Data Sheet
Twin-T Notch Filter
C = 7500pF
60Hz: R = 375kΩ
50Hz: R = 422kΩ
R
AD8244
20
10
0
R
1/4
MAGNITUDE (dB)
AD8244
2C
1/4
V
IN
V
OUT
(1 – K) × R'
–10
–20
–26dB FROM 57Hz TO 63Hz
–30
–40
–50
AD8244
R/2
K × R'
11689-145
–60
–70
–80
10
SINGLE STAGE NOTCH
TWO STAGE CASCADED NOTCH
11689-046
C
C
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:
f
O
= 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 f
O
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.
100
FREQUENCY (Hz)
1k
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
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
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.
GUARD
R
F
C
F
1/4
AD8244
I
PHD
A1
V
OUT
Figure 46.
in a Photodiode Application
Rev. 0 | Page 17 of 20
11689-044
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