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Data Sheet

AD8224

Rev. D | Page 21 of 28

THEORY OF OPERATION

NODE A

NODE B

NODE C

NODE D

–VS

+VS

–VS

+IN

VPINCH

+VS

–VS

–IN

VPINCH

+VS

–VS

+VS

–VS

20kΩ

+VS

–VS

+VS

–VS

REF

OUTPUT

NODE E

NODE F

24.7kΩ

Figure 56. Simplified Schematic

The AD8224 is a JFET input, monolithic instrumentation amplifier

based on the classic three op amp topology (see Figure 56). 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, and R1 and J2, A2,

and R2 can be viewed as precision current feedback amplifiers. 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 a gain error of less than 0.04%. New trim

techniques were developed to ensure that the CMRR exceeds 86 dB

(G = 1).

Using JFET transistors, the AD8224 offers an 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 175 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, an excessive

milliampere input bias current can result, and the output can

undergo phase reversal.

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 because the amplifier cannot output 100 V.

The AD8224 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 AD8224 has extremely low load induced nonlinearity. All

amplifiers that comprise the AD8224 have rail-to-rail output

capability for enhanced dynamic range. The input of the AD8224

can amplify signals with wide common-mode voltages even slightly

lower than the negative supply rail. The AD8224 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 AD8224 is

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 AD8224 gain bandwidth product

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

as much bandwidth loss as voltage feedback architectures at

higher gains.

GAIN SELECTION

Placing a resistor across the RG terminals sets the gain of the

AD8224. This is calculated by referring to Table 13 or by using

the following gain equation

kΩ

49.4





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