Datasheet

AD8426
Rev. 0 | Page 25 of 28
APPLICATIONS INFORMATION
PRECISION STRAIN GAGE
The low offset and high CMRR over frequency of the AD8426
make it an excellent candidate for bridge measurements. The
bridge can be connected directly to the inputs of the amplifier
(see Figure 68).
5
V
2.5V
10µF 0.1µF
AD8426
+IN
–IN
R
G
350
350350
350
+
–
09490-010
Figure 68. Precision Strain Gage
DIFFERENTIAL DRIVE
The differential output configuration of the AD8426 has the
same excellent dc precision specifications as the single-ended
output configuration.
Differential Output Using Both AD8426 Amplifiers
The circuit configuration is shown in Figure 69. The differential
output specifications in Table 2, Table 4, Table 5, and Table 7
refer to this configuration only. The circuit includes an RC filter
that maintains the stability of the loop.
+IN1
–IN1
AD8426
+
–
AD8426
+
–
100pF
+INx
V
OUT–
V
OUT+
10k
REF2
09490-163
R
G
Figure 69. Differential Circuit Schematic
The differential output voltage is set by the following equation:
V
DIFF_OUT
= V
OUT+
− V
OUT−
= G × (V
IN+
− V
IN−
)
where:
G
R
G
Ω
+=
k4.49
1
The common-mode output voltage is set by the average of +IN2
and REF2. The transfer function is
V
CM_OUT
= (V
OUT+
+ V
OUT−
)/2 = (V
+IN2
+ V
REF2
)/2
A common application sets the common-mode output voltage
to the midscale of a differential ADC. In this case, the ADC
reference voltage is sent to the +IN2 terminal, and ground is
connected to the REF2 terminal. This produces a common-
mode output voltage of half the ADC reference voltage.
2-Channel Differential Output Using a Dual Op Amp
Another differential output topology is shown in Figure 70.
Instead of a second in-amp, one-half of a dual op amp creates
the inverted output. The recommended dual op amps (the
AD8642 and the AD822) are packaged in an MSOP. This
configuration allows the creation of a dual-channel, precision
differential output in-amp with little board area.
Figure 70 shows how to configure the AD8426 for differential
output.
+IN
–IN
REF
AD8426
V
BIAS
R
+
–
OP AMP
V
OUT+
V
OUT–
R
RECOMMENDED OP AMPS: AD8642, AD822.
RECOMMENDED R VALUES: 5k TO 20k.
09490-009
Figure 70. Differential Output Using an Op Amp
The differential output voltage is set by the following equation:
V
DIFF_OUT
= V
OUT+
− V
OUT−
= G × (V
IN+
− V
IN−
)
where:
G
R
G
Ω
+=
k4.49
1
The common-mode output voltage is set by the following
equation:
V
CM_OUT
= (V
OUT+
− V
OUT−
)/2 = V
BIAS
The advantage of this circuit is that the dc differential accuracy
depends on the AD8426 and not on the op amp or the resistors.
This circuit takes advantage of the precise control of the AD8426
over its output voltage relative to the reference voltage. Op amp
dc performance and resistor matching do 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.
For best ac performance, an op amp with gain bandwidth of at
least 2 MHz and a slew rate of at least 1 V/µs is recommended.
Good choices for op amps are the AD8642 and the AD822.