improved internal

reference temperature coefficient. No external compo-

nent value changes are required to upgrade existing

designs.

6.0

COMPONENT VALUE

SELECTION

6.1

Auto-Zero Capacitor (CAZ)

The CAZ capacitor size has some influence on system

noise. A 0.47

µF capacitor is recommended for 200mV

full scale applications where 1LSB is 100

µV. A 0.033µF

capacitor is adequate for 2.0V full scale applications. A

mylar type dielectric capacitor is adequate.

6.2

Reference Voltage Capacitor (CREF)

The reference voltage, used to ramp the integrator out-

put voltage back to zero during the reference integrate

phase, is stored on CREF.A 0.1µF capacitor is accept-

able when VREF- is tied to analog common. If a large

Common mode voltage exists (VREF- – analog com-

mon) and the application requires a 200mV full scale,

increase CREF to 1µF. Rollover error will be held to less

than 0.5 count. A Mylar type dielectric capacitor is

adequate.

6.3

Integrating Capacitor (CINT)

CINT should be selected to maximize integrator output

voltage swing without causing output saturation. Due to

the TC7126A's superior analog common 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 per second (FOSC = 48kHz), a 0.047µF

value is suggested. For 1 reading per second, 0.15

µF

is recommended. 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 6-1:

At 3 readings per second, a 750

Ω resistor should be

placed in series with CINT. This increases accuracy by

compensating for comparator delay. CINT must have

low dielectric absorption to minimize rollover error. A

polypropylene capacitor is recommended.

6.4

Integrating Resistor (RINT)

The input buffer amplifier and integrator are designed

with Class A output stages. The output stage idling cur-

rent is 6µA. The integrator and buffer can supply 1µA

drive current with negligible linearity errors. RINT is cho-

sen to remain in the output stage linear drive region, but

not so large that PC board leakage currents induce

errors. For a 200mV full scale, RINT is 180kΩ.A 2V full

scale requires 1.8M

Ω.

Note:

FOSC = 48kHz (3 readings per sec).

TSI = 4000

FOSC

Where: FOSC is the externally set clock frequency.

Component

Value

Nominal Full Scale Voltage

200mV

CAZ

0.33

µF

0.033

µF

RINT

180k

Ω

1.8M

Ω

CINT

0.047

µF

0.047

µF

CINT =

(4000)

FOSC

VFS

RINT

















VINT

Where:

FOSC = Clock frequency at Pin 38

VFS = Full scale input voltage

RINT = Integrating resistor

VINT = Desired full scale integrator output swing

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