AD8231

Data Sheet

Rev. E | Page 22 of 24

When operating the AD8231 on dual supplies, a level-shift is

typically needed from standard single-supply control logic. One

easy way to accomplish the level-shift is through a single-pole,

double-throw switch, such as the ADG633. Figure 55 shows an

application schematic for ±2.5 V operation.

+2.5V

–2.5V

+2.5V

–2.5V

+2.5V

–2.5V

A0

A1

A2

EN

GND

–2.5V

GND

DIGITAL

CONTROL

(FPGA,

MICROCONTROLLER,

ETC.)

ADG633

AD8231

A0 A1 A2

SDN

CS

+2.5V

ANY VOLTAGE BETWEEN 2.5V AND 9.5V.

–2.5V

Figure 55. Converting Single-Supply Control Signals to Dual Supply.

SALLEN KEY FILTER

The extra op amp in the AD8231 can be used to create a 2-pole

Sallen Key filter. Such a filter can remove excess noise or

perform antialiasing before an analog-to-digital converter.

Figure 56 shows how to create a 2-pole low-pass Butterworth

filter. Components R1, R2, C1, and C2 set the frequency of the

filter. The ratio of R3 and R4 sets the peaking of the filter. If R4

equals 10 kΩ, R3 should equal 5.9 kΩ for an optimum 2-pole

response.

Depending on the circuitry before and after the AD8231,

a 3-pole filter can be possible. If the previous stage has a small

output impedance, an additional pole can be added before the

in amp (R6, R7, and C4). If the following stage has a high input

impedance, an additional pole can be added after the op amp

(R5 and C3). Peaking from the Sallen Key stage should be

higher to compensate for the extra attenuation of the third pole;

both R3 and R4 should be 10 kΩ for optimum response.

Note that in addition to setting the peaking of the filter, the

ratio R3/R4 also sets the dc gain: G = 1 + R3/R4. If lower dc

gain is required, replace R1 with a voltage divider, where the

output resistance of the divider is equal to the required value of R1.

Figure 56 shows a bias point connected to R4 and the in-amp

reference. The filter stage amplifies the signal around this bias

point. The bias point is typically midsupply and should be low

impedance.

Table 10. Recommended Component Values for Butterworth

Low-Pass Filter in Figure 56

3 dB

Freq

Sallen Key

Optional Poles

Before In-Amp

After Op Amp

R1, R2

(kΩ)

C1, C2

(nF)

R6, R7

(kΩ)

C4

(nF)

R5

(kΩ)

C3

(nF)

32 Hz

499

10

499

4.7

49.9

100

100 Hz

158

10

158

4.7

16

100

320 Hz

49.9

10

49.9

4.7

4.99

100

1 kHz

158

1

158

0.47

1.6

100

3.2 kHz

49.9

1

49.9

0.47

0.499

100

10 kHz

15.8

1

15.8

0.47

0.16

100

32 kHz

4.99

1

4.99

0.47

0.049

100

IN-AMP

OP AMP

REF

BIAS

POINT

BIAS

POINT

R7

R1

R2

R6

R5

C4

SALLEN KEY

(TWO POLE)

OPTIONAL POLE

OPTIONAL POLE

C3

R3

R4

C2

C1

Figure 56. Butterworth Low-Pass Filter (Dotted Sections Indicate Optional Poles)