3-278

TELCOM SEMICONDUCTOR, INC.

TC7650

CHOPPER-STABILIZED

OPERATIONAL AMPLIFIER

Figure 6.

Input Guard Connection

INPUT

OUTPUT

R2

Inverting Amplifier

INPUT

OUTPUT

Follower

INPUT

OUTPUT

R2

R1

Noninverting Amplifier

R3*

R3*

SHOULD BE LOW

IMPEDANCE FOR

OPTIMUM GUARDING.

NOTE: R

=

3

R1 R2

R1 R2

+

R3*

R1

Other operational amplifiers may use this pin as an offset

or compensation point.

The minor modifications needed to retrofit a TC7650

into existing sockets operating at reduced power supply

voltages make prototyping and circuit verification straight-

forward.

Input Guarding

High impedance, low leakage CMOS inputs allow the

TC7650 to make measurements of high-impedance sources.

Stray leakage paths can increase input currents and de-

crease input resistance unless inputs are guarded. A guard

is a conductive PC trace surrounding the input terminals.

The ring connects to a low-impedance point at the same

potential as the inputs. Stray leakages are absorbed by the

low-impedance ring. The equal potential between ring and

inputs prevents input leakage currents. Typical guard con-

nections are shown in Figure 6.

The 14-pin DIP configuration has been specifically

designed to ease input guarding. The pins adjacent to the

inputs are unused.

In applications requiring low leakage currents, boards

should be cleaned thoroughly and blown dry after soldering.

Protective coatings will prevent future board contamination.

Component Selection

values, depending on the clock or chopping frequency. For

the preset internal clock, the correct value is 0.1

µF. To

maintain the same relationship between the chopping fre-

quency and the nulling time constant, the capacitor values

should be scaled in proportion to the external clock, if used.

High-quality film-type capacitors (such as Mylar) are pre-

ferred; ceramic or other lower-grade capacitors may be

suitable in some applications. For fast settling on initial turn-

on, low dielectric absorption capacitors (such as polypro-

pylene) should be used. With ceramic capacitors, several

seconds may be required to settle to 1

µV.