Data Sheet
AD823A
APPLICATIONS INFORMATION
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
+VS − 0.7 V, t h e input current reverses direction as internal device
junctions become forward biased. This is illustrated in Figure 11.
INPUT CHARACTERISTICS
In the AD823A, N-channel JFETs provide a low offset, low noise,
high impedance input stage. Minimum input common-mode
voltage extends from 0.2 V below −VS to 1.2 V < +VS. Driving
the input voltage closer to the positive rail causes a loss of
amplifier bandwidth and increased common-mode voltage error.
A current limiting resistor should be used in series with the input
of the AD823A if the input voltage can be driven over 300 mV
more positive than +Vs or 300 mV more negative than –Vs. The
amplifier will be damaged if either condition persists for more than
10 seconds. A 1 kΩ resistor in series with the AD823A input allows
the amplifier to withstand up to 10 V of continuous overvoltage
and increases input voltage noise by a negligible amount.
The AD823A does not exhibit phase reversal for input voltages
up to and including +VS. Figure 39 shows the response of an
AD823A voltage follower to a 0 V to 5 V (+VS) square wave
input. The input and output are superimposed. The output
polarity tracks the input polarity up to +VS, with no phase
reversal. The reduced bandwidth above a 4 V input causes the
rounding of the output waveform. For input voltages greater
than +VS, a resistor (RP) in series with the AD823A noninverting
input prevents phase reversal, at the expense of greater input
voltage noise. The value of RP ranges from 1 kΩ to 10 kΩ. This
is illustrated in Figure 40.
The AD823A is designed for 14 nV/√Hz wideband input voltage
noise (see Figure 19). This noise performance, along with the
AD823A low input current and current noise, means that the
AD823A contributes negligible noise for applications with high
source resistances. Figure 41 shows that the source resistance
contributes to negligible noise for source impedances lower
than 10 kΩ. The low input capacitance of 0.6 pF also means that
one can use a source impedance up to 13 kΩ without cutting
into the G = +1 small signal bandwidth region.
5.0V
2.5V
100
TOTAL AMPLIFIER NOISE
INPUT
OUTPUT
10
0V
AMPLIFIER VOLTAGE AND
CURRENT NOISE
1V
2µs
SOURCE RESISTANCE
NOISE
Figure 39. Input and Output Response: RP = 0 kΩ, VIN = 0 V to +VS
OUTPUT
INPUT
6V
3V
0V
1
10
100
1k
10k
100k
SOURCE RESISTANCE (Ω)
Figure 41. RTI Noise vs. Source Resistance
OUTPUT CHARACTERISTICS
The unique bipolar rail-to-rail output stage of the amplifier swings
within 20 mV of the supplies with no external resistive load.
The approximate output saturation resistance of the AD823A
is 33 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 165 m V.
1V
10µs
5V
R
P
V
IN
AD823A
V
OUT
Figure 40. Input and Output Response: VIN = 0 V to +VS + 1 V,
OUT = 0 V to +VS + 400 mV, RP = 4.99 kΩ
V
Rev.B | Page 15 of 20
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