Datasheet

AD8228
Rev. 0 | Page 19 of 24
APPLICATIONS INFORMATION
DIFFERENTIAL DRIVE
Figure 48 shows how to configure the AD8228 for differential
output. The advantage of this circuit is that the dc differential
accuracy depends on the AD8228 and not on the op amp or the
resistors. This circuit takes advantage of the precise control the
AD8228 has of its output voltage relative to the reference voltage.
The ideal equation for the differential output is as follows:
V
DIFF_OUT
= V
OUT+
− V
OUT−
= Gain × (V
IN+
− V
IN−
)
Op amp dc performance and resistor matching determine 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. The ideal equation for the common-mode
output is as follows:
V
CM_OUT
=
2
−+
+
OUTOUT
VV
= V
REF
For best ac performance, an op amp with at least 3 MHz gain
bandwidth product and 2 V/µs slew rate is recommended.
+IN
–IN
REF
AD8228
V
REF
10kΩ
+
–
AD8641
+OUT
–OUT
07035-017
10kΩ
Figure 48. Differential Output Using an Op Amp
PRECISION STRAIN GAGE
The low offset and high CMRR over frequency of the AD8228
make it an excellent candidate for bridge measurements. As shown
in
Figure 49, the bridge can be connected directly to the inputs
of the amplifier.
5
V
2.5V
07035-011
10µF 0.1µF
AD8228
+IN
–IN
350Ω
350Ω350Ω
350Ω
+
–
Figure 49. Precision Strain Gage
DRIVING A DIFFERENTIAL ADC
Figure 50 shows how the AD8228 can be used to drive a
differential ADC. The AD8228 is configured with an op amp and
two resistors for differential drive. The 510  resistors and 2200
pF capacitors isolate the instrumentation amplifier from the
switching transients produced by the switched capacitor front
end of a typical SAR converter. These components between the
ADC and the amplifier also create a filter at 142 kHz, which
provides antialiasing and noise filtering. The advantage of this
configuration is that it uses less power than a dedicated ADC
driver: the
AD8641 typically consumes 200 µA, and the current
through the two 10 kΩ resistors is 250 µA at full output voltage.
With the
AD7688, this configuration gives excellent dc perform-
ance and a THD of 71 dB (10 kHz input). For applications that
need better distortion performance, a dedicated ADC driver, such
as the
ADA4941-1 or ADA4922-1, is recommended.
07035-032
IN+
VDDREF
GND
AD7688
IN–
0.1µF
+5V
ADR435
GND
V
IN
V
OUT
0.1µF
+8
V
0.1µF
10µF
X5R
10kΩ
10kΩ
0.1µF
510Ω
0.1µF
510Ω
0.1µF
0.1µF
AD8228
+
IN
–
IN
REF
+8V
–8V
0.1µF
0.1µF
–8V
+8V
AD8641
10kΩ
10kΩ
Figure 50. Driving a Differential ADC