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AD8278 Datasheet(PDF) 16 Page - Analog Devices

Part No. AD8278
Description  Low Power, Wide Supply Range, Low Cost Difference Amplifier, G = ½, 2
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD8278 Datasheet(HTML) 16 Page - Analog Devices

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AD8278
Rev. 0 | Page 16 of 24
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD8278 consists of a low power, low noise op amp and
four laser-trimmed on-chip resistors. These resistors can be
externally connected to make a variety of amplifier confi-
gurations, including difference, noninverting, and inverting
configurations. Taking advantage of the integrated resistors
of the AD8278 provides the designer with several benefits
over a discrete design, including smaller size, lower cost, and
better ac and dc performance.
2
5
3
1
6
7
4
40kΩ
20kΩ
40kΩ
–VS
+VS
–IN
+IN
SENSE
OUT
REF
20kΩ
AD8278
Figure 45. Functional Block Diagram
DC Performance
Much of the dc performance of op amp circuits depends on the
accuracy of the surrounding resistors. Using superposition to
analyze a typical difference amplifier circuit, as is shown in
Figure 46, the output voltage is found to be
⎛ +
+
=
+
R3
R4
V
R3
R4
R2
R1
R2
V
V
IN
IN
OUT
1
This equation demonstrates that the gain accuracy and common-
mode rejection ratio of the AD8278 is determined primarily by
the matching of resistor ratios. Even a 0.1% mismatch in one
resistor degrades the CMRR to 69 dB for a G = 2 difference
amplifier.
The difference amplifier output voltage equation can be reduced to
()
+
=
IN
IN
OUT
V
V
R3
R4
V
as long as the following ratio of the resistors is tightly matched:
R3
R4
R1
R2 =
The resistors on the AD8278 are laser trimmed to match accurately.
As a result, the AD8278 provides superior performance over a
discrete solution, enabling better CMRR, gain accuracy, and
gain drift, even over a wide temperature range.
AC Performance
Component sizes and trace lengths are much smaller in an IC
than on a PCB, so the corresponding parasitic elements are also
smaller. This results in better ac performance of the AD8278.
For example, the positive and negative input terminals of the
AD8278 op amp are intentionally not pinned out. By not
connecting these nodes to the traces on the PCB, their capacitance
remains low and balanced, resulting in improved loop stability
and excellent common-mode rejection over frequency.
DRIVING THE AD8278
Care should be taken to drive the AD8278 with a low impedance
source: for example, another amplifier. Source resistance of even
a few kilohms (kΩ) can unbalance the resistor ratios and,
therefore, significantly degrade the gain accuracy and common-
mode rejection of the AD8278. Because all configurations present
several kilohms (kΩ) of input resistance, the AD8278 does not
require a high current drive from the source and so is easy to
drive.
INPUT VOLTAGE RANGE
The AD8278 is able to measure input voltages beyond the supply
rails. The internal resistors divide down the voltage before it
reaches the internal op amp, and provide protection to the op
amp inputs. Figure 46 shows an example of how the voltage
division works in a difference amplifier configuration. For the
AD8278 to measure correctly, the input voltages at the input
nodes of the internal op amp must stay below 1.5 V of the
positive supply rail and can exceed the negative supply rail by
0.1 V. Refer to the Power Supplies section for more details.
R4
VIN+
VIN–
R3
R1
R2
R2
R1 + R2
(VIN+)
R2
R1 + R2
(VIN+)
Figure 46. Voltage Division in the Difference Amplifier Configuration
The AD8278 has integrated ESD diodes at the inputs that provide
overvoltage protection. This feature simplifies system design by
eliminating the need for additional external protection circuitry,
and enables a more robust system.
The voltages at any of the inputs of the parts can safely range
from +VS − 40 V up to −VS + 40 V. For example, on ±10 V
supplies, input voltages can go as high as ±30 V. Care should be
taken to not exceed the +VS − 40 V to −VS + 40 V input limits
to avoid risking damage to the parts.


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