Datasheet

AD820
REV. A
–14–
Direct capacitive load 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 38 shows the AD820’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 39 is a plot of capacitive load that will result in a
20 degree phase margin versus noise gain for the AD820. Noise
gain is the inverse of the feedback attenuation factor provided
by the feedback network in use.
10
0%
20mV
2µs
90
100
Figure 38. Small Signal Response of AD820 as Unity Gain
Follower Driving 350 pF Capacitive Load
5
1
300 30k
4
2
1k
3
3k 10k
CAPACITIVE LOAD FOR 20 ° PHASE MARGIN – pF
NOISE GAIN – 1+ –––
R
F
R
I
R
F
R
I
Figure 39. Capacitive Load Tolerance vs. Noise Gain
Figure 40 shows a possible configuration 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.
3
2
6
7
4
0.01µF
20pF
20kΩ
100Ω
V
OUT
V
IN
+V
S
–V
S
AD820
0.01µF
Figure 40. Extending Unity Gain Follower Capacitive Load
Capability Beyond 350 pF
OFFSET VOLTAGE ADJUSTMENT
The AD820’s offset voltage is low, so external offset voltage
nulling is not usually required. Figure 41 shows the recom-
mended technique for AD820’s packaged in plastic DIPs.
Adjusting offset voltage in this manner will change the offset
voltage temperature drift by 4 µV/°C for every millivolt of in-
duced offset. The null pins are not functional for AD820s in the
SO-8 “R” package.
3
2
6
7
4
+V
S
–V
S
20k
1
AD820
5
Figure 41. Offset Null
APPLICATIONS
Single Supply Half-Wave and Full-Wave Rectifiers
An AD820 configured as a unity gain follower and operated
with a single supply can be used as a simple half-wave rectifier.
The AD820’s inputs maintain picoamp level input currents even
when driven well below the minus supply. The rectifier puts that
behavior to good use, maintaining an input impedance of over
10
11
Ω for input voltages from 1 volt from the positive supply to
20 volts below the negative supply.
The full and half-wave rectifier shown in Figure 42 operates as
follows: when V
IN
is above ground, R1 is bootstrapped through
the unity gain follower A1 and the loop of amplifier A2. This
forces the inputs of A2 to be equal, thus no current flows
through R1 or R2, and the circuit output tracks the input. When
V
IN
is below ground, the output of A1 is forced to ground. The
noninverting input of amplifier A2 sees the ground level output
of A1, therefore A2 operates as a unity gain inverter. The output
at node C is then a full-wave rectified version of the input. Node
B is a buffered half-wave rectified version of the input. Input
voltages up to ±18 volts can be rectified, depending on the
voltage supply used.