AD8220ARMZ-RL View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD8220ARMZ-RL

JFET Input Instrumentation Amplifier with Rail-to-Rail Output in MSOP Package

Analog Devices 

AD8220

THEORY OF OPERATION

+VS

+VS

+VS

+VS

NODE A

RG

NODE B

R1

24.7kΩ –VS

R2

24.7kΩ

–VS

+VS

+IN

–VS

J1 Q1

C1

NODE C

NODE D

C2

Q2 J2

VPINCH

A1

A2

VPINCH

I

VB

I

–VS

Figure 55. Simplified Schematic

20kΩ

20kΩ

+VS

–IN

–VS

20kΩ

NODE F

A3

+VS

OUTPUT

NODE E +VS –VS

20kΩ

REF

–VS

The AD8220 is a JFET input, monolithic instrumentation amplifier

based on the classic three 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

change accordingly; the input signal creates a current through RG

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

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

impedance, extremely low bias currents of 10 pA maximum,

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,3 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 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 = 1 + 49.4 kΩ

RG

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

standard resistor. Since 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.

–IN 1

RG 2

RG 3

+IN 4

AD8220

8 +VS

7 VOUT

6 REF

5 –VS

3 Overdriving the input at high gains refers to when the input signal is within

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.

TOP VIEW

(Not to Scale)

Figure 56. Pin Configuration

Rev. 0 | Page 19 of 28

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