Datasheet AD8244 (Analog Devices) - 17

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Data Sheet AD8244 Twin-T Notch Filter PHOTODIODE AMPLIFIER C = 7500pF 60Hz: R = 357kΩ
Photodiodes in precision circuits are typically measured in
50Hz: R = 422kΩ
photovoltaic mode, in which there is no reverse bias voltage.
R R 1/4 AD8244 VOUT
Two benefits to this measurement mode are that there is no dark current, and the output is linearly related to the light intensity.
2C (1 – K) × R'
However, in photovoltaic mode, the signal current can be very
1/4
small, requiring a high gain transimpedance amplifier (TIA).
V AD8244 IN
There are a limited number of amplifiers suited for building
R/2 K × R'
TIAs for measuring photodiodes or other low current sensors, which can make it difficult to achieve high performance. Using an 145 9-
C C
AD8244 as the interface to the photodiode eliminates the need 1168 Figure 44. Twin-T Notch Filter for a low bias current op amp, allowing optimization of other parameters, such as precision, slew rate, output drive, board The following equations describe the parameters of the Twin-T space, and cost. As with any composite amplifier, it is important notch filter with active feedback shown in Figure 44: to pay special attention to stability. The unity-gain crossover fO = 1/(2πRC) frequency of the op amp must be less than the AD8244 bandwidth Q = 0.25/(1 − K) for this configuration to be unity-gain stable. The noise gain of the op amp varies with the shunt resistance of the diode, which where K is an attenuation factor from 0 to 1, as shown in Figure 44. is temperature dependent. A K of either 0 or 1 can be achieved with only one buffer.
GUARD
One of the best things about this filter is that fO and Q are
R
independent, which allows for easy tuning of filter characteristics.
F
However, designers use the Twin-T notch filter sparingly in
CF
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
1/4 AD8244
frequency has sufficient attenuation independent of component
VOUT I A1 PHD
variance and drift; however, reducing the Q also linearly increases the distance between the pass bands. The notch depth can be 4 04 improved and the stop-band width decreased simultaneously by 9- 68 1 cascading multiple filter stages. 1 Figure 46. AD8244 in a Photodiode Application 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.
20 10 0 –10 ) B d –20 E ( D –26dB FROM 57Hz TO 63Hz U –30 IT N G –40 MA –50 –60 –70 SINGLE STAGE NOTCH TWO STAGE CASCADED NOTCH –80
6
10 100 1k
04 89-
FREQUENCY (Hz)
16 1 Figure 45. Cascading Notch Filters Rev. A | Page 17 of 20 Document Outline FEATURES APPLICATIONS PIN CONFIGURATION GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION OVERVIEW GUARDING INPUT PROTECTION LAYOUT CONSIDERATIONS DIFFERENTIAL SIGNAL CHAINS LOW OUTPUT IMPEDANCE vs. FREQUENCY APPLICATIONS INFORMATION ELECTROCARDIOGRAM (ECG) FILTERING Sallen-Key Low-Pass Filter Sallen-Key High-Pass Filter Twin-T Notch Filter PHOTODIODE AMPLIFIER LOW NOISE, JFET INPUT BUFFER OUTLINE DIMENSIONS ORDERING GUIDE