AD822
The AD822 is designed for 13 nV/√Hz wideband input voltage
noise and maintains low noise performance to low frequencies
(refer to Figure 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 kΩ and signal bandwidths greater
than 1 kHz. This is illustrated in Figure 40.
100k
WHENEVER JOHNSON NOISE IS GREATER THAN
AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE
CONSIDERED NEGLIGIBLE FOR APPLICATION.
1kHz
1k
RESISTOR JOHNSON
NOISE
100
20mV
100
90
2µ s
10
0%
INPUT VOLTAGE NOISE –
µV
RMS
10k
Figure 41. Small Signal Response of AD822 as Unity Gain
Follower Driving 350 pF Capacitive Load
5
10
10Hz
1
AMPLIFIER GENERATED
NOISE
0.1
10k
100k
1M
10M
100M
SOURCE IMPEDANCE –
Ω
1G
10G
4
Figure 40. Total Noise vs. Source Impedance
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 saturation resistance is 40
Ω
sourcing and
20
Ω
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.
The amplifier’s open-loop gain characteristic will change as a
function of resistive load, as shown in Figures 7 through 10. For
load resistances over 20 kΩ, the AD822’s input error voltage is
virtually unchanged until the output voltage is driven to 180 mV
of either supply.
If the AD822’s output is overdriven so as to saturate either of
the output devices, the amplifier will recover within 2
µs
of its
input returning to the amplifier’s linear operating region.
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 41 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. Figure 42 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 feedback network in use.
R
F
NOISE GAIN – 1+ –––
R
I
3
2
1
300
1k
3k
10k
CAPACITIVE LOAD FOR 20
°
PHASE MARGIN – pF
30k
R
F
C
L
R
I
Figure 42. Capacitive Load Tolerance vs. Noise Gain
Figure 43 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.
+V
S
8
V
IN
0.01µF
100Ω
0.01µF
4
–V
S
20pF
20kΩ
C
L
V
OUT
1/2
AD822
Figure 43. Extending Unity Gain Follower Capacitive Load
Capability Beyond 350 pF
–14–
REV. A
AD [ ANALOG DEVICES ]
