AD820A Analog Devices, AD820A Datasheet - Page 13

• Manufacturer Part Number: AD820A
• Manufacturer: Analog Devices

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 16 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 sensi-
tive to capacitance load effects. Figure 17 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 attenu–
ation factor provided by the feedback network in use.
5
4
3
2
1
300
CAPACITIVE LOAD FOR 20 PHASE MARGIN – pF
1k
R
I
3k
R
F
10k
30k
Figure 18 shows a possible configuration for extending capaci-
tance load drive capability for a unity gain follower. With these
component values, the circuit will drive 5,000 pF with a 10%
overshoot.
OFFSET VOLTAGE ADJUSTMENT
The AD820’s offset voltage is low, so external offset voltage null-
ing is not usually required. Figure 19 shows the recommended
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 induced offset. The null pins
are not functional for AD820s in the SO-8 “R” package.
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 behav-
ior to good use, maintaining an input impedance of over 10
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 20 operates as
follows: when V
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
the output of A1 is forced to ground. The noninverting input of
amplifier A2 sees the ground level output of A1, therefore, A2 oper-
ates 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.
V
IN
IN
3
2
AD820
is above ground, R1 is bootstrapped through
+V
–V
7
4
S
S
0.01 F
0.01 F
20pF
3
2
6
4
–V
AD820
+V
7
S
5
S
20k
100
20k
1
6
IN
is below ground,
AD820
V
OUT
11
Ω