TC811CKW datasheet(9/14 Pages) MICROCHIP

Part #	TC811CKW

Description	3-1/2 Digit Analog-To-Digital Converter with Hold and Differential Reference Inputs

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Manufacturer	MICROCHIP [Microchip Technology]

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9

TC811

TC811-7 11/5/96

DS21472A

3-1/2 Digit Analog-To-Digital Converter with

Hold and Differential Reference Inputs

Figure 6b.

Figure 7. Common-Mode Voltage Removed in Battery Operation With V

–

IN = Analog Common

facing with sensors such as load cells and temperature

sensors. The TC811 is ideally suited to applications in hand-

held multimeters, panel meters, and portable instrumenta-

tion. The reference voltage can be generated anywhere

within the V+ to V– power supply range.

To prevent rollover type errors from being induced by

large common-mode voltages, CREF should be large com-

pared to stray node capacitance. A 0.1

µF capacitor is a

typical value.

The TC811 offers a significantly improved Analog Com-

mon temperature coefficient. This provides a very stable

voltage suitable for use as a voltage reference. The tem-

perature coefficient of Analog Common is typically

35ppm/

°C.

DEVICE PIN FUNCTIONAL DESCRIPTION

Differential Signal Inputs (VIN

+

(Pin 31),

VIN

– (Pin 30))

The TC811 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– +

1.5V. Common-mode voltages are removed from the sys-

tem when the TC811 operates from a battery or floating

power source (isolated from measured system) and VIN

– is

connected to Analog Common. (see Figure 8)

In systems where common-mode voltages exist, the

86dB common-mode rejection ratio minimizes error. Com-

mon-mode voltages do, however, affect the integrator out-

put level. A worse case condition exists if a large positive

VCM exists in conjunction with a full-scale negative differen-

tial signal. The negative signal drives the integrator output

positive along with VCM (Figure 8). For such applications the

integrator output swing can be reduced below the recom-

mended 2.0V full-scale swing. The integrator output will

swing within 0.3V of V+ or V– without increased linearity

error.

Reference (V

+

REF (Pin 36), V

–

REF (Pin 33))

Unlike the ICL7116, the TC811 has a differential refer-

ence as well as the “hold” function. The differential reference

inputs permit ratiometric measurements and simplify inter-

Oscillator

Full-Scale Voltage (VFS)

Freq. (kHz)

200mV

2.0V

RINT

CINT

RINT

CINT

32.768

180k

0.068

µF

1.8M

0.068

µF

40

150k

0.068

µF

1.5M

0.068

µF

VBUF

CAZ VINT

BP

POL

SEGMENT

DRIVE

OSC1

OSC2

V

–

V

+

VREF

+

VREF

–

ANALOG

COMMON

V

–

V

+

V

–

V

+

GND

MEASURED

SYSTEM

POWER

SOURCE

9V

LCD DISPLAY

TC811

+

V

–

V

+

470k

V +

20M

Ω

V +

20 pF

10pF

40kHZ

Figure 8. Common-Mode Voltage Reduces Available

Integrator Swing. (VCOM ≠ VIN)

RI

+

–

VIN

V

C I

INTEGRATOR

VI =

[

VCM

VIN

–

INPUT

BUFFER

CI =

=

RI

INTEGRATION CAPACITOR

INTEGRATION RESISTOR

4000

f

INTEGRATION TIME

TI ==

Where:

VI

CM

OSC

–

+

–

+

TI

RI CI

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