AD8232ACPZ-R7 datasheet(20/29 Pages) AD

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Data Sheet

AD8232

Rev. B | Page 19 of 28

AC Leads Off Detection

The ac leads off detection mode is useful when using two

electrodes only (it does not require the use of a driven electrode).

In this case, a conduction path must exist between the two

electrodes, which is usually formed by two resistors, as shown

in Figure 51.

These resistors also provide a path for bias return on each input.

Connect each resistor to REFOUT or RLD to maintain the inputs

within the common-mode range of the instrumentation

amplifier.

Figure 51. Circuit Configuration for AC Leads Off Detection

The AD8232 detects when an electrode is disconnected by

forcing a small 100 kHz current into the input terminals. This

current flows through the external resistors from IN+ to IN−

and develops a differential voltage across the inputs, which is

then synchronously detected and compared to an internal

threshold. The recommended value for these external resistors

is 10 MΩ. Low resistance values make the differential drop too

low to be detected and lower the input impedance of the

amplifier. When the electrodes are attached to the subject, the

impedance of this path should be less than 3 MΩ to maintain

the drop below the comparator’s threshold.

As opposed to the dc leads off detection mode, the AD8232 is

able to determine only that an electrode has lost its connection,

not which one. During such an event, the LOD+ pin goes high.

In this mode, the LOD− pin is not used and remains in a logic

low state. To use the ac leads off mode, tie the AC/DC pin to the

positive supply rail.

Note that while REFOUT is at a constant voltage value, using

the RLD output as the input bias may be more effective in

rejecting common-mode interference.

STANDBY OPERATION

The AD8232 includes a shutdown pin (SDN) that further

enhances the flexibility and ease of use in portable applications

where power consumption is critical. A logic level signal can be

applied to this pin to switch to shutdown mode, even when the

supply is still on.

Driving the SDN pin low places the AD8232 in shutdown mode

and draws less than 200 nA of supply current, offering considerable

power savings. To enter normal operation, drive SDN high; when

not using this feature, permanently tie SDN to +VS.

During shutdown operation, the AD8232 is not able to maintain

the REFOUT voltage, but it does not drain the REFIN voltage,

thereby maintaining this additional conduction path from the

supply to ground.

When emerging from a shutdown condition, the charge stored

in the capacitors on the high-pass filters can saturate the instru-

mentation amplifier and subsequent stages. The use of the fast

restore feature helps reduce the recovery time and, therefore,

minimize on time in power sensitive applications.

INPUT PROTECTION

All terminals of the AD8232 are protected against ESD. In

addition, the input structure allows for dc overload conditions

that are a diode drop above the positive supply and a diode drop

below the negative supply. Voltages beyond a diode drop of the

supplies cause the ESD diodes to conduct and enable current to

flow through the diode. Therefore, use an external resistor in

series with each of the inputs to limit current for voltages beyond

the supplies. In either scenario, the AD8232 safely handles a

continuous 5 mA current at room temperature.

For applications where the AD8232 encounters extreme over-

load voltages, such as in cardiac defibrillators, use external series

resistors and gas discharge tubes (GDT). Neon lamps are com-

monly used as an inexpensive alternative to GDTs. These devices

can handle the application of large voltages but do not maintain

the voltage below the absolute maximum ratings for the AD8232.

A complete solution includes further clamping to either supply

using additional resistors and low leakage diode clamps, such as

BAV199 or FJH1100.

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

the electrode that is connected to the subject to ensure that the

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.

REFOUT

10MΩ

+VS

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