AD8293G80/AD8293G160

Rev. 0 | Page 11 of 16

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

0.1µF

+5V

1

8

7

5

6

4

ADC OUT

OUTPUT TO ADC

WITH ANTIALIASING

FILTER

FILT

–IN

+IN

R1

4kΩ

R2

320kΩ

R3

5kΩ

100kΩ

100kΩ

C3

39nF

OUT

C2

680pF

AD8293G160

REF

GND

3

2

IN-AMP

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

The REF pin current is approximately 10 pA, and as a result, an

external buffer is not required.

1µF

0.1µF

0.1µF

VOLTAGE

REFERENCE

+5V

0.1µF

1

8

7

5

6

4

ADC OUT

OUTPUT

FILT

–IN

+IN

R1

4kΩ

R2

R3

5kΩ

OUT

C2

AD8293Gxx

REF

GND

3

2

IN-AMP

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

R2 (kΩ)

AD8293G80

160

AD8293G160

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)