3

TC901

Dual Auto-Zeroed Operational Amplifiers

TC901- 7 9/5/96

© 2001 Microchip Technology Inc.

DS21480A

Figure 7. Unwanted Thermocouple Errors Eliminated by Reducing

Thermal Gradients and Balancing Junctions

Avoiding Latch-Up

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

excessive device power supply current and excessive power

dissipation. TC901's power supply should be established at

the same time (or before) input signals are applied. If this is

not possible, input current should be limited to 100

µA to

avoid triggering the p-n-p-n structure.

Pin Compatibility

The CMOS TC901 is pin compatible with other chopper-

stabilized amplifiers, such as the 7650, 7652 and 1052.

Amplifiers such as the 7650 require 0.1

µF external capaci-

tors connected to pins 1 and 8. The TC901 includes the

chopper capacitors on-chip, so external capacitors are not

required. Since pins 1, 5 and 8 of the TC901 are not

connected, the TC901 can directly replace other chopper-

stabilized amplifiers in existing circuits.

The TC901 pinout also matches many popular bipolar

and JFET op amps, such as the OP-07, OP-20, LM101,

LM108, 356 and 741. In many applications that operate from

±15V power supplies, the TC901 offers superior electrical

performance and is a functional pin-compatible replace-

ment. Offset voltage correction potentiometers, compensa-

tion capacitors, and chopper-stabilization capacitors can be

removed when retrofitting existing equipment designs.

System parts count, assembly time, and system cost are

reduced, while reliability and performance are improved.

Overload Recovery

The TSC90X Series recovers quickly from output

saturation. Typical recovery time from positive output satu-

ration is 20msec. Negative output saturation recovery time

is typically 5msec.

Overload Recovery

The TC901 recovers quickly from output saturation.

Typical recovery time from positive output saturation is 20

msec. Negative output saturation recovery time is typically

msec.

Thermocouple Errors

Heating a junction made from two different metallic

wires causes current flow. This is known as the Seebeck

effect. An open-circuit voltage (Seebeck voltage) can be

measured across the junction. 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 TC901's offset voltage drift.

Unless unwanted thermocouple potentials can be con-

trolled, system performance will be less than optimum.

Unwanted thermocouple junctions are created when

leads are soldered or sockets/connectors are used. Low

thermoelectric coefficient solder can reduce errors. A 60%

Cd/40% Sn Pb solder has one-tenth 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 7).

Shielding precision analog circuits from air currents —

especially those caused by power dissipating components

and fans — will minimize temperature gradients and mini-

mize thermocouple-induced errors.

J3

J4

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

J5

J6

V = 0

T

V = V + V + V – V – V – V = 0

T1

2

3

4

5

6