AD8227 Datasheet, PDF (22/24 Pages) Analog Devices – Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier

AD8227
APPLICATIONS INFORMATION
DIFFERENTIAL DRIVE
Figure 64 shows how to configure the AD8227 for differential
output.
+IN
AD8227
–IN
REF
R
+OUT
V_BIAS
–
+
R
OP AMP
–OUT
RECOMMENDED OP AMPS: AD8515, AD8641, AD820.
RECOMMENDED R VALUES: 5kΩ to 20kΩ.
Figure 64. Differential Output Using an Op Amp
The differential output is set by the following equation:
VDIFF_OUT = VOUT+ − VOUT− = Gain × (VIN+ − VIN−)
The common-mode output is set by the following equation:
VCM_OUT = (VOUT+ − VOUT−)/2 = VBIAS
The advantage of this circuit is that the dc differential accuracy
depends on the AD8227 and not on the op amp or the resistors.
This circuit takes advantage of the AD8227’s precise control of
its output voltage relative to the reference voltage. Op amp dc
performance and resistor matching affect the dc common-mode
output accuracy. However, because common-mode errors are
likely to be rejected by the next device in the signal chain, these
errors typically have little effect on overall system accuracy.
Tips for Best Differential Output Performance
For best ac performance, an op amp with at least 2 MHz gain
bandwidth and 1 V/μs slew rate is recommended. Good choices
for op amps are the AD8641, AD8515, or AD820.
Keep trace lengths from resistors to the inverting terminal of
the op amp as short as possible. Excessive capacitance at this
node can cause the circuit to be unstable. If capacitance cannot
be avoided, use lower value resistors.
PRECISION STRAIN GAGE
The low offset and high CMRR over frequency of the AD8227
make it an excellent choice for bridge measurements. The
bridge can be connected directly to the inputs of the amplifier
(see Figure 65).
5V
10µF 0.1µF
350Ω
350Ω
350Ω
350Ω
+IN ₊
R_G AD8227
–
–IN
2.5V
Figure 65. Precision Strain Gage
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