AD8233

Data Sheet

Rev. 0 | Page 24 of 29

Table 6. Comparison of High-Pass Filtering Options

Figure to

Reference

Filter

Order

Component Count

Low Frequency Rejection

Capacitor

Sizes/Values

Signal

Output

Figure 61

1

2

Good

Large

Low

Low

Figure 63

2

4

Better

Large

Medium

Higher

Figure 64

2

5

Better

Smaller

Medium

Low

Figure 65

3

7

Best

Smaller

Highest

Higher

of values for a low-pass filter, as explained in the Low-Pass Filtering and Gain section.

The design of the high-pass filter involves trade-offs between

signal distortion, component count, low frequency rejection,

and component size. For example, a single-pole, high-pass filter

results in the least distortion to the signal, but the associated

rejection of low frequency artifacts is the lowest of the available

filter options. Table 6 compares the recommended filtering options.

LOW-PASS FILTERING AND GAIN

The AD8233 includes an uncommitted op amp that can be used

for extra gain and filtering. For applications that do not require

a high order filter, a simple RC low-pass filter is sufficient, and

the op amp can buffer or further amplify the signal.

REFOUT

FILTERED

SIGNAL

A1

FROM IN-AMP

STAGE

C

R

Figure 67. Schematic for a Single-Pole, Low-Pass Filter and Additional Gain

A Sallen-Key filter topology can be implemented for applications

that require a steeper roll-off or a sharper cutoff frequency, as

shown in Figure 68.

REFOUT

FILTERED

SIGNAL

A1

FROM IN-AMP

STAGE

C2

C1

R2

R3

R4

R1

Figure 68. Schematic for a Two-Pole, Low-Pass Filter

The following equations describe the low-pass cutoff frequency

Gain = 1 + R3/R4

Q =

)

1

(

Gain

C1

R1

C2

R2

C2

R1

C2

R2

C1

R1



















Note that changing the gain has an effect on Q and vice versa.

Common values for Q are 0.5, to avoid peaking, or 0.7 for max-

imum flatness and a sharp cutoff frequency. Use a high Q value

in narrow-band applications to increase peaking and the

selectivity of the band-pass filter.

A common design procedure is to set R1 = R2 = R and C1 = C2 =

C, simplifying the expressions for the cutoff frequency and Q to

Q =

Gain



3

1

Note that Q can be controlled by setting the gain with R3 and

R4; however, this limits the gain to be less than 3. For gain

values equal to or greater than 3, the circuit becomes unstable.

A simple modification that allows higher gains is to make the

value of C2 at least four times larger than C1.

Note that these design equations only hold true in a case where

the output impedance of the previous stage is much lower than

the input impedance of the Sallen-Key filter. The design equations

do not hold true when using an ac coupling network between

the instrumentation amplifier output and the input of the low-

pass filter without a buffer.

To connect these two filtering stages properly without a buffer,

make the value of R1 at least 10 times larger than the resistor of

the ac coupling network (labeled as R2 in Figure 63).