TC911B datasheet(4/14 Pages) MICROCHIP

4 / 14 page

TC911A/TC911B

DS21481C-page 4

2.0

PIN DESCRIPTIONS

The descriptions of the pins are listed in <Blue

References>Table 2-1.

TABLE 2-1:

PIN FUNCTION TABLE

3.0

DETAILED DESCRIPTION

3.1

Pin Compatibility

The CMOS TC911 is pin compatible with the industry

standard

ICL7650 chopper stabilized amplifier. The

ICL7650 must use external 0.1

μF capacitors con-

nected at pins 1 and 8. With the TC911, external off-

set voltage error canceling capacitors are not

required. On the TC911 pins 1, 8 and 5 are not con-

nected internally. The ICL7650 uses pin 5 as an

optional output clamp connection. External chopper

capacitors and clamp connections are not necessary

with the TC911. External circuits connected to pins 1, 8

and 5 will have no effect. The TC911 can be quickly

evaluated in existing ICL7650 designs. Since external

capacitors are not required, system part count, assem-

bly time and total system cost are reduced. Reliability

is increased and PC board layout eased by having the

error storage capacitors integrated on the TC911 chip.

The TC911 pinout matches many existing op amps:

741, LM101, LM108, OP05–OP08, OP-20, OP-21,

ICL7650 and ICL7652. In many applications operating

from +5V supplies, the TC911 offers superior electrical

performance and can be a functional pin compatible

replacement. Offset voltage correction potentiometers,

compensation capacitors, and chopper stabilization

capacitors can be removed when retro-fitting existing

equipment designs.

3.2

Thermocouple Errors

Heating one joint of a loop made from two different

metallic wires causes current flow. This is known as the

Seebeck effect. By breaking the loop, an open circuit

voltage (Seebeck voltage) can be measured. Junction

temperature and metal type determine the magnitude.

Typical values are 0.1

μV/°C to 10μV/°C. Thermal

induced voltages can be many times larger than the

TC911 offset voltage drift. Unless unwanted thermo-

couple potentials can be controlled, system perfor-

mance will be less than optimum.

Unwanted thermocouple junctions are created when

leads are soldered or sockets/connectors are used.

Low thermo-electric coefficient solder can reduce

errors. A 60% Sn/36% Pb solder has 1/10 the thermal

voltage of common 64% Sn/36% Pb solder at a copper

junction.

The number and type of dissimilar metallic junctions in

the input circuit loop should be balanced. If the junc-

tions are kept at the same temperature, their summa-

tion will add to zero-canceling errors (Figure 3-1).

Shielding precision analog circuits from air currents -

especially those caused by power dissipating compo-

nents and fans - will minimize temperature gradients

and thermocouple induced errors.

FIGURE 3-1:

UNWANTED THERMOCOUPLE ERRORS ELIMINATED BY REDUCING

THERMAL GRADIENTS AND BALANCING JUNCTIONS

Pin Number

Symbol

Description

1, 5, 8

No Internal Connection.

-INPUT

Inverting Input

+INPUT

Non-inverting Input

4VSS

Negative Power Supply

OUTPUT

Output

7VDD

Positive Power Supply

Package

J3 = J4

J2 = J5

J1 = J6

No Temperature Differential and same

Metallic Connection

–

+–

VT = 0

VT = V1 + V2 + V3 – V4 – V5 – V6 = 0

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