AD820ARMZ-R7 datasheet(17/24 Pages) AD

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AD820

Rev. H | Page 17 of 24

OUTPUT CHARACTERISTICS

The AD820 unique bipolar rail-to-rail output stage swings

within 5 mV of the negative supply and 10 mV of the positive

supply with no external resistive load. The approximate output

saturation resistance of the AD820 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

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 100 mV.

The open-loop gain characteristic of the amplifier changes

as a function of resistive load, as shown in Figure 10 through

Figure 13. For load resistances over 20 kΩ, the AD820 input

error voltage is virtually unchanged until the output voltage is

driven to 180 mV of either supply.

If the AD820 output is driven hard against the output saturation

voltage, it recovers within 2 μs of the input returning to the

linear operating region of the amplifier.

Direct capacitive load interacts 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 stability. The worst case occurs when the

amplifier is used as a unity-gain follower. Figure 40 shows

AD820 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, are much less sensitive to capacitance load effects.

Figure 41 is a plot of noise gain vs. the capacitive load that results

in a 20 degree phase margin for the AD820. Noise gain is the

inverse of the feedback attenuation factor provided by the

feedback network in use.

20mV

2µs

100

Figure 40. Small Signal Response of AD820 as Unity-Gain Follower Driving

350 pF Capacitive Load

300

30k

CAPACITIVE LOAD FOR 20º PHASE MARGIN (pF)

10k

Figure 41. Noise Gain vs. Capacitive Load Tolerance

Figure 42 shows a possible configuration for extending

capacitance load drive capability for a unity-gain follower. With

these component values, the circuit drives 5000 pF with a 10%

overshoot.

AD820

–

+VS

–VS

0.01µF

20pF

20kΩ

100Ω

VOUT

VIN

–

Figure 42. Extending Unity-Gain Follower Capacitive Load Capability

Beyond 350 pF

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 inputs maintain picoamp level input currents even

when driven well below the negative supply. The rectifier puts

that behavior to good use, maintaining an input impedance of

over 1011 Ω for input voltages from 1 V from the positive supply

to 20 V below the negative supply.

The full- and half-wave rectifier shown in Figure 43 operates as

follows: when VIN 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 VIN is

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

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