Datasheet

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.
06527-049
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
06527-050
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
V
IN_SDA
AND V
IN_SDB
(V)
SUPPLY CURRENT (µA)
06527-051
V
SY
= 5V
V
SY
= 2.7V
Figure 50. Supply Current Shutdown Mode, AD8647