Datasheet AD8244 (Analog Devices) - 16

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AD8244 Data Sheet APPLICATIONS INFORMATION ELECTROCARDIOGRAM (ECG) Sallen-Key Low-Pass Filter C1
In an ECG system, mismatches between the source impedance
2nF
of different leads, working against the input impedance of the front-end amplifier, can create unbalanced voltage dividers that reduce the system CMRR. When presented to a moderately high input impedance amplifier, the combined impedance of the
1/4 R1 R2 AD8244 VOUT
skin, electrolyte, electrodes, and the protection resistors can be
C2 1nF V
enough to cause power line noise pickup, current noise issues, and
IN
signal division. Dry electrode systems, which are becoming
NOTES
143
1. R1 = R2 = R
1689- increasingly common and have significantly higher source
2. R = 112.5MΩ/f
1
C, Q = 0.707
impedance, are especial y sensitive to these errors. Typically, a high Figure 42. Sallen-Key Low-Pass Filter input impedance, low bias current, FET input op amp is used to The fol owing equations describe the corner frequency, fC, and buffer the electrode signal before it is presented to an quality factor, Q, for the low-pass filter case of the Sallen-Key instrumentation amplifier. This buffer solves the majority of topology, shown in Figure 42: these problems; however, when an instrument is in the field, it fC = 1/(2π R1 R2 × C1 × C2 × ) can be subject to dust pickup and humidity. If the op amp input is not guarded, these environmental factors can create unwanted Q = ( R1 R2 × C1 × C2 × )/(C2 × (R1 + R2)) leakage currents that bring back the aforementioned issues from For an example of a design with this topology, choose a filter insufficient input impedance. The AD8244 pinout is configured to where Q = 0.707 and R1 = R2 = R. This requires that C1 = 2 × C2. make it simple to guard the inputs from parasitic resistance and The corner frequency equation can now be simplified to capacitance while it also drives the instrumentation amplifier inputs, creating a more robust design, while saving power and fC = 1/(2π × R × C2 × √2) board space. The CMRR of the AD8244 driving an instrumentation If an available capacitor, such as 1 nF, is chosen for C2, R can be amplifier initially depends on the gain matching for the chosen written in terms of the desired cutoff frequency: supplies and voltage range, as wel as the instrumentation R = 1/(2√2 × π × 1 nF × fC) = 112.5 MΩ/fc (that is, amplifier used, but it can be improved with design techniques R = 750 kΩ for fC = 150 Hz) such as right leg drive (RLD) or digital filtering.
Sallen-Key High-Pass Filter FILTERING R1
In filtering applications, it is generally recommended to use capacitors such as C0G or NP0 ceramics for distortion and dielectric absorption performance. These types of capacitors
C1 C2 22nF 22nF 1/4
do not have a high volumetric efficiency and are only available
AD8244 VOUT
in values less than a few tens of nanofarads, depending on the
R2 VIN
case size and voltage rating. For a given cutoff frequency, using
NOTES
144
1. R2 = R, R1 = R/2
smaller capacitors requires larger resistor values. At low
2. R = 10.2MΩ/fC, Q = 0.707
11689- frequencies where the resistor values become very large, the bias Figure 43. Sallen-Key High-Pass Filter current of a typical op amp can introduce significant offsets and The high-pass filter case of the Sallen-Key topology has the additional noise. The subpicoampere bias current of the same corner frequency equation as the low-pass filter. However, AD8244 allows resistor values in the tens of megaohms with no the equation for Q changes to additional error while providing an excellent low power, small footprint solution for filter design. Between the four channels of Q = ( R1 R2 × C1 × C2 × )/(R1 × (C1 + C2)) the AD8244, a filter with more than eight poles can be In this case, a Q of 0.707 is achieved with C1 = C2 = C, and 2 × implemented while using less space than the same filter with a R1 = R2 = R, which is a symmetrical result to the low-pass filter quad op amp. case. The corner frequency then simplifies to fC = 1/(√2 × π × R × C) For a low corner frequency, a larger available capacitor such as 22 nF can be chosen, yielding the following expression for R: R = 10.2 MΩ/fc (that is, a 0.5 Hz filter requires R1 = 10 MΩ and R2 = 20 MΩ) Rev. A | Page 16 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