AD822
APPLICATION NOTES
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 stage. Minimum input
common-mode voltage extends from 0.2 V below −VS to ± V
less than +VS. 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 20.
1kHz
RESISTOR JOHNSON
NOISE
100
10
10Hz
1
The AD822 does not exhibit phase reversal for input voltages up
to and including +VS. Figure 42 shows the response of an
AD822 voltage follower to a 0 V to ꢀ V (+VS) square wave input.
The input and output are superimposed. The output tracks the
input up to +VS without 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 in series with the
AD822’s noninverting input prevents phase reversal, at the
expense of greater input voltage noise. This is illustrated in
Figure 42.
AMPLIFIER-GENERATED
NOISE
0.1
10k
100k
10M
100M
1G
10G
1M
SOURCE IMPEDANCE (Ω)
Figure 43. Total Noise vs. Source Impedance
OUTPUT CHARACTERISTICS
The AD822’s unique bipolar rail-to-rail output stage swings within
ꢀ mV of the negative supply and ±0 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 ꢀ mA, the saturation
voltage to the positive supply rail is 200 mV; when sinking ꢀ mA,
the saturation voltage to the negative rail is ±00 mV.
Since 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 – 0.4 V, then the input current reverses direction as
internal device junctions become forward biased. This is
illustrated in Figure 7.
The amplifier’s open-loop gain characteristic changes as a
function of resistive load, as shown in Figure ±0 to Figure ±3.
For load resistances over 20 kΩ, the AD822’s input error voltage
is virtually unchanged until the output voltage is driven to
±80 mV of either supply.
A current limiting resistor should be used in series with the
input of the AD822 if there is a possibility of the input voltage
exceeding the positive supply by more than 300 mV, or if an
input voltage is applied to the AD822 when ±VS = 0. The
amplifier is damaged if left in that condition for more than
±0 seconds. A ± kΩ resistor allows the amplifier to withstand up
to ±0 V of continuous overvoltage and increases the input
voltage noise by a negligible amount.
If the AD822’s output is overdriven so as to saturate either of
the output devices, the amplifier recovers within 2 ꢁs of its
input returning to the amplifier’s linear operating region.
Direct capacitive loads 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 44 shows the AD822’s pulse
response as a unity gain follower driving 3ꢀ0 pF. This amount of
overshoot indicates approximately 20° of phase margin—the
system is stable, but nearing the edge. Configurations with less
loop gain, and as a result less loop bandwidth, are much less
sensitive to capacitance load effects.
Input voltages less than –VS are a completely different story. The
amplifier can safely withstand input voltages 20 V below the
negative supply voltage as long as the total voltage from the
positive supply to the input terminal is less than 36 V. In
addition, the input stage typically maintains picoampere (pA)
level input currents across that input voltage range.
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’s 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 ± kHz. This is illustrated in Figure 43.
Rev. G | Page 20 of 28
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