5.0 Printed-Circuit-Board Layout

The designer should be aware that when it is inappropriate

to lay out a PC board for the sake of just a few circuits, there

is another technique which is even better than a guard ring

on a PC board: Don’t insert the amplifier’s input pin into the

board at all, but bend it up in the air and use only air as an in-

sulator. Air is an excellent insulator. In this case you may

have to forego some of the advantages of PC board con-

struction, but the advantages are sometimes well worth the

effort of using point-to-point up-in-the-air wiring. See

Figure

13.

6.0 Powerdown

6.1 PINOUT FOR THE LMC6681/LMC6682/LMC6684

For the LMC6681/2/4, the input, output, and power pins are

the same as those used in the standard configuration. One

of the other pins, pin 5 in the case of the LMC6681, is used

to enable the powerdown mode. The connection diagrams

for the LMC6681/2/4 are on the front page of the datasheet.

The LMC6684 has 2 powerdown options. Each of the power-

down pins disables two amplifiers. If both the powerdown

pins are pulled high, all four amplifiers will be disabled. Re-

ferring to the connection diagrams on the front page of the

datasheet, Pin 5 disables amplifiers B and C and Pin 13 dis-

ables amplifiers A and D.

6.2 EXPLANATION OF DATASHEET PARAMETERS

The LMC6681/2/4 is ON (meets all the datasheet specs)

when the voltage applied to the powerdown pin, V

logic low. The device is OFF when V

logic levels are indicated in the test conditions in the

datasheet tables. Summarizing these numbers:

Supply

Logic High [V]

Logic Low [V]

Voltage

at room

over temp

at room

over temp

2.2V

3V

5V

10V

In applications where the powerdown pin is not connected

externally, it is pulled to a logic low internally through a cur-

rent source. The t

same for a V

LMC6681/2/4 will typically be fully operational 50 µs after a

logic low has been applied to the powerdown pin. Please

note that the frequency of V

5 Hz.

DS012042-19

Inverting Amplifier

DS012042-20

Non-Inverting Amplifier

DS012042-21

Follower

FIGURE 12. Typical Connections of Guard Rings

DS012042-22

(Input pins are lifted out of PC board and soldered directly to components.

All other pins are connected to PC board.)

FIGURE 13. Air Wiring

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