Datasheet

Data Sheet AD8276/AD8277
Rev. C | Page 17 of 20
The differential output voltage and common-mode voltage of
the AD8226 is shown in the following equations:
V
DIFF_OUT
= V
+OUT
− V
−OUT
= Gain
AD8226
× (V
+IN
– V
−IN
)
V
CM
= (V
S+
− V
S−
)/2 = V
BIAS
Refer to the AD8226 data sheet for additional information.
07692-056
2
12
3
14
13
11
40kΩ 40kΩ
40kΩ
+VS
–IN
+IN
+OUT
40kΩ
AD8277
6
10
5
8
9
4
40kΩ 40kΩ
40kΩ
–VS
40kΩ
–OUT
Figure 48. AD8277 Differential Output Configuration
The two difference amplifiers of the AD8277 can be configured
to provide a differential output, as shown in Figure 48. This
differential output configuration is suitable for various applications,
such as strain gage excitation and single-ended-to-differential
conversion. The differential output voltage has a gain of 2 as
shown in the following equation:
V
DIFF_OUT
= V
+OUT
− V
−OUT
= 2 × (V
+IN
– V
−IN
)
CURRENT SOURCE
The AD8276 difference amplifier can be implemented as part
of a voltage-to-current converter or a precision constant current
source as shown in Figure 49. Using an integrated precision
solution such as the AD8276 provides several advantages over
a discrete solution, including space-saving, improved gain accuracy,
and temperature drift. The internal resistors are tightly matched
to minimize error and temperature drift. If the external resistors,
R1 and R2, are not well-matched, they become a significant
source of error in the system, so precision resistors are recom-
mended to maintain performance. The ADR821 provides a
precision voltage reference and integrated op amp that also
reduces error in the signal chain.
The AD8276 has rail-to-rail output capability that allows higher
current outputs.
REF
1
2
3
4
5
10
9
8
7
6
V–
V+
ADR821
40kΩ
40kΩ
R
LOAD
R1
R2
2N3904
40kΩ
40kΩ
V+
7
4
5
6
2
3
1
AD8276
–2.5V
I
O
= 2.5V(1/40kΩ + 1/R1)
R1 = R2
07692-046
Figure 49. Constant Current Source
VOLTAGE AND CURRENT MONITORING
Voltage and current monitoring is critical in the following
applications: power line metering, power line protection, motor
control applications, and battery monitoring. The AD8276/
AD8277 can be used to monitor voltages and currents in a
system, as shown in Figure 50. As the signals monitored by the
AD8276/AD8277 rise above or drop below critical levels, a
circuit event can be triggered to correct the situation or raise
a warning.
OP1177
07692-057
I
1
R
AD8276
I
3
I
C
R
AD8276
V
1
R
AD8276
V
3
R
AD8276
V
C
R
AD8276
8:1
ADC
Figure 50.Voltage and Current Monitoring in 3-Phase Power Line Protection
Using the AD8276
Figure 50 shows an example of how the AD8276 can be used to
monitor voltage and current on a 3-phase power supply. I
1
through I
3
are the currents to be monitored, and V
1
through V
3
are the voltages to be monitored on each phase. I
C
and V
C
are
the common or zero lines. Couplers or transformers interface
the power lines to the front-end circuitry and provide
attenuation, isolation, and protection.
On the current monitoring side, current transformers (CTs)
step down the power-line current and isolate the front-end
circuitry from the high voltage and high current lines. Across
the inputs of each difference amplifier is a shunt resistor that
converts the coupled current into a voltage. The value of the