AD623 datasheet(19/20 Pages) AD

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Data Sheet

AD8428

Rev. A | Page 19 of 20

Notch Filter

In cases where the frequency of the interfering signal is well

known, a notch filter can be implemented to help minimize the

impact of the known signal on the measurement. The filter can

be realized by adding a series LC network between the filter

pins, as shown in Figure 48.

–

AD8428

OUT

+FIL

–FIL

Figure 48. Notch Filter Example

The inductor and capacitor form a resonant circuit that rejects

frequencies near the notch. The center frequency can be

calculated using the following equation:

The Q factor of the filter is given by the following equation:

6000

The accuracy of the center frequency, fN, depends only on the

tolerance of the capacitor and inductor values, not on the value

of the internal resistors. However, the Q of the circuit depends on

both the tolerance of the external components and the absolute

tolerance of the internal resistors, which is typically 10%.

The Q factor is a filter parameter that indicates how narrow the

notch filter is. It is defined as follows:

−

where fA and fB are the frequencies at which there is −3 dB

attenuation on each side of the notch.

This equation indicates that the higher the Q, the narrower the

notch—that is, high values of Q increase the selectivity of the

notch. In other words, although high values of Q reduce the

effect of the notch on the amplitude and phase in neighboring

frequencies, the ability to reject the undesired frequency may

also be reduced due to mismatch between it and the actual

center frequency. This mismatch can be caused by frequency

variations on the affecting source and the tolerance of the filter

inductor and capacitor values.

In contrast, low values of Q work better to ensure that the

interfering frequency is attenuated, but these low values also

affect the signal of interest if it is located close to the center

frequency of the notch.

For example, if the goal is to attenuate the interfering signal by

20 dB, a large Q value reduces the frequency range where the

notch is effective, as shown in Figure 49.

In contrast, a small Q value increases the range for the same

attenuation, which relaxes the tolerance requirements between

the inductor and capacitor and the frequency uncertainty of the

undesired signal. However, the lower Q value has a significant

effect on signal bandwidth one decade before the notch

frequency.

0.01fN

0.1fN

10fN

100fN

FREQUENCY (Hz)

Q = 0.1

Q = 1

–3dB

–20dB

Figure 49. Notch Filter Attenuation with Q = 0.1 and Q = 1

Around the Center Frequency

The maximum attenuation that can be achieved with a notch

filter is at its center frequency, fN. This maximum attenuation

(or depth of the notch) depends on the equivalent series

resistance of the inductor and capacitor at the center frequency.

Choosing components with high quality factors improves the

rejection at the filter’s center frequency. For information about

calculating the maximum allowed series resistance at the frequency

of interest to obtain the desired attenuation, see the Setting the

Amplifier to Different Gains section.

Extracting the Common-Mode Voltage of the Input

The common-mode signal present at the input terminals can be

extracted by inserting two resistors between the filter terminals

and tapping from the center, as shown in Figure 50. The common-

mode voltage, VCM, is the average of the voltages present at the

two inputs minus a 0.6 V drop.

+IN

–IN

–

AD8428

OUT

+FIL

–FIL

VCM

Figure 50. Extracting the Common-Mode Voltage

Use resistor values that are high enough to minimize the impact

on gain accuracy. For example, resistor values of 2 MΩ introduce

an additional gain error of less than 0.2%. For information about

the impact of these resistors on the gain of the amplifier, see the

Effect of Passive Network Across the Filter Terminals section.

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