3-266

TELCOM SEMICONDUCTOR, INC.

AUTO-ZEROED MONOLITHIC

OPERATIONAL AMPLIFIERS

TC911A

TC911B

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 connected at pins 1 and

8. With the TC911, external offset voltage error cancel-

ing capacitors are not required. On the TC911 pins 1, 8

and 5 are not connected 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, assembly time, and total sys-

tem cost are reduced. Reliability is increased and PC board

layout eased by having the error storage capacitors inte-

grated 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 capaci-

tors, and chopper-stabilization capacitors can be removed

when retrofitting existing equipment designs.

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 thermocouple potentials can be controlled, sys-

tem performance 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 junctions are

kept at the same temperature, their summation will add to

zero-canceling errors (Figure 1).

Shielding precision analog circuits from air currents —

especially those caused by power dissipating components

and fans — will minimize temperature gradients and ther-

mocouple-induced errors.

Avoiding Latch-Up

Junction-isolated CMOS circuits inherently contain a

parasitic p-n-p-n transistor circuit. Voltages exceeding the

supplies by 0.3V should not be applied to the device pins.

Larger voltages can turn the p-n-p-n device on, causing

excessive device power supply current and excessive power

dissipation. TC911 power supplies should be established at

the same time or before input signals are applied. If this is not

possible input current should be limited to 0.1mA to avoid

triggering the p-n-p-n structure.

Overload Recovery

The TC911 recovers quickly from the output saturation.

Typical recovery time from positive output saturation is

20msec. Negative output saturation recovery time is typi-

cally 5msec.

Figure 1. Unwanted Thermocouple Errors Eliminated by

Reducing Thermal Gradients and Balancing Junctions

J3

J4

J5

J2

J1

J6

PACKAGE

PIN

J = J

J = J

J = J

3

2

1

4

5

6

NO TEMPERATURE DIFFERENTIAL

AND SAME METALLIC CONNECTION

+

–

–

+

+–

+–

+–

+–

V2

V1

V5

V6

V4

V3

J4

J3

J2

J1

J5

J6

V = 0

T

V = V + V + V – V – V – V = 0

T1

2

3

4

5

6