Datasheet

Data Sheet AD8232
Rev. A | 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:
f
C
= 1/(2π√(R1 C1 R2 C2))
Gain = 1 + R3/R4

GainC1R1CR2C2R1
C2R2C1R1
Q



12
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
f
C
= 1/(2πRC)
Gain
Q


3
1
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
A1
FROM IN-AMP
STAGE
C
R
10866-158
REFOUT
FILTERED
SIGNAL
A1
FROM IN-AMP
STAGE
C2
C1
R2
R3
R4
10866-159
R1
A1
C
R
ADC10
AD8232
10866-261