3-277

TELCOM SEMICONDUCTOR, INC.

7

6

5

4

3

1

2

8

Figure 4.

Noninverting Amplifier With Optional Clamp

Figure 3.

Internal Clamp Circuit

INTERNAL

POSITIVE CLAMP BIAS

V –V

V – 0.7V

++

T

P-CHANNEL

OUTPUT

CLAMP PIN

N-CHANNEL

INTERNAL

NEGATIVE CLAMP BIAS

V + V

–

T

V + 0.7V

–

TC7650

OUTPUT PIN

≈≈

≈

≈

TC7650

+

–

C

R

C

OUTPUT

INPUT

FOR FULL CLAMP EFFECT.

R2

R1

+ (

/

)

R2

R1

≥ 100k Ω

R3

0.1µF

CONNECT TO V

ON 8-PIN DIP.

*

SS

*

R

CLAMP

3

eliminates the overload recovery problem by reducing the

feedback network gain before the output voltage reaches

either supply rail.

The output clamp circuit is shown in Figure 3, with typical

inverting and noninverting circuit connections shown in

Figures 4 and 5. Output voltage versus clamp circuit current

characteristics are shown in the typical operating curves.

For the clamp to be fully effective, the impedance across the

clamp output should be greater than 100k

Ω.

Latch-Up Avoidance

Junction-isolated CMOS circuits inherently include a

parasitic 4-layer (p-n-p-n) structure which has characteris-

tics similar to an SCR. Under certain circumstances this

junction may be triggered into a low-impedance state, result-

ing in excessive supply current. To avoid this condition, no

voltage greater than 0.3V beyond the supply rails should be

applied to any pin. In general, the amplifier supplies must be

established either at the same time or before any input

signals are applied. If this is not possible, the drive circuits

must limit input current flow to under 0.1mA to avoid latch-

up.

Thermoelectric Potentials

Precision DC measurements are ultimately limited by

thermoelectric potentials developed in thermocouple junc-

tions of dissimilar metals, alloys, silicon, etc. Unless all

junctions are at the same temperature, thermoelectric volt-

ages, typically around 0.1

µV/°C, but up to tens of µV/°C for

some materials, will be generated. In order to realize the

benefits extremely-low offset voltages provide, it is essential

to take special precautions to avoid temperature gradients.

All components should be enclosed to eliminate air move-

ment, especially those caused by power-dissipating ele-

ments in the system. Low thermoelectric-coefficient con-

nections should be used where possible and power supply

voltages and power dissipation should be kept to a mini-

mum. High-impedance loads are preferable, and separation

from surrounding heat-dissipating elements is advised.

Pin Compatibility

On the 8-pin mini-DIP TC7650, the external null storage

capacitors are connected to pins 1 and 8. On most other

operational amplifiers these are left open or are used for

offset potentiometer or compensation capacitor connec-

tions.

For OP05 and OP07 operational amplifiers, the replace-

ment of the offset null potentiometer between pins 1 and 8

07 pin configurations for TC7650 operation. For LM108

devices, the compensation capacitor is replaced by the

external nulling capacitors. The LM101/748/709 pinouts

are modified similarly by removing any circuit connections to

pin 5. On the TC7650, pin 5 is the output clamp connection.

Figure 5. Inverting Amplifier with Optional Clamp

TC7650

CLAMP

+

–

C

R

C

R1

OUTPUT

0.1 F

µ

0.1 F

µ

INPUT

R2

FOR FULL CLAMP

EFFECT.

(

)

R2

R1

≥ 100kΩ

*

CONNECT TO V

ON 8-PIN DIP.

*

R

–

CHOPPER-STABILIZED

OPERATIONAL AMPLIFIER

TC7650