Data Sheet AD8232
Rev. A | Page 17 of 28
RIGHT LEG DRIVE AMPLIFIER
The right leg drive (RLD) amplifier inverts the common-mode
signal that is present at the instrumentation amplifier inputs.
When the right leg drive output current is injected into the
subject, it counteracts common-mode voltage variations, thus
improving the common-mode rejection of the system.
The common-mode signal that is present across the inputs of
the instrumentation amplifier is derived from the transconduct-
ance amplifier, GM1. It is then connected to the inverting input
of A2 through a 150 kΩ resistor.
An integrator can be built by connecting a capacitor between the
RLD FB and RLD terminals. A good starting point is a 1 nF
capacitor, which places the crossover frequency at about 1 kHz
(the frequency at which the amplifier has an inverting unity
gain). This configuration results in about 26 dB of loop gain
available at a frequency range from 50 Hz to 60 Hz for
common-mode line rejection. Higher capacitor values reduce
the crossover frequency, thereby reducing the gain that is
available for rejection and, consequently, increasing the line
noise. Lower capacitor values move the crossover frequency to
higher frequencies, allowing increased gain. The tradeoff is that
with higher gain, the system can become unstable and saturate
the output of the right leg amplifier.
Note that when using this amplifier to drive an electrode, there
should be a resistor in series with the output to limit the current
to be always less than 10uA even in fault conditions. For
example, if the supply used is 3.0V, this resistor should be
greater than 330kΩ to account for component and supply
variations.
Figure 46. Typical Configuration of Right-Leg Drive Circuit
In two-electrode configurations, RLD can be used to bias the
inputs through 10MΩ resistors as described in the Leads Off
Detection section. If left unused, it is recommended to configure
A2 as a follower by connecting RLDFB directly to RLD.
REFERENCE BUFFER
The AD8232 operates from a single supply. To simplify the
design of single-supply applications, the AD8232 includes a
reference buffer to create a virtual ground between the supply
voltage and the system ground. The signals present at the out-
put of the instrumentation amplifier are referenced around this
voltage. For example, if there is zero differential input voltage,
the voltage at the output of the instrumentation amplifier is this
reference voltage.
The reference voltage level is set at the REFIN pin. It can be set
with a voltage divider or by driving the REFIN pin from some
other point in the circuit (for example, from the ADC reference).
The voltage is available at the REFOUT pin for the filtering
circuits or for an ADC input.
Figure 47. Setting the Internal Reference
To limit the power consumption of the voltage divider, the use
of large resistors is recommended, such as 10 MΩ. The designer
must keep in mind that high resistor values make it easier for
interfering signals to appear at the input of the reference buffer.
To minimize noise pickup, it is recommended to place the resistors
close to each other and as near as possible to the REFIN terminal.
Furthermore, use a capacitor in parallel with the lower resistor
on the divider for additional filtering, as shown in Figure 47.
Keep in mind that a large capacitor results in better noise
filtering but it takes longer to settle the reference after power-up.
The total time it takes the reference to settle within 1% can be
estimated with the formula
R2R1
C1R2R1
t
REFERENCESETTLE
+
×= 5
_
Note that disabling the AD8232 with the shutdown terminal
does not discharge this capacitor.
FAST RESTORE CIRCUIT
Because of the low cutoff frequency used in high-pass filters in
ECG applications, signals may require several seconds to settle.
This settling time can result in a frustrating delay for the user
after a step response: for example, when the electrodes are first
connected.
This fast restore function is implemented internally, as shown in
Figure 48. The output of the instrumentation amplifier is connec-
ted to a window comparator. The window comparator detects a
saturation condition at the output of the instrumentation amplifier
when its voltage approaches 50 mV from either supply rail.
Figure 48. Fast Restore Circuit
RLD
1nF
R*
*LIMI T CURRENT TO LE S S THAN 10µA.
RLDFB
A2
REFOUT
TO DRIV E N
ELECTRODE
150kΩVCM
185
4
10866-146
REFIN A3
18
R1
R2C1
+VS
10866-046
SWITCH
TIMING
S1
S2
LOD+
LOD–
FR 15
IAOUT0.05V
+IN
–IN
IA
2
3
+VS – 0.05V
10866-047