TC7107 datasheet(14/26 Pages) MICROCHIP

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TC7106/A/TC7107/A

DS21455B-page 14

Thedifferential reference canalsobe usedwhenadig-

ital zero reading is required when VIN is not equal to

zero. This is common in temperature measuring instru-

mentation. A compensating offset voltage can be

applied between analog common and VIN-. The trans-

ducer output is connected between VIN+ and analog

common.

The internal voltage reference potential available at

analog common will normally be used to supply the

converter's reference. This potential is stable when-

ever the supply potential is greater than approximately

7V. In applications where an externally generated ref-

erence voltage is desired, refer to Figure 7-1.

FIGURE 7-1:

EXTERNAL REFERENCE

8.0

DEVICE PIN FUNCTIONAL

DESCRIPTION

8.1

Differential Signal Inputs

VIN+(Pin31), VIN-(Pin 30)

The TC7106A/7017A is designed with true differential

inputs and accepts input signals within the input stage

common mode voltage range (VCM). The typical range

is V+ – 1.0 to V+ + 1V. Common mode voltages are

removed from the system when the TC7106A/

TC7107A operates from a battery or floating power

source (isolated from measured system) and VIN-is

connected to analog common (VCOM) (see Figure 8-2).

In systems where Common mode voltages exist, the

86dB Common mode rejection ratio minimizes error.

Common mode voltages do, however, affect the inte-

grator output level. Integrator output saturation must be

prevented. A worst case condition exists if a large pos-

itive VCM exists in conjunction with a full scale negative

differential signal. The negative signal drives the inte-

grator output positive along with VCM (see Figure 8-1).

For such applications the integrator output swing can

be reduced below the recommended 2.0V full scale

swing. The integrator output will swing within 0.3V of

V+ or V- without increasing linearity errors.

FIGURE 8-1:

COMMON MODE

VOLTAGE REDUCES

AVAILABLE INTEGRATOR

SWING (VCOM ≠ VIN)

8.2

Differential Reference

VREF+(Pin36), VREF-(Pin 35)

The reference voltage can be generated anywhere

within the V+ to V- power supply range.

To prevent rollover type errors being induced by large

Common mode voltages, CREF should be large com-

pared to stray node capacitance.

The TC7106A/TC7107A circuits have a significantly

lower analog common temperature coefficient. This

gives a very stable voltage suitable for use as a refer-

ence. The temperature coefficient of analog common is

20ppm/°C typically.

8.3

Analog Common(Pin32)

The analog common pin is set at a voltage potential

approximately 3.0V below V+. The potential is between

2.7V and 3.35V below V+. Analog common is tied inter-

nally to the N channel FET capable of sinking 20mA.

This FET will hold the common line at 3.0V should an

external load attempt to pull the common line toward

V+. Analog common source current is limited to 10

µA.

Analog common is, therefore, easily pulled to a more

negative voltage (i.e., below V+ – 3.0V).

The TC7106A connects the internal VIN+and VIN-

inputs to analog common during the auto-zero cycle.

During the reference integrate phase, VIN- is con-

nected to analog common. If VIN- is not externally con-

nected to analog common, a Common mode voltage

exists. This is rejected by the converter's 86dB Com-

mon mode rejection ratio. In battery operation, analog

common and VIN- are usually connected, removing

Common mode voltage concerns. In systems where V-

is connected to the power supply ground, or to a given

voltage, analog common should be connected to VIN-.

TC7106A

TC7107A

6.8V

Zener

V+

1.2V

Ref

Common

TC7106A

TC7107A

6.8k

Ω

20k

Ω

VREF+

VREF-

VREF+

VREF-

(a)(b)

V+

–

VIN

VCM

Integrator

VI =

[

VCM – VIN

Input Buffer

CI = Integration Capacitor

RI = Integration Resistor

4000

FOSC

TI = Integration Time =

Where:

–

RI CI

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