 2001-2012 Microchip Technology Inc.

DS21463C-page 7

TC7650

3.9

Pin Compatibility

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

age 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 capaci-

tor connections.

For OP05 and OP07 operational amplifiers, the

replacement of the offset null potentiometer between

convert the OP05/07 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.

Other operational amplifiers may use this pin as an off-

set or compensation point.

The minor modifications needed to retrofit a TC7650

into existing sockets operating at reduced power sup-

ply voltages make prototyping and circuit verification

straightforward.

3.10

Input Guarding

High impedance, low leakage CMOS inputs allow the

TC7650 to make measurements of high-impedance

sources. Stray leakage paths can increase input cur-

rents and decrease input resistance unless inputs are

guarded. A guard is a conductive PC trace surrounding

the input terminals. The ring connects to a low imped-

ance 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 connections are shown

in Figure 3-6.

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 sol-

dering. Protective coatings will prevent future board

contamination.

3.11

Component Selection

values, depending on the clock or chopping frequency.

For the preset internal clock, the correct value is 0.1

F.

To maintain the same relationship between the chop-

ping frequency 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 preferred; ceramic or other lower

grade capacitors may be suitable in some applications.

For fast settling on initial turn-on, low dielectric absorp-

tion capacitors (such as polypropylene) should be

used. With ceramic capacitors, several seconds may

be required to settle to 1

V.

FIGURE 3-6:

INPUT GUARD CONNECTION

Input

Output

Inverting Amplifier

Input

Output

Follower

Input

Output

Noninverting Amplifier

Should Be Low

Impedence For

Optimum Guarding