Datasheet

AD8278/AD8279
Rev. C | Page 19 of 24
The reference must be driven with a low impedance source to
maintain the internal resistor ratio. An example using the low
power, low noise OP1177 as a reference is shown in Figure 58.
INCORRECT
V
CORRECT
AD8278
OP1177
+
–
V
REF
AD8278
REF
0
8308-054
Figure 58. Driving the Reference Pin
DIFFERENTIAL OUTPUT
The two difference amplifiers of the AD8279 can be configured
to provide a differential output, as shown in Figure 59. 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 twice that
of a single AD8279 channel, as shown in the following equation:
V
DIFF_OUT
= V
+OUT
− V
−OUT
= 2 × G
AD8279
× (V
IN+
– V
IN−
)
If the AD8279 amplifiers are each configured for G = ½, the
differential gain is 1×; if the AD8279 amplifiers are each
configured for G = 2, the differential gain is 4×.
08308-061
12
2
14 3
13
11
20k 40k
20k
+VS
–IN
+IN
+OUT
40k
AD8279
10
6
8 5
9
4
20k 40k
20k
–VS
40k
–OUT
Figure 59. AD8279 Differential Output G = 4 Configuration
INSTRUMENTATION AMPLIFIER
The AD8278 and AD8279 can be used as building blocks for a
low power, low cost instrumentation amplifier. An instrumentation
amplifier provides high impedance inputs and delivers high
common-mode rejection. Combining the AD8278 with an Analog
Devices, Inc., low power amplifier (see Table 11) creates a precise,
power efficient voltage measurement solution suitable for power
critical systems.
R
G
R
F
R
F
–IN
+IN
A1
A2
AD8278/
AD8279
40k
20k
20k
40k
REF
V
OUT
V
OUT
= (1 + 2R
F
/R
G
) (V
IN+
– V
IN–
) × 2
08308-056
Figure 60. Low Power Precision Instrumentation Amplifier
Table 11. Low Power Op Amps
Op Amp (A1, A2) Features
AD8506 Dual micropower op amp
AD8607 Precision dual micropower op amp
AD8617 Low cost CMOS micropower op amp
AD8667 Dual precision CMOS micropower op amp
It is preferable to use dual op amps for the high impedance inputs
because they have better matched performance and track each
other over temperature. The AD8278 and AD8279 difference
amplifiers cancel out common-mode errors from the input op
amps, if they track each other. The differential gain accuracy of
the in-amp is proportional to how well the input feedback
resistors (R
F
) match each other. The CMRR of the in-amp
increases as the differential gain is increased (1 + 2R
F
/R
G
), but a
higher gain also reduces the common-mode voltage range.
Refer to
A Designer’s Guide to Instrumentation Amplifiers for
more design ideas and considerations at www.analog.com,
under Technical Documentation.