TC7117ACPL datasheet(8/22 Pages) MICROCHIP

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TC7116/A/TC7117/A

DS21457B-page 8

FIGURE 3-1:

ANALOG SECTION OF TC7116/TC7117A AND TC7117/TC7117A

3.1.3

REFERENCE INTEGRATE PHASE

The final phase is reference integrate, or de-integrate.

Input low is internally connected to analog common

and input high is connected across the previously

charged reference capacitor. Circuitry within the chip

ensures that the capacitor will be connected with the

correct polarity to cause the integrator output to return

to zero. The time required for the output to return to

zero is proportional to the input signal. The digital

reading displayed is:

EQUATION 3-1:

3.1.4

REFERENCE

The positive reference voltage (VREF+) is referred to

analog common.

3.1.5

DIFFERENTIAL INPUT

This input can accept differential voltages anywhere

within the Common mode range of the input amplifier

or, specifically, from 1V below the positive supply to 1V

above the negative supply. In this range, the system

has a CMRR of 86dB, typical. However, since the inte-

grator also swings with the Common mode voltage,

care must be exercised to ensure that the integrator

output does not saturate. A worst case condition would

be a large, positive Common mode voltage with a near

full scale negative differential input voltage. The nega-

tive input signal drives the integrator positive, when

most of its swing has been used up by the positive

Common mode voltage. For these critical applications,

the integrator swing can be reduced to less than the

recommended 2V full scale swing with little loss of

accuracy. The integrator output can swing within 0.3V

of either supply without loss of linearity.

3.1.6

ANALOG COMMON

This pin is included primarily to set the Common mode

voltage for battery operation (TC7116/TC7116A), or for

any system where the input signals are floating, with

respect to the power supply. The analog common pin

sets a voltage approximately 2.8V more negative than

the positive supply. This is selected to give a minimum

end of life battery voltage of about 6V. However, analog

common has some attributes of a reference voltage.

When the total supply voltage is large enough to cause

the zener to regulate (>7V), the analog common volt-

age will have a low voltage coefficient (0.001%), low

output impedance (

≅15Ω), and a temperature coeffi-

cient of less than 20ppm/°C, typically, and 50 ppm max-

imum. The TC7116/TC7117 temperature coefficients

are typically 80ppm/°C.

An external reference may be used, if necessary, as

shownin Figure3-2.

FIGURE 3-2:

USING AN EXTERNAL

REFERENCE

TC7116

TC7116A

TC7117

TC7117A

CREF

CREF+

CREF-

RINT

V+

CAZ

Auto-Zero

VINT

µA

INT

AZ & DE (±)

INT

Integrator

V+ -3V

Comparator

Digital

Section

DE (+)

(–)

(+)

DE (–)

V+

Analog

Common

VIN+

VIN-

VBUFF

CINT

VREF+

Low

Temp.

Drift

Zener

VREF

V-

–

1000 =

VIN

VREF

V+

1.2V REF

COMMON

TC7116

TC7116A

TC7117

TC7117A

6.8k

Ω

VREF+

20k

Ω

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