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AD8293G80 Datasheet PDF : 16 Pages
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APPLICATIONS INFORMATION
OVERVIEW
The AD8293G80/AD8293G160 reduce board area by integrating
filter components, such as Resistors R1, R2, and R3, as shown in
Figure 19. Two outputs are available to the user: OUT (Pin 6) and
ADC OUT (Pin 4). The difference between the two is the inclusion
of a series 5 kΩ resistor at ADC OUT. With the addition of an
external capacitor, C3, ADC OUT forms a second filter, comprising
of the 5 kΩ resistor and C3, which can be used as an ADC anti-
aliasing filter. In contrast, OUT is the direct output of the instru-
mentation amplifier. When using the antialiasing filter, there is
slightly less switching ripple at ADC OUT than when obtaining
the signal directly from OUT.
+5V
0.1µF
C2
680pF
7
+VS
5
6
FILT OUT
+IN
8
R1
4kΩ
–IN
1
R2
320kΩ
IN-AMP
OUTPUT TO ADC
R3
WITH ANTIALIASING
5kΩ
FILTER
4
ADC OUT
C3
39nF
GND REF
2
3
AD8293G160
+5V
100kΩ
0.1µF 100kΩ
Figure 19. AD8293G160 with Antialiasing Filter and Level-Shifted Output
(Using the Resistor Divider at the REF Pin, the Output Is Biased at 2.5 V)
REFERENCE CONNECTION
Unlike traditional 3-op-amp instrumentation amplifiers, parasitic
resistance in series with REF (Pin 3) does not degrade CMR
performance. The AD8293G80/AD8293G160 can attain extremely
high CMR performance without the use of an external buffer
amplifier to drive the REF pin, which is required by industry-
standard instrumentation amplifiers. Reducing the need for
buffer amplifiers to drive the REF pin helps to save valuable
printed circuit board (PCB) space and minimizes system costs.
For optimal performance in single-supply applications, REF
should be set with a low noise precision voltage reference, such
as the ADR44x (see Figure 20). However, for a lower system cost,
the reference voltage can be set with a simple resistor voltage
divider between the supply and GND (see Figure 19). This
configuration results in degraded output offset performance if
the resistors deviate from their ideal values. In dual-supply
applications, VREF can simply be connected to GND.
The REF pin current is approximately 10 pA, and as a result, an
external buffer is not required.
AD8293G80/AD8293G160
+5V
0.1µF
7
+VS
C2
OUTPUT
5
6
FILT OUT
+IN
8
R1
4kΩ
–IN
1
R2
IN-AMP
R3
5kΩ
4
ADC OUT
GND REF
2
3
AD8293Gxx
0.1µF
1µF
VOLTAGE
REFERENCE
0.1µF
Figure 20. Operating on a Single Supply Using an External Voltage Reference
(The Output Can Be Used Without an Antialiasing Filter if the Signal
Bandwidth Is <10 Hz)
OUTPUT FILTERING
The output of the AD8293G80/AD8293G160 can be filtered to
reduce switching ripple. Two filters can be used in conjunction
to set the filter frequency. In the example that follows, two 700 Hz
filters are used in conjunction to form a 500 Hz (recommended)
bandwidth. Because the filter resistors are integrated in the
AD8293G80/AD8293G160, only external capacitors are needed
to set the filter frequencies.
The primary filter is needed to limit the amount of switching
noise at the output. Regardless of the output that is being used,
OUT or ADC OUT, the primary filter comprising R2 and C2
must be implemented. The R2 value depends on the model; Table 7
shows the R2 value for each model.
Table 7. Internal R2 Values
Model
AD8293G80
AD8293G160
R2 (kΩ)
160
320
The following equation results in the C2 value needed to set a
700 Hz primary filter. For a gain of 160, substitute R2 with
320 kΩ; for a gain of 80, substitute R2 with 160 kΩ.
C2 = 1/(700 × 2 × π × R2)
Adding an external capacitor, C3, and measuring the output from
ADC OUT further reduces the correction ripple. The internal
5 kΩ resistor, labeled R3 in Figure 18, forms a low-pass filter
with C3. This low-pass filter is the secondary filter. Set to
700 Hz, the secondary filter equation for C3 is as follows:
C3 = 1/(700 × 2 × π × 5 kΩ)
Rev. 0 | Page 11 of 16
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