AD8232ACPZ-R7 datasheet(24/29 Pages) AD

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Data Sheet

AD8232

Rev. B | Page 23 of 28

LOW-PASS FILTERING AND GAIN

The AD8232 includes an uncommitted op amp that can be used

for extra gain and filtering. For applications that do not require

a high-order filter, a simple RC low-pass filter should suffice,

and the op amp can buffer or further amplify the signal.

Figure 59. Schematic for a Single-Pole Low-Pass Filter and Additional Gain

Applications that require a steeper roll off or a sharper cut off, a

Sallen-Key filter topology can be implemented, as shown in

Figure 60.

Figure 60. Schematic for a Two-Pole Low-Pass Filter

The following equations describe the low-pass cut off frequency,

gain, and Q:

fC = 1/(2π√(R1 C1 R2 C2))

Gain = 1 + R3/R4

(

)

Gain

−

Note that changing the gain has an effect on Q and vice versa.

Common values for Q are 0.5 to avoid peaking or 0.7 for

maximum flatness and sharp cut off. A high value of Q can be

used in narrow-band applications to increase peaking and the

selectivity of the band-pass filter.

A common design procedure is to set R1 = R2 = R and C1 = C2 =

C, which simplifies the expressions for cutoff frequency and Q to

fC = 1/(2πRC)

Gain

−

Note that Q can be controlled by setting the gain with R3 and

R4; however, this limits the gain to be less than 3. For gain

values equal to or greater than 3, the circuit becomes unstable.

A simple modification that allows higher gains is to make the

value of C2 at least four times larger than C1.

It is important to note that these design equations only hold

true in the case that the output impedance of the previous stage

is much lower than the input impedance of the Sallen-Key filter.

This is not the case when using an ac coupling network between

the instrumentation amplifier output and the input of the low-

pass filter without a buffer.

To connect these two filtering stages properly without a buffer,

make the value of R1 at least ten times larger than the resistor of

the ac coupling network (labeled as R2 in Figure 55).

DRIVING ANALOG-TO-DIGITAL CONVERTERS

The ability of AD8232 to drive capacitive loads makes it ideal to

drive an ADC without the need for an additional buffer. However,

depending on the input architecture of the ADC, a simple low-

pass RC network may be required to decouple the transients

from the switched-capacitor input typical of modern ADCs.

This RC network also acts as an additional filter that can help

reduce noise and aliasing. Follow the recommended guidelines

from the ADC data sheet for the selection of proper R and C values.

Figure 61. Driving an ADC

DRIVEN ELECTRODE

A driven lead (or reference electrode) is often used to minimize

the effects of common-mode voltages induced by the power line

and other interfering sources. The AD8232 extracts the common-

mode voltage from the instrumentation amplifier inputs and

makes it available through the RLD amplifier to drive an opposing

signal into the patient. This functionality maintains the voltage

between the patient and the AD8232 at a near constant, greatly

improving the common-mode rejection ratio.

As a safety measure, place a resistor between the RLD pin and

the electrode connected to the subject to ensure that current

flow never exceeds 10 µA. Calculate the value of this resistor to

be equal to the supply voltage across the AD8232 divided by 10 µA.

The AD8232 implements an integrator formed by an internal

150 kΩ resistor and an external capacitor to drive this electrode.

Choice of the integrator capacitor is a tradeoff between line rejec-

tion capability and stability. The capacitor should be small to

maintain as much loop gain as possible, around 50 Hz and 60 Hz,

which are typical line frequencies. For stability, the gain of the

integrator should be less than unity at the frequency of any

other poles in the loop, such as those formed by the patient’s

capacitance and the safety resistors. The suggested application

circuits use a 1 nF capacitor, which results in a loop gain of about

20 at line frequencies, with a crossover frequency of about 1 kHz.

In a two-lead configuration, the RLD amplifier can be used to

drive the bias current resistors on the inputs. Although not as

effective as a true driven electrode, this configuration can

provide some common-mode rejection improvement if the

sense electrode impedance is small and well matched.

REFOUT

FILTERED

SIGNAL

FROM IN-AMP

STAGE

REFOUT

FILTERED

SIGNAL

FROM IN-AMP

STAGE

ADC

AD8232

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