AD8223ARM Analog Devices Inc, AD8223ARM Datasheet - Page 14

• Manufacturer Part Number: AD8223ARM
• Description: Single Supply Low Cost Intrumentation Am
• Manufacturer: Analog Devices Inc

Specifications of AD8223ARM

• Amplifier Type: Instrumentation
• Number Of Circuits: 1
• Output Type: Rail-to-Rail
• Slew Rate: 0.3 V/µs
• -3db Bandwidth: 200kHz
• Current - Input Bias: 12nA
• Voltage - Input Offset: 250µV
• Current - Supply: 650µA
• Voltage - Supply, Single/dual (±): 3 V ~ 24 V, ±2 V ~ 12 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Peak Reflow Compatible (260 C): No
• Rohs Compliant: No
• Input Offset Voltage: 250µV
• Bandwidth: 200kHz
• Amplifier Output: Rail To Rail
• Cmrr: 90dB
• Supply Current: 350µA
• Amplifier Case Style: MSOP
• No. Of Pins: 8
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output / Channel: -
• Gain Bandwidth Product: -
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant, Contains lead / RoHS non-compliant

AD8223
THEORY OF OPERATION
AMPLIFIER ARCHITECTURE
The AD8223 is an instrumentation amplifier based on a
classic 3-op amp approach, modified to ensure operation
even at common-mode voltages at the negative supply rail.
The architecture allows lower voltage offsets, better CMRR,
and higher gain accuracy than competing instrumentation
amplifiers in its class.
Figure 31 shows a simplified schematic of the AD8223. The
AD8223 has three stages. In the first stage, the input signal is
applied to PNP transistors. These PNP transistors act as voltage
buffers and allow input voltages below ground. The second
stage consists of a pair of 8 kΩ resistors, the R
pair of amplifiers. This stage allows the amplification of the
AD8223 to be set with a single external resistor. The third stage
is a differential amplifier composed of an op amp, two 10 kΩ
resistors, and two 50 kΩ resistors. This stage removes the
common-mode signal and applies an additional gain of 5.
The transfer function of the AD8223 is
where:
V
G
OUT

INVERTING
INVERTING
5 
= G(V
GAIN
NON-
2
1
8
3
0 8
NEGATIVE SUPPLY
POSITIVE SUPPLY
R
IN+
kΩ
G
Figure 31. Simplified Schematic
− V
7
4
4
7
IN−
) + V
+
+
–
–
8kΩ
8kΩ
REF
10kΩ
10kΩ
–
+
50kΩ
50kΩ
G
resistor, and a
OUT
REF
6
5
Rev. 0 | Page 14 of 20
GAIN SELECTION
Placing a resistor across the R
AD8223, which can be calculated by referring to Table 7 or by
using the following gain equation:
Table 7. Gains Achieved Using 1% Resistors
1% Standard Table
Value of R
26.7 k
15.8 k
5.36 k
2.26 k
1.78 k
845
412
162
80.6
The AD8223 defaults to G = 5 when no gain resistor is used. Add
the tolerance and gain drift of the R
of the AD8223 to determine the total gain accuracy of the system.
When the gain resistor is not used, gain depends only on
internal resistor matching, so gain error and gain drift are
minimal.
INPUT VOLTAGE RANGE
The 3-op amp architecture of the AD8223 applies gain and then
removes the common-mode voltage. Therefore, internal nodes
in the AD8223 experience a combination of both the gained
signal and the common-mode signal. This combined signal can
be limited by the voltage supplies even when the individual input
and output signals are not. To determine whether the signal can be
limited, refer to Figure 18 through Figure 21. Alternatively, use
the parameters in the Specifications section to verify that the input
and output are not limited and then use the following formula to
make sure the internal nodes are not limited.
To check if it is limited by the internal nodes,
If more common-mode range is required, a solution is to apply less
gain in the instrumentation amplifier and more in a later stage.
R

V
G
S


G
80
. 0
G
(Ω)
01

kΩ
V
5

0
6 .

V
CM
Desired Gain
8
10
20
40
50
100
200
500
1000

G
V
terminals sets the gain of the
DIFF
G
resistor to the specifications
10

Gain

Calculated Gain
7.99
10.1
19.9
40.4
49.9
99.7
199
499
998

V
S

0
1 .
V