Data Sheet AD823
Rev. E | Page 15 of 20
APPLICATION NOTES
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 causes 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
, with 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 noninverting input prevents phase
reversal, at the expense of greater input voltage noise. This is
illustrated in Figure 39.
GND
100
0%
1V
2µs
1V
90
00901-038
10
Figure 38. AD823 Input Response: R
P
= 0, V
IN
= 0 to +V
S
+V
S
GND
5V
V
IN
R
P
V
OUT
AD823
1V
10µs1V
00901-039
90
100
10
0%
Figure 39. AD823 Input Response:
V
IN
= 0 to +V
S
+ 200 mV, V
OUT
= 0 to +V
S
, R
P
= 49.9 kΩ
Because 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 reverses 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 is applied to the AD823 when ±V
S
= 0. The
amplifier becomes 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 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 V below
−V
S
as long as the total voltage from the positive supply to the
input terminal is less than 36 V. In addition, the input stage
typically maintains picoamp 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
instance, when driving 5 mA, the saturation voltage to the rails
is approximately 125 mV.
If the AD823’s output is driven hard against the output
saturation voltage, it recovers 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.
