ADA4638-1ACPZ-R7 datasheet(18/25 Pages) AD

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Data Sheet

ADA4638-1

Rev. 0 | Page 17 of 24

THEORY OF OPERATION

Figure 57 shows the ADA4638-1 amplifier block diagram. The

noninverting and inverting amplifier inputs are +IN and –IN,

respectively. The transconductance amplifiers, A1 and A2, are

the two input gain stages; the A3 and A4 transconductance

amplifiers are the nulling amplifiers used to correct the offsets

of A1 and A2, and AOUT is the output amplifier. A four-phase

cycle (φ1 to φ4) controls the switches. In Phase 1 (φ1), A1 is

auto-zeroed where both the inputs of A1 are connected to +IN.

A1 produces a differential output current of VOS1 × gm1, where

VOS1 is the input offset voltage of A1, and gm1 is the differential

transconductance of A1. The outputs of A1 are then connected

to the inputs and outputs of A3. A3 is designed to have an

equivalent resistance of 1/gm3, where gm3 is the transconduct-

ance of A3. The amplified version of VOS1, which is VOS1 ×

gm1/gm3, is stored on Capacitors C1 and C2. These capacitors,

together with A3, are used to null out the offset of A1 when A1

amplifies the signal during the φ3 and φ4 phases.

While A1 is being auto-zeroed, A2 (nulled by A4, C3, and C4)

is used for signal amplification. The ADA4638-1 differs from

traditional auto-zero amplifiers in that the input offset voltage

is also chopped during signal amplification. During φ1, +IN

and −IN are applied to the noninverting and inverting inputs,

respectively, of A2. However, during φ2, both the inputs and

outputs of A2 are inverted, and the input offset voltage of A2 is

chopped.

The combination of auto-zeroing and chopping offers two major

benefits. First, any residual offset following the auto-zeroing

process is reduced. During φ1, the output offset voltage of A2 is

+VOSAZ2 and during φ2, it is –VOSAZ2, producing a theoretical

average of zero. Second, the aliased noise spectrum density at dc

due to auto-zeroing is modulated up to the chopping frequency,

and the prechopped noise spectrum density at the chopping

frequency is modulated down to dc. This noise transformation

lowers the noise spectrum density at dc, thus making zero-drift

amplifiers ideal for low frequency signal amplification.

During φ3 and φ4, the roles of A1 and A2 are reversed. A2

offset is nulled, and the input signal is chopped and amplified

using A1.

+IN

–IN

Ф4

Ф3

Ф1

Ф2

Ф1

Ф3

Ф1

Ф3

Ф2

Ф1

Ф2

Ф1

Ф3

Ф1

Ф4

Ф3

Ф4

Ф1

Ф3

Ф4

AOUT

OUT

Figure 57. ADA4638-1 Amplifier Block Diagram

INPUT PROTECTION

The ADA4638-1 has internal ESD protection diodes that are

connected between the inputs and each supply rail. These diodes

protect the input transistors in the event of electrostatic dis-

charge and are reverse-biased during normal operation. However,

if either input exceeds one of the supply rails, these ESD diodes

become forward-biased and large amounts of current begin to

flow through them. Without current limiting, this excessive

fault current causes permanent damage to the device. If the

inputs are expected to be subject to overvoltage conditions,

insert a resistor in series with each input to limit the input

current to 10 mA maximum. However, consider the resistor

thermal noise effect on the entire circuit.

NO OUTPUT PHASE REVERSAL

An undesired phenomenon, phase reversal (also known as

phase inversion) occurs in many amplifiers when one or both of

the inputs are driven beyond the specified input common-mode

voltage range, in effect reversing the polarity of the output. In

some cases, phase reversal can induce lockups and cause

equipment damage as well as self destruction.

The ADA4638-1 has been carefully designed to prevent any

output phase reversal, provided that both inputs are maintained

within the supply voltages. If either one or both inputs may

exceed either supply voltage, place resistors in series with the

inputs to limit the current to less than 10 mA.

The ADA4638-1 features rail-to-rail output with a supply volt-

age from 4.5 V to 30 V. Figure 58 shows the input and output

waveforms of the ADA4638-1 configured as a unity-gain buffer

with a supply voltage of ±15 V and a resistive load of 10 kΩ.

The ADA4638-1 does not exhibit phase reversal.

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