–14–

AD822

REV. A

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 k

Ω and signal bandwidths greater

than 1 kHz. This is illustrated in Figure 40.

100k

0.1

10G

100

1

100k

10

10k

10k

1k

1G

100M

10M

1M

SOURCE IMPEDANCE –

Ω

WHENEVER JOHNSON NOISE IS GREATER THAN

AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE

CONSIDERED NEGLIGIBLE FOR APPLICATION.

RESISTOR JOHNSON

NOISE

AMPLIFIER GENERATED

NOISE

1kHz

10Hz

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 k

Ω, 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.

10

0%

20mV

2

µ s

90

100

Figure 41. Small Signal Response of AD822 as Unity Gain

Follower Driving 350 pF Capacitive Load

5

1

300

30k

4

2

1k

3

3k

10k

CAPACITIVE LOAD FOR 20

R

F

R

I

C

L

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.

8

4

0.01

µF

20pF

20k

Ω

100

Ω

V

OUT

V

IN

+V

S

–V

S

0.01

µF

C

L

1/2

AD822

Figure 43. Extending Unity Gain Follower Capacitive Load

Capability Beyond 350 pF