AD822
Input voltages less than –VS 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 picoamp level input
currents across that input voltage range.
20mV
. . . . . . . .
2µs
. . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . . . .
100
90
The AD822 is designed for 13 nV/÷Hz wideband input voltage
noise and maintains low noise performance to low frequencies
(refer to TPC 11). This noise performance, along with the
AD822’s low input current and current noise, means that the
AD822 contributes negligible noise for applications with source
resistances greater than 10 kW and signal bandwidths greater
than 1 kHz. This is illustrated in Figure 3.
10
. . . . . . . .
. . . .
. . . . . . . . . . . .
. . . . . . . . . . . . . . . .
0%
100k
WHENEVER JOHNSON NOISE IS GREATER THAN
AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE
Figure 4. Small Signal Response of AD822 as
Unity Gain Follower Driving 350 pF
CONSIDERED NEGLIGIBLE FOR APPLICATION.
10k
1kHz
Figure 5 is a plot of capacitive load that will result in a 20 degree
phase margin versus noise gain for the AD822. Noise gain is the
inverse of the feedback attenuation factor provided by the feed-
back network in use.
1k
RESISTOR JOHNSON
NOISE
100
5
10
10Hz
1
4
3
AMPLIFIER-GENERATED
NOISE
0.1
10k
100k
10M
100M
10G
1M
1G
SOURCE IMPEDANCE – ꢃ
Figure 3. Total Noise vs. Source Impedance
2
1
OUTPUT CHARACTERISTICS
The AD822’s unique bipolar rail-to-rail output stage swings within
5 mV of the minus supply and 10 mV of the positive supply with
no external resistive load. The AD822’s approximate output satu-
ration resistance is 40 W sourcing and 20 W sinking. This can be
used to estimate output saturation voltage when driving heavier
current loads. For instance, when sourcing 5 mA, the saturation
voltage to the positive supply rail will be 200 mV; when sinking
5 mA, the saturation voltage to the minus rail will be 100 mV.
300
1k
3k
10k
30k
O
CAPACITIVE LOAD FOR 20 PHASE MARGIN – pF
C
L
R
F
The amplifier’s open-loop gain characteristic will change as a
function of resistive load, as shown in TPCs 7 through 10. For
load resistances over 20 kW, the AD822’s input error voltage is
virtually unchanged until the output voltage is driven to 180 mV
of either supply.
R1
Figure 5. Capacitive Load Tolerance vs. Noise Gain
Figure 6 shows a method for extending capacitance load drive
capability for a unity gain follower. With these component values,
the circuit will drive 5,000 pF with a 10% overshoot.
If the AD822’s output is overdriven so as to saturate either of the
output devices, the amplifier will recover within 2 ms of its input
returning to the amplifier’s linear operating region.
+V
S
Direct capacitive loads will interact with the amplifier’s effective
output impedance to form an additional pole in the amplifier’s
feedback loop which can cause excessive peaking on the pulse
response or loss of stability. Worst case is when the amplifier is
used as a unity gain follower. Figure 4 shows the AD822’s pulse
response as a unity gain follower driving 350 pF. This amount
of overshoot indicates approximately 20 degrees of phase margin—
the system is stable but is nearing the edge. Configurations with
less loop gain, and as a result less loop bandwidth, will be much
less sensitive to capacitance load effects.
0.01ꢁF
0.01ꢁF
20pF
8
100ꢃ
1/2
V
IN
AD822
V
OUT
C
4
L
–V
S
20kꢃ
Figure 6. Extending Unity Gain Follower
Capacitive Load Capability Beyond 350 pF
REV. D
–14–
ETC [ ETC ]
