AD8224 Analog Devices, AD8224 Datasheet - Page 19

• Manufacturer Part Number: AD8224
• Description: Precision, Dual-Channel, JFET Input Rail-to-Rail Instrumentation Amplifier
• Manufacturer: Analog Devices

THEORY OF OPERATION
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.
+IN
+V
–V
S
S
+V
J1
S
V
PINCH
I
Q1
C1
24.7kΩ
NODE A
R1
A1
NODE C
+V
–V
S
S
–V
RG
S
VB
NODE D
Figure 52. Simplified Schematic
+V
–V
S
S
NODE B
A2
Rev. 0 | Page 19 of 28
R2
24.7kΩ
C2
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
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 R
AD8224. This is calculated by referring to Table 11 or by using
the following gain equation:
Q2
V
PINCH
I
J2
G
+V
R
G
S
=
=
1 +
49.4
+V
–V
G
49.4
S
S
−
RG
kΩ
1
20kΩ
20kΩ
kΩ
–IN
NODE F
NODE E
20kΩ
A3
G
20kΩ
terminals sets the gain of the
+V
–V
S
S
+V
–V
S
S
OUTPUT
REF
AD8224