AD822
1V
100
90
2µs
100
90
1V
2µs
10
GND
10
0%
1V
GND
0%
a
1V
100
1V
10µs
+V
S
90
Figure 37. V
S
= +5 V, 0 V; Gain of Two Inverter Response
to 2.5 V Step Centered –1.25 V Below Ground, R
L
= 10 k
Ω
10
GND
0%
1V
500mV
100
90
10µs
b.
+5V
R
P
V
IN
V
OUT
10
GND
0%
Figure 39. (a) Response with R
P
= 0; V
IN
from 0 to +V
S
(b) V
IN
= 0 to +V
S
+ 200 mV
V
OUT
= 0 to +V
S
R
P
= 49.9 k
Ω
Figure 38. V
S
= 3 V, 0 V; Gain of Two Inverter, V
IN
= 1.25 V,
25 kHz, Sine Wave Centered at –0.75 V, R
L
= 600
Ω
APPLICATION NOTES
INPUT CHARACTERISTICS
In the AD822, n-channel JFETs are used to provide a low
offset, low noise, high impedance input stage. Minimum input
common-mode voltage extends from 0.2 V below –V
S
to 1 V
less than +V
S
. Driving the input voltage closer to the positive
rail will cause a loss of amplifier bandwidth (as can be seen by
comparing the large signal responses shown in Figures 31 and
34) and increased common-mode voltage error as illustrated in
Figure 17.
The AD822 does not exhibit phase reversal for input voltages
up to and including +V
S
. Figure 39a shows the response of an
AD822 voltage follower to a 0 V to +5 V (+V
S
) square wave
input. The input and output arc superimposed. The output
tracks the input up to +V
S
without phase reversal. The reduced
bandwidth above a 4 V input causes the rounding of the output
wave form. For input voltages greater than +V
S
, a resistor in
series with the AD822’s noninverting input will prevent phase
reversal, at the expense of greater input voltage noise. This is
illustrated in Figure 39b.
Since the input stage uses n-channel JFETs, input current
during normal operation is negative; the current flows out from
the input terminals. If the input voltage is driven more positive
than +V
S
– 0.4 V, the input current will reverse direction as
internal device junctions become forward biased. This is
illustrated in Figure 4.
A current limiting resistor should be used in series with the
input of the AD822 if there is a possibility of the input voltage
exceeding the positive supply by more than 300 mV, or if an
input voltage will be applied to the AD822 when
±
V
S
= 0. The
amplifier will be damaged if left in that condition for more than
10 seconds. A 1 kΩ resistor allows the amplifier to withstand up
to 10 volts of continuous overvoltage, and increases the input
voltage noise by a negligible amount.
Input voltages less than –V
S
are a completely different story.
The amplifier can safely withstand input voltages 20 volts below
the minus supply voltage as long as the total voltage from the
positive supply to the input terminal is less than 36 volts. In
addition, the input stage typically maintains picoamp level input
currents across that input voltage range.
REV. A
–13–
AD [ ANALOG DEVICES ]
