AD8273
Rev. B | Page 12 of 16
THEORY OF OPERATION
The AD8273 has two channels, each consisting of a high
precision, low distortion op amp and four trimmed resistors.
Although such a circuit can be built discretely, placing the
resistors on the chip offers advantages to board designers that
include better dc specifications, better ac specification, and
lower production costs.
The resistors on the AD8273 are laser trimmed and tightly
matched. Specifications that depend on the resistor matching,
such as gain drift, common-mode rejection, and gain accuracy,
are better than can be achieved with standard discrete resistors.
The positive and negative input terminals of the AD8273 op amp
are not pinned out intentionally. Keeping these nodes internal
means their capacitance is considerably lower than it would be
in discrete designs. Lower capacitance at these nodes means
better loop stability and improved common-mode rejection vs.
frequency.
The internal resistors of the AD8273 lower production costs.
One part rather than several is placed on the board, which
improves both board build time and reliability.
CONFIGURATIONS
The AD8273 can be configured in several different ways; see
Figure 39 to Figure 46. Because these configurations rely on the
internal, matched resistors, these configurations have excellent
gain accuracy and gain drift.
POWER SUPPLIES
Use a stable dc voltage to power the AD8273. Noise on the
supply pins can adversely affect performance. Place a bypass
capacitor of 0.1 μF between each supply pin and ground, as
close to each pin as possible. Also, use a tantalum capacitor of
10 μF between each supply and ground. It can be farther away
from the AD8273 and typically can be shared by other precision
integrated circuits.
The AD8273 is specified at ±15 V, but it can be used with
unbalanced supplies as well, for example, −V
S
= 0 V, +V
S
= 20 V.
The difference between the two supplies must be kept below 36 V.
06981-012
12kΩ
2
3
12
14
13
6kΩ
12kΩ 6kΩ
–IN1
OUT1
+IN1
12kΩ
6
5
10
8
9
6kΩ
12kΩ 6kΩ
–IN2
OUT2
+IN2
V
OUT
= ½ (V
IN+
− V
IN−
)
Figure 39. Difference Amplifier, G = ½
6981-016
6kΩ
3
2
14
13
12kΩ
6kΩ 12kΩ
–IN1
OUT1
+IN1
6kΩ
6
5
10
8
9
12kΩ
6kΩ 12kΩ
–IN2
OUT2
+IN2
12
V
OUT
= 2 (V
IN+
− V
IN−
)
Figure 40. Difference Amplifier, G = 2
06981-013
12kΩ
2
3
12
14
13
6kΩ
12kΩ
6kΩ
IN1
OUT1
5
8
12kΩ
6kΩ
12kΩ
610
9
6kΩ
IN2
OUT2
V
OUT
= −½ V
IN
Figure 41. Inverting Amplifier, G = ½
06981-017
6kΩ
2
3
12
14
13
12kΩ
6kΩ
12kΩ
IN1
OUT1
5
8
6kΩ
12kΩ
6kΩ
610
9
12kΩ
IN2
OUT2
V
OUT
= −2 V
IN
Figure 42. Inverting Amplifier, G = 2
