AD822
APPLICATIONS INFORMATION
100k
10k
1k
INPUT CHARACTERISTICS
WHENEVER JOHNSON NOISE IS GREATER THAN
AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE
CONSIDERED NEGLIGIBLE FOR APPLICATION.
In the AD822, N-channel JFETs are used to provide a low offset,
low noise, high impedance input stageꢀ Minimum input common-
mode voltage extends from 0ꢀ2 V below −VS to ± V less than +VSꢀ
Driving the input voltage closer to the positive rail causes a loss
of amplifier bandwidth (as can be seen by comparing the large
signal responses shown in Figure 34 and Figure 37) and increased
common-mode voltage error as illustrated in Figure 20ꢀ
1kHz
RESISTOR JOHNSON
NOISE
100
10
10Hz
The AD822 does not exhibit phase reversal for input voltages
up to and including +VSꢀ Figure 42 shows the response of an
AD822 voltage follower to a 0 V to 5 V (+VS) square wave inputꢀ
The input and output are superimposedꢀ The output tracks the
input up to +VS without phase reversalꢀ The reduced bandwidth
above a 4 V input causes the rounding of the output waveformꢀ
For input voltages greater than +VS, a resistor in series with the
AD822 noninverting input prevents phase reversal, at the expense
of greater input voltage noiseꢀ This is illustrated in Figure 42ꢀ
1
AMPLIFIER-GENERATED
NOISE
0.1
10k
100k
10M
100M
10G
1M
1G
SOURCE IMPEDANCE (Ω)
Figure 43. Total Noise vs. Source Impedance
OUTPUT CHARACTERISTICS
The AD822 unique bipolar rail-to-rail output stage swings within
5 mV of the negative supply and ±0 mV of the positive supply with
no external resistive loadꢀ The approximate output saturation
resistance of the AD822 is 40 Ω sourcing and 20 Ω sinking, which
can be used to estimate output saturation voltage when driving
heavier current loadsꢀ For instance, when sourcing 5 mA, the
saturation voltage to the positive supply rail is 200 mV; when
sinking 5 mA, the saturation voltage to the negative rail is ±00 mVꢀ
Because the input stage uses N-channel JFETs, input current
during normal operation is negative; the current flows out from
the input terminalsꢀ If the input voltage is driven more positive
than +VS − 0ꢀ4 V, then the input current reverses direction as
internal device junctions become forward biasedꢀ This is illu-
strated in Figure 7ꢀ
A current limiting resistor should be used in series with the input
of the AD822 if there is a possibility of the input voltage exceed-
ing the positive supply by more than 300 mV, or if an input voltage
is applied to the AD822 when +VS or −VS = 0 Vꢀ The amplifier is
damaged if left in that condition for more than ±0 secondsꢀ A ± kΩ
resistor allows the amplifier to withstand up to ±0 V of conti-
nuous overvoltage and increases the input voltage noise by a
negligible amountꢀ
The open-loop gain characteristic of the amplifier changes as a
function of resistive load, as shown in Figure ±0 to Figure ±3ꢀ
For load resistances over 20 kΩ, the AD822 input error voltage
is virtually unchanged until the output voltage is driven to ±80 mV
of either supplyꢀ
If the AD822 output is overdriven so that either of the output
devices are saturated, the amplifier recovers within 2 ꢁs of its
input returning to the linear operating region of the amplifierꢀ
Input voltages less than −VS are a completely different storyꢀ The
amplifier can safely withstand input voltages 20 V below the
negative supply voltage if the total voltage from the positive
supply to the input terminal is less than 36 Vꢀ In addition, the
input stage typically maintains picoampere (pA) level input
currents across that input voltage rangeꢀ
Direct capacitive loads interact with the effective output imped-
ance of the amplifier to form an additional pole in the amplifier
feedback loop, which can cause excessive peaking on the pulse
response or loss of stabilityꢀ The worst case occurs when the
amplifier is used as a unity-gain followerꢀ Figure 44 shows the
AD822 pulse response as a unity-gain follower driving 350 pFꢀ
This amount of overshoot indicates approximately 20° of phase
margin—the system is stable, but nearing the edgeꢀ Configurations
with less loop gain, and as a result less loop bandwidth, are
much less sensitive to capacitance load effectsꢀ
The AD822 is designed for ±3 nV/√Hz wideband input voltage
noise and maintains low noise performance to low frequencies
(refer to Figure ±4)ꢀ This noise performance, along with the
AD822 low input current and current noise, means that the
AD822 contributes negligible noise for applications with source
resistances greater than ±0 kΩ and signal bandwidths greater
than ± kHzꢀ This is illustrated in Figure 43ꢀ
Rev. I | Page 18 of 24
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