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TC811

3-1/2 DIGIT A/D CONVERTER WITH HOLD AND DIFFERENTIAL REFERENCE INPUTS

TelCom Semiconductor Inc => Microchip 

3-1/2 DIGIT A/D CONVERTER WITH HOLD AND

DIFFERENTIAL REFERENCE INPUTS

1

TC811

Oscillator

Freq. (kHz)

32.768

40

Full-Scale Voltage (VFS)

200mV

2.0V

RINT CINT RINT CINT

180k 0.068µF

150k 0.068µF

Figure 6b.

1.8M

1.5M

0.068µF

0.068µF

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-

2 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 0.1µF capacitor is a

3 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.

INPUT

CI

BUFFER

+

+

RI

–

VIN

–

+

4

VI

INTEGRATOR

–

VCM

[ VI

Where:

=

TI

RI CI

VCM – VIN

TI

=

INTEGRATION TIME =

4000

fOSC

5

CI = INTEGRATION CAPACITOR

RI = INTEGRATION RESISTOR

Figure 8. Common-Mode Voltage Reduces Available

Integrator Swing. (VCOM ≠ VIN)

6

SEGMENT

DRIVE

LCD DISPLAY

MEASURED

SYSTEM

V+

V–

GND

V+ V– GND

POWER

SOURCE

VBUF

V+

CAZ VINT

POL BP

OSC1

10pF

V+

V–

TC811

ANALOG

COMMON

VRE– F VR+ EF

V+

20MΩ

40kHZ

OV –SC2

20 pF

V+

470k

+

9V

Figure 7. Common-Mode Voltage Removed in Battery Operation With V–IN = Analog Common

TELCOM SEMICONDUCTOR, INC.

7

8

3-145

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