AD8244

Data Sheet

APPLICATIONS INFORMATION

ELECTROCARDIOGRAM (ECG)

In an ECG system, mismatches between the source impedance

of different leads, working against the input impedance of the

front-end amplifier, can create unbalanced resistor dividers

that potentially reduce the system CMRR. When presented to

a moderately high input impedance amplifier, the combined

impedance of the skin, electrolyte, electrodes, and the protection

resistors can be enough to cause power line noise pickup, current

noise issues, and signal division. Dry electrode systems, which

are becoming increasingly common and have significantly higher

source impedance, are especially sensitive to these errors. Typically,

a high input impedance, low bias current, FET input op amp is

used to buffer the electrode signal before it is presented to an

instrumentation amplifier. This buffer solves the majority of

these problems; however, when an instrument is in the field, it

can be subject to dust pickup and humidity. If the op amp input

is not guarded, these environmental factors can create unwanted

leakage currents that bring back the previous issues from input

impedance that is not sufficiently high. The AD8244 is configured

to make it simple to guard the inputs from parasitic resistance and

capacitance while it also drives the instrumentation amplifier

inputs, creating a more robust design, while saving power and

board space. The CMRR of the AD8244 driving an instrumentation

amplifier initially depends on the gain matching for the chosen

supplies and voltage range, as well as the instrumentation

amplifier used, but it can be improved with design techniques

such as right leg drive (RLD) or digital filtering.

FILTERING

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

do not have a high volumetric efficiency and are available in

values up to the tens of nanofarads, depending on the case size

and voltage rating. For a given cutoff frequency, using smaller

capacitors requires larger resistor values. At low frequencies

where the resistor values become very large, the bias current of

a typical op amp can introduce significant offsets and additional

noise. The subpicoampere bias current of the AD8244 allows

resistor values in the tens of megaohms with no additional error

while providing an excellent low power, small footprint solution

for filter design. Between the four channels of the AD8244, a

filter with more than eight poles can be implemented while

using less space than the same filter with a quad op amp.

Sallen-Key Low-Pass Filter

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AD8244

Figure 42. Sallen-Key Low-Pass Filter

quality factor, Q, for the low-pass filter case of the Sallen-Key

topology, shown in Figure 42:

C2

C1

R2

R1

×

×

×

)

Q = (

C2

C1

R2

R1

×

×

×

)/(C2 × (R1 + R2))

For an example of a design with this topology, choose a filter

where Q = 0.707 and R1 = R2 = R. This requires that C1 = 2 × C2.

The corner frequency equation can now be simplified to

If an available capacitor, such as 1 nF, is chosen for C2, R can be

written in terms of the desired cutoff frequency:

Sallen-Key High-Pass Filter

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AD8244

Figure 43. Sallen-Key High-Pass Filter

The high-pass filter case of the Sallen-Key topology has the

same corner frequency equation as the low-pass filter. However,

the equation for Q changes to

Q = (

C2

C1

R2

R1

×

×

×

)/(R1 × (C1 + C2))

In this case, a Q of 0.707 is achieved with C1 = C2 = C, and R1 =

½ R2, which is a symmetrical result to the low-pass filter case.

The corner frequency then simplifies to

For a low corner frequency, a larger available capacitor such as

22 nF can be chosen, yielding the following expression for R2:

R2 = 10.2 MΩ × Hz (that is, a 0.5 Hz filter requires

R1 = 10 MΩ and R2 = 20 MΩ)

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