AD8027ARTZ-R2 datasheet(21/27 Pages) AD

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Data Sheet

AD8027/AD8028

THEORY OF OPERATION

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Rev. E | 21 of 27

OUTPUT STAGE

The AD8027/AD8028 use a common emitter output structure to

achieve rail-to-rail output capability. The output stage is designed

to drive 50 mA of linear output current, 40 mA within 200 mV

of the rail, and 2.5 mA within 35 mV of the rail. Loading of the

output stage, including any possible feedback network, lowers the

open-loop gain of the amplifier. Refer to Figure 53 for the loading

behavior. Capacitive load can degrade the phase margin of the

amplifier. The AD8027/AD8028 can drive up to 20 pF, G = +1, as

shown in Figure 14. Include a small (25 Ω to 50 Ω) series resistor,

RSNUB, if the capacitive load is to exceed 20 pF for a gain of 1.

Increasing the closed-loop gain increases the amount of capacitive

load that can be driven before a series resistor must be included.

DC ERRORS

The AD8027/AD8028 use two complementary input stages to ach-

ieve rail-to-rail input performance, as described in the Input Stage

section. To use the dc performance over the entire common-mode

range, the input bias current and input offset voltage of each pair

must be considered.

Referring to Figure 61, the output offset voltage of each pair is

calculated by

VOS,PNP,OUT=VOS,PNP RG+RFRG

VOS,NPN,OUT=VOS,NPN RG+RFRG

(4)

where the difference of the two input stages is the discontinuity

experienced when going through the crossover region.

The size of the discontinuity is defined as

VDIS= VOS,PNP−VOS,NPN × RG+RFRG (5)

Using the crossover select feature of the AD8027/AD8028 helps to

avoid this region. In the event that the region cannot be avoided,

the quantity (VOS, PNP − VOS, NPN) is trimmed to minimize this effect.

Because the input pairs are complementary, the input bias current

reverses polarity when going through the crossover region shown in

Figure 41. The offset between pairs is described by

VOS,PNP−VOS,NPN= IB,PNP−IB,NPN ×

RS RG+RFRG −RF

(6)

where:

IB, PNP is the input bias current of either input when the PNP input

pair is active.

IB, NPN is the input bias current of either input pair when the NPN

pair is active.

If RS is sized so that it equals RF when multiplied by the gain factor,

this effect is eliminated. It is strongly recommended to balance the

impedances in this manner when traveling through the crossover

region to minimize the dc error and distortion. As an example,

assuming that the PNP input pair has an input bias current of 6 µA

and the NPN input pair has an input bias current of −2 µA, a 200 µV

shift in offset occurs when traveling through the crossover region

with RF equal to 0 Ω and RS equal to 25 Ω.

In addition to the input bias current shift between pairs, each input

pair has an input bias current offset that contributes to the total

offset in the following manner:

∆VOS=IB+RS RG+RFRG −IB−RF

(7)

Figure 61. Op Amp DC Error Sources

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