AD8270 Analog Devices, AD8270 Datasheet - Page 13

• Manufacturer Part Number: AD8270
• Description: Precision Dual-Channel, Difference Amplifier
• Manufacturer: Analog Devices

THEORY OF OPERATION
CIRCUIT INFORMATION
The AD8270 has two channels, each consisting of a high precision,
low distortion op amp and seven trimmed resistors. These resis-
tors can be connected to make a wide variety of amplifier
configurations: difference, noninverting, inverting, and more.
The resistors on the chip can be connected in parallel for a wider
range of options. Using the on-chip resistors of the AD8270
provides the designer 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 AD8270
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 AD8270 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 PCB board, the corresponding parasitics are smaller, as well.
The smaller feature size helps the ac performance of the AD8270.
For example, the positive and negative input terminals of the
AD8270 op amp are not pinned out intentionally. By not
connecting these nodes to the traces on the PCB board, 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
board, the board can be built more quickly.
+IN2A
+IN1A
–IN1A
–IN2A
1
2
3
4
10kΩ
10kΩ
10kΩ
10kΩ
Figure 40. Functional Block Diagram
20kΩ
_
+
10kΩ
20kΩ
AD8270
20kΩ
10kΩ
20kΩ
+
_
10kΩ
10kΩ
10kΩ
10kΩ
12 –IN1B
11 –IN2B
10 +IN2B
9 +IN1B
Rev. 0 | Page 13 of 20
Size
The AD8270 fits two op amps and 14 resistors in a 4 mm ×
4 mm package.
DRIVING THE AD8270
The AD8270 is easy to drive, with all configurations presenting
at least several kilohms (kΩ) of input resistance. The AD8270
should be driven with a low impedance source: for example,
another amplifier. The gain accuracy and common-mode rejection
of the AD8270 depend on the matching of its resistors. Even
source resistance of a few ohms can have a substantial effect on
these specifications.
PACKAGE CONSIDERATIONS
The AD8270 is packaged in a 4 mm × 4 mm LFCSP. Beware of
blindly copying the footprint from another 4 mm × 4 mm LFCSP
part; it may not have the same thermal pad size and leads. Refer
to the Outline Dimensions section to verify that the PCB symbol
has the correct dimensions.
The 4 mm × 4 mm LFCSP of the AD8270 comes with a thermal
pad. This pad is connected internally to −V
pad is not necessary for electrical performance; the pad can be
left unconnected or can be connected to the negative supply rail.
Connecting the pad to the negative supply rail is recommended
in high vibration applications or when good heat dissipation is
required (for example, with high ambient temperatures or when
driving heavy loads). For best heat dissipation performance, the
negative supply rail should be a plane in the board. See the
Absolute Maximum Ratings section for thermal coefficients
with and without the pad soldered.
Space between the leads and thermal pad should be as wide as
possible to minimize the risk of contaminants affecting perform-
ance. A thorough washing of the board is recommended after the
soldering process, especially if high accuracy performance is
required at high temperatures.
POWER SUPPLIES
A stable dc voltage should be used to power the AD8270. 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 AD8270 is specified at ±15 V and ±5 V, but it can be used with
unbalanced supplies, as well. For example, −V
The difference between the two supplies must be kept below 36 V.
S
. Connecting to this
S
= 0 V, +V
AD8270
S
= 20 V.