AD8276/AD8277 Data Sheet
Rev. C | Page 14 of 20
THEORY OF OPERATION
Each channel of the AD8276/AD8277 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
configurations, including difference, noninverting, and inverting
configurations. Taking advantage of the integrated resistors of
the AD8276/AD8277 provides the designer with several benefits
over a discrete design, including smaller size, lower cost, and
better ac and dc performance.
Figure 38. Functional Block Diagram
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 39, the output voltage is found to be
This equation demonstrates that the gain accuracy and common-
mode rejection ratio of the AD8276/AD8277 is determined
primarily by the matching of resistor ratios. Even a 0.1% mismatch
in one resistor degrades the CMRR to 66 dB for a G = 1 difference
The difference amplifier output voltage equation can be reduced to
as long as the following ratio of the resistors is tightly matched:
The resistors on the AD8276/AD8277 are laser trimmed to match
accurately. As a result, the AD8276/AD8277 provide superior
performance over a discrete solution, enabling better CMRR,
gain accuracy, and gain drift, even over a wide temperature range.
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 AD8276/
AD8277. For example, the positive and negative input terminals
of the AD8276/AD8277 op amps are intentionally not pinned
out. By not connecting these nodes to the traces on the PCB, the
capacitance remains low, resulting in improved loop stability
and excellent common-mode rejection over frequency.
DRIVING THE AD8276/AD8277
Care should be taken to drive the AD8276/AD8277 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 AD8276/AD8277. Because all
configurations present several kilohms of input resistance, the
AD8276/AD8277 do not require a high current drive from the
source and so are easy to drive.
INPUT VOLTAGE RANGE
The AD8276/AD8277 are 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 39 shows an example of how the
voltage division works in a difference amplifier configuration.
For the AD8276/AD8277 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.
R1 + R2 (V
R1 + R2 (V
Figure 39. Voltage Division in the Difference Amplifier Configuration
The AD8276/AD8277 have 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.