AD8232ACPZ-R7 datasheet(18/29 Pages) AD

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Data Sheet

AD8232

Rev. B | 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

REFERENCE

SETTLE

= 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

*LIMIT CURRENT TO LESS THAN 10µA.

RLDFB

REFOUT

TO DRIVEN

ELECTRODE

150k

Ω

VCM

REFIN

+VS

SWITCH

TIMING

LOD+

LOD–

FR 15

IAOUT

0.05V

+IN

–IN

+VS – 0.05V

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