AD820

REV. B

–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 40 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 ef-

fects. Figure 41 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.

Figure 40. Small Signal Response of AD820 as Unity Gain

Follower Driving 350 pF Capacitive Load

CAPACITIVE LOAD FOR 20 PHASE MARGIN – pF

5

4

1

300

3

2

1k

3k

10k

30k

Figure 41. Capacitive Load Tolerance vs. Noise Gain

Figure 42 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.

100

20k

AD820

0.01 F

0.01 F

3

2

4

7

6

20pF

Figure 42. 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 43 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.

20k

1

AD820

3

2

4

7

6

5

Figure 43. 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 44 operates as

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

below ground, the output of A1 is forced to ground. The non-

inverting 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 volt-

ages up to

±18 volts can be rectified, depending on the voltage

supply used.