Datasheet

AD823
Rev. A | Page 15 of 20
APPLICATIONS
INPUT CHARACTERISTICS
In the AD823, 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 <
+V
S
. Driving the input voltage closer to the positive rail will
cause a loss of amplifier bandwidth and increased common-
mode voltage error.
The AD823 does not exhibit phase reversal for input voltages up
to and including +V
S
. Figure 38 shows the response of an
AD823 voltage follower to a 0 V to 5 V (+V
S
) square wave input.
The input and output are superimposed. The output polarity
tracks the input polarity up to +V
S
, no 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 AD823’s plus input will prevent phase
reversal, at the expense of greater input voltage noise. This is
illustrated in Figure 39.
GND
100
0%
1V
2µs
1V
10
90
00901-A-038
Figure 38. AD823 Input Response: Response with R
P
= 0, V
IN
from 0 to V
S
+V
S
GND
90
100
5V
V
IN
R
P
V
OUT
AD823
10
0%
1V
10µs
1V
1V
00901-A-046
Figure 39. AD823 Input Response:
V
IN
= 0 to +V
S
+ 200 mV, V
OUT
= 0 to +V
S
, R
P
= 49.9 kΩ
Since the input stage uses N-Channel JFETs, input current dur-
ing 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 7.
A current limiting resistor should be used in series with the
input of the AD823 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 AD823 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 V of continuous overvoltage and increases the input volt-
age noise by a negligible amount.
Input voltages less than −V
S
are a completely different story. The
amplifier can safely withstand input voltages 20 V below the
minus supply voltage as long as the total voltage from the
positive supply to the input terminal is less than 36 V. In addi-
tion, the input stage typically maintains pico-amp level input
currents across that input voltage range.
The AD823 is designed for 16 nV/√Hz wideband input voltage
noise and maintains low noise performance to low frequencies
(see Figure 16). This noise performance, along with the AD823’s
low input current and current noise, means that the AD823
contributes negligible noise for applications with source
resistances greater than 10 kΩ and signal bandwidths greater
than 1 kHz.
OUTPUT CHARACTERISTICS
The AD823’s unique bipolar rail-to-rail output stage swings
within 25 mV of the supplies with no external resistive load.
The AD823’s approximate output saturation resistance is 25 Ω
sourcing and sinking. This can be used to estimate the output
saturation voltage when driving heavier current loads. For in-
stance, when driving 5 mA, the saturation voltage to the rails
will be approximately 125 mV.
If the AD823’s output is driven hard against the output satura-
tion voltage, it will recover within 250 ns of the input returning
to the amplifier’s linear operating region.
A/D Driver
The rail-to-rail output of the AD823 makes it useful as an A/D
driver in a single-supply system. Because it is a dual op amp, it
can be used to drive both the analog input of the A/D as well as
its reference input. The high impedance FET input of the
AD823 is well suited for minimal loading of high output
impedance devices.