AD8274
Rev. C | Page 12 of 16
THEORY OF OPERATION
2
5
3 1
6
7
4
12kΩ 6kΩ
12kΩ
–V
S
+
S
6kΩ
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Figure 38. Functional Block Diagram
CIRCUIT INFORMATION
The AD8274 consists of a high precision, low distortion op amp
and four trimmed resistors. These resistors can be connected to
make a wide variety of amplifier configurations, including
difference, noninverting, and inverting configurations. Using
the on-chip resistors of the AD8274 provides the designer with
several advantages over a discrete design.
DC Performance
Much of the dc performance of op amp circuits depends on the
accuracy of the surrounding resistors. The resistors on the AD8274
are laid out to be tightly matched. The resistors of each part are
laser trimmed and tested for their matching accuracy. Because
of this trimming and testing, the AD8274 can guarantee high
accuracy for specifications such as gain drift, common-mode
rejection, and gain error.
AC Performance
Because feature size is much smaller in an integrated circuit than
on a printed circuit board (PCB), the corresponding parasitics are
also smaller. The smaller feature size helps the ac performance of
the AD8274. For example, the positive and negative input terminals
of the AD8274 op amp are not pinned out intentionally. By not
connecting these nodes to the traces on the PCB, the capacitance
remains low, resulting in both improved loop stability and
common-mode rejection over frequency.
Production Costs
Because one part, rather than several, is placed on the PCB, the
board can be built more quickly.
Size
The AD8274 fits a precision op amp and four resistors in one
8-lead MSOP or SOIC package.
DRIVING THE AD8274
The AD8274 is easy to drive, with all configurations presenting
at least several kilohms (kΩ) of input resistance. The AD8274
should be driven with a low impedance source: for example,
another amplifier. The gain accuracy and common-mode rejection
of the AD8274 depend on the matching of its resistors. Even
source resistance of a few ohms can have a substantial effect on
these specifications.
POWER SUPPLIES
A stable dc voltage should be used to power the AD8274. Noise
on the supply pins can adversely affect performance. A bypass
capacitor of 0.1 µF should be placed between each supply pin
and ground, as close as possible to each supply pin. A tantalum
capacitor of 10 µF should also be used between each supply and
ground. It can be farther away from the supply pins and, typically,
it can be shared by other precision integrated circuits.
The AD8274 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.
INPUT VOLTAGE RANGE
The AD8274 can measure voltages beyond the rails. For the G = ½
and G = 2 difference amplifier configurations, see the input voltage
range in Table 2 for specifications.
The AD8274 is able to measure beyond the rail because the
internal resistors divide down the voltage before it reaches the
internal op amp. Figure 39 shows an example of how the voltage
division works in the difference amplifier configuration. For the
AD8274 to measure correctly, the input voltages at the internal
op amp must stay within 1.5 V of either supply rail.
07362-061
R4
R3
R1
R2
R2
R1 + R2
(V
IN+
)
R2
R1 + R2
(V
IN+
)
Figure 39. Voltage Division in the Difference Amplifier Configuration
For best long-term reliability of the part, voltages at any of the
part’s inputs (Pin 1, Pin 2, Pin 3, or Pin 5) should stay within
+V
S
– 40 V to −V
S
+ 40 V. For example, on ±10 V supplies,
input voltages should not exceed ±30 V.
