AD8646/AD8647/AD8648

Rev. D | Page 15 of 20

THEORY OF OPERATION

POWER-DOWN OPERATION

The shutdown function of the AD8647 is referenced to the

negative supply voltage of the operational amplifier. A logic

level high (> 2.0 V) enables the device, while a logic level low

(< 0.8 V) disables the device and places the output in a high

impedance condition. Several outputs can be wire-OR’ed, thus

eliminating a multiplexer. The logic input is a high impedance

CMOS input. If dual or split supplies are used, the logic signals

must be properly referred to the negative supply voltage.

MULTIPLEXING OPERATION

Because each op amp has a separate logic input enable pin, the

outputs can be connected together if it can be guaranteed that

only one op amp is active at any time. By connecting the op amps

as shown in Figure 48, a multiplexer can be eliminated. With

the reasonably short turn-on and turn-off times, low frequency

signal paths can be smoothly selected. The turn-off time is

slightly faster than the turn-on time so, even when using

sections from two different packages, the overlap is less than

300 nanoseconds.

1/2

AD8647

1/2

AD8647

9

1

1

10

5V

6

7

8

2

3

4

5

2

13kHz

5kHz

2kHz

Figure 48. AD8647 Output Switching

TIME (200µs/DIV)

0V

5V

0V

2V

1V

Figure 49. Switching Waveforms

80

70

60

50

40

30

20

10

0

–10

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Figure 50. Supply Current Shutdown Mode, AD8647