AD8224ACPZ-R7 datasheet(19/28 Pages) AD

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AD8224

Rev. 0 | Page 19 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 52. Simplified Schematic

The AD8224 is a JFET input, monolithic instrumentation amplifier

based on the classic three op amp topology (see Figure 52). 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 with 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

0.06%. New trim techniques were developed to ensure that CMRR

exceeds 78 dB (G = 1).

Using JFET transistors, the AD8224 offers extremely high input

impedance, extremely low bias currents of 25 pA maximum,

low offset current of 2 pA maximum, and no input bias current

noise. In addition, input offset is less than 300 μV and drift is

less than 10 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. Since 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 11 or by using

the following gain equation:

kΩ

49.4

−

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