AD8220

Rev. B | Page 19 of 28

THEORY OF OPERATION

Q2

Q1

NODE A

NODE B

NODE C

NODE D

VB

C1

C2

A1

A2

J1

+IN

J2

–IN

20kΩ

20kΩ

20kΩ

20kΩ

REF

OUTPUT

A3

NODE E

NODE F

I

I

R2

24.7kΩ

R1

24.7kΩ

Figure 55. Simplified Schematic

The AD8220 is a JFET input, monolithic instrumentation amplifier

based on the classic 3-op amp topology (see Figure 55). Input

Transistor J1 and Input Transistor J2 are biased at a fixed current so

that any input signal forces the output voltages of A1 and A2 to

that flows in R1 and R2 such that the outputs of A1 and A2 provide

the correct, gained signal. Topologically, J1, A1, and R1 and J2, A2,

and R2 can be viewed as precision current feedback amplifiers that

have a gain bandwidth of 1.5 MHz. The common-mode voltage

and amplified differential signal from A1 and A2 are applied to a

difference amplifier that rejects the common-mode voltage but

amplifies the differential signal. The difference amplifier employs

20 kΩ laser-trimmed resistors that result in an in-amp with gain

error less than 0.04%. New trim techniques were developed to

ensure that CMRR exceeds 86 dB (G = 1).

Using JFET transistors, the AD8220 offers an extremely high

input impedance, extremely low bias currents of 10 pA

maximum, a low offset current of 0.6 pA maximum, and no

input bias current noise. In addition, input offset is less than

125 μV and drift is less than 5 μV/°C. Ease of use and robustness

were considered. A common problem for instrumentation

amplifiers is that at high gains, when the input is overdriven,1

an excessive milliampere input bias current can result and the

output can undergo phase reversal. The AD8220 has none of

these problems; its input bias current is limited to less than

10 μA, and the output does not phase reverse under overdrive

fault conditions.

the supply voltages but the amplifier cannot output the gained signal. For

example, at a gain of 100, driving the amplifier with 10 V on ±15 V constitutes

overdriving the inputs since the amplifier cannot output 100 V.

The AD8220 has extremely low load-induced nonlinearity. All

amplifiers that comprise the AD8220 have rail-to-rail output

capability for enhanced dynamic range. The input of the AD8220

can amplify signals with wide common-mode voltages even

slightly lower than the negative supply rail. The AD8220 operates

over a wide supply voltage range. It can operate from either a

single +4.5 V to +36 V supply or a dual ±2.25 V to ±18 V. The

transfer function of the AD8220 is

G

R

G

kΩ

49.4

1



Users can easily and accurately set the gain using a single,

standard resistor. Because the input amplifiers employ a current

feedback architecture, the AD8220 gain-bandwidth product

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

much bandwidth loss as voltage feedback architectures at higher

gains. A unique pinout enables the AD8220 to meet a CMRR

specification of 80 dB through 5 kHz (G = 1). The balanced

pinout, shown in Figure 56, reduces parasitics that adversely

affect 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.

TOP VIEW

(Not to Scale)

–IN

1

+IN

4

8

7

REF

6

5

AD8220

Figure 56. Pin Configuration