Data Sheet

AD8244

Rev. A | Page 17 of 20

Twin-T Notch Filter

1/4

AD8244

VOUT

C

2C

VIN

C

(1 – K) × R'

K × R'

R

R

C = 7500pF

60Hz: R = 357kΩ

50Hz: R = 422kΩ

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AD8244

R/2

Figure 44. Twin-T Notch Filter

The following equations describe the parameters of the Twin-T

notch filter with active feedback shown in Figure 44:

fO = 1/(2πRC)

Q = 0.25/(1 − K)

where K is an attenuation factor from 0 to 1, as shown in Figure 44.

A K of either 0 or 1 can be achieved with only one buffer.

One of the best things about this filter is that fO and Q are

independent, which allows for easy tuning of filter characteristics.

However, designers use the Twin-T notch filter sparingly in

production designs because of its sensitivity to component

tolerances, which affect both the depth and the frequency of the

notch. Reducing the Q is one way to ensure that the desired

frequency has sufficient attenuation independent of component

variance and drift; however, reducing the Q also linearly increases

the distance between the pass bands. The notch depth can be

improved and the stop-band width decreased simultaneously by

cascading multiple filter stages.

To illustrate the benefit of cascading stages, Figure 45 shows the

response of two filters, both designed to provide greater than

26 dB of attenuation at 60 Hz ± 5%, which allows for component

tolerance. The single stage filter requires a Q of 0.5 and results

in a −3 dB notch bandwidth of 120 Hz. The two stage filter has

a Q of 2.25 for each stage, and the −3 dB notch bandwidth is

reduced to about 40 Hz.

–80

–70

–60

–50

–40

–30

–20

–10

0

10

20

10

100

1k

FREQUENCY (Hz)

–26dB FROM 57Hz TO 63Hz

SINGLE STAGE NOTCH

TWO STAGE CASCADED NOTCH

Figure 45. Cascading Notch Filters

PHOTODIODE AMPLIFIER

Photodiodes in precision circuits are typically measured in

photovoltaic mode, in which there is no reverse bias voltage.

Two benefits to this measurement mode are that there is no dark

current, and the output is linearly related to the light intensity.

However, in photovoltaic mode, the signal current can be very

small, requiring a high gain transimpedance amplifier (TIA).

There are a limited number of amplifiers suited for building

TIAs for measuring photodiodes or other low current sensors,

which can make it difficult to achieve high performance. Using an

AD8244 as the interface to the photodiode eliminates the need

for a low bias current op amp, allowing optimization of other

parameters, such as precision, slew rate, output drive, board

space, and cost. As with any composite amplifier, it is important

to pay special attention to stability. The unity-gain crossover

frequency of the op amp must be less than the AD8244 bandwidth

for this configuration to be unity-gain stable. The noise gain of

the op amp varies with the shunt resistance of the diode, which

is temperature dependent.

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AD8244

VOUT

CF

RF

IPHD

GUARD

A1

Figure 46. AD8244 in a Photodiode Application