TC7107RCPL datasheet(16/34 Pages) MICROCHIP

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TC7106/A/TC7107/A

DS21455D-page 16

8.0

DEVICE PIN FUNCTIONAL

DESCRIPTION

8.1

Differential Signal Inputs

VIN+ (Pin 31), VIN- (Pin 30)

The

TC7106A/TC7107A

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

86 dB Common mode rejection ratio minimizes error.

Common mode voltages do, however, affect the

integrator output level. Integrator output saturation

must be prevented. A worst-case condition exists if a

large positive VCM exists in conjunction with a full scale

negative differential signal. The negative signal drives

the integrator 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 (V

COM ≠ VIN).

8.2

Differential Reference

VREF+ (Pin 36), 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

compared 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

reference. The temperature coefficient of analog

common is 20 ppm/°C typically.

8.3

Analog Common (Pin 32)

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

internally to the N channel FET capable of sinking

20 mA. 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

connected to analog common. If VIN- is not externally

connected to analog common, a Common mode

voltage exists. This is rejected by the converter’s 86 dB

Common 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-.

–

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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