AD8221ARZ-R7 datasheet(17/24 Pages) AD

17 / 24 page

AD8221

Rev. C | Page 17 of 24

THEORY OF OPERATION

+VS

+IN

–IN

–VS

10k

Ω

10k

Ω

10k

Ω

400

Ω

400

Ω

10k

Ω

REF

OUTPUT

IB COMPENSATION

+VS

–VS

+VS

–VS

+VS

24.7k

Ω

24.7k

Ω

–VS

+VS

–VS

Figure 43. Simplified Schematic

The AD8221 is a monolithic instrumentation amplifier based

on the classic 3-op amp topology. Input transistors Q1 and Q2

are biased at a fixed current so that any differential input signal

forces the output voltages of A1 and A2 to change accordingly.

A signal applied to the input creates a current through RG, R1,

and R2, such that the outputs of A1 and A2 deliver the correct

voltage. Topologically, Q1, A1, R1 and Q2, A2, R2 can be

viewed as precision current feedback amplifiers. The amplified

differential and common-mode signals are applied to a

difference amplifier that rejects the common-mode voltage

but amplifies the differential voltage. The difference amplifier

employs innovations that result in low output offset voltage as

well as low output offset voltage drift. Laser-trimmed resistors

allow for a highly accurate in-amp with gain error typically less

than 20 ppm and CMRR that exceeds 90 dB (G = 1).

Using superbeta input transistors and an IB compensation

scheme, the AD8221 offers extremely high input impedance,

low IB, low IB drift, low IOS, low input bias current noise, and

extremely low voltage noise of 8 nV/√Hz.

The transfer function of the AD8221 is

kΩ

Users can easily and accurately set the gain using a single

standard resistor.

Because the input amplifiers employ a current feedback

architecture, the gain-bandwidth product of the AD8221

increases with gain, resulting in a system that does not suffer

from the expected bandwidth loss of voltage feedback

architectures at higher gains.

To maintain precision even at low input levels, special attention

was given to the design and layout of the AD8221, resulting in

an in-amp whose performance satisfies the most demanding

applications.

A unique pinout enables the AD8221 to meet a CMRR

specification of 80 dB at 10 kHz (G = 1) and 110 dB at 1 kHz

(G = 1000). The balanced pinout, shown in Figure 44, reduces

the parasitics that had, in the past, adversely affected CMRR

performance. In addition, the new pinout simplifies board

layout because associated traces are grouped together. For

example, the gain setting resistor pins are adjacent to the

inputs, and the reference pin is next to the output.

–IN

+VS

VOUT

REF

–VS

+IN

TOP VIEW

AD8221

Figure 44. Pinout Diagram

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