AD8233ACBZ-R7 datasheet(20/30 Pages) AD

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Data Sheet

AD8233

Rev. 0 | Page 19 of 29

LEADS OFF

LEADS ON

SATURATION DETECTED

NO SATURATION

tS1

tS2

tRST

Figure 54. Timing Diagram for Fast Restore Switches (Time Base Not to Scale)

If this saturation condition is present when both input electrodes

are attached to the subject, the comparator triggers a timing

circuit that automatically closes Switch S1 and Switch S2 (see

Figure 54 for a timing diagram).

These two switches (S1 and S2) enable two different 10 kΩ

resistor paths: one between HPSENSE and IAOUT and another

between SW and REFOUT. During the time Switch S1 and

Switch S2 are enabled, these internal resistors appear in parallel

with their corresponding external resistors forming high-pass

filters. The result is that the equivalent lower resistance shifts

the pole to a higher frequency, delivering a quicker settling

time. Note that the fast restore settling time depends on how

quickly the internal 10 kΩ resistors of the AD8233 can drain the

capacitors in the high-pass circuit. Smaller capacitor values

result in a shorter settling time.

If, by the end of the timing, the saturation condition persists,

the cycle repeats. Otherwise, the AD8233 returns to its normal

operation. If either of the leads off comparator outputs is indi-

cating that an electrode is disconnected, the timing circuit is

prevented from triggering because it is assumed that no valid

signal is present. To disable fast restore, drive the FR pin low or

tie it permanently to GND.

LEADS ON/OFF DETECTION

The AD8233 includes leads off detection. It features ac and dc

detection modes that both work with two and three electrode

configurations. Ultralow power comparators allow the leads

on/off detection to remain functional in shutdown mode,

allowing power savings at the system level when the LOD

output is used as a wake-up signal for the microcontroller.

DC Leads On/Off Detection

The dc leads off detection mode can be used in two or three

electrode configurations. It works by sensing when either

instrumentation amplifier input voltage is within 0.27 V from

the positive rail. The lowest power use case for the AD8233 is

two electrode dc mode. A pull-up resistor on +IN and a pull-

down resistor on −IN creates a voltage divider when the

electrodes are connected, setting the input common mode to

midsupply. When the electrodes disconnect, the comparator

monitoring +IN sets LOD high when the input pulls to +VS.

10MΩ

+VS

Figure 55. Circuit Configuration for Two Electrode DC Leads Off Detection

For three electrode dc mode, each input must have a pull-up

resistor connected to the positive supply. During normal operation,

the potential of the subject must be inside the common-mode

range of the instrumentation amplifier, which is only possible if

a third electrode is connected to the output of the right leg drive

amplifier.

RLD

10MΩ

TO DRIVEN

ELECTRODE

+VS

Figure 56. Circuit Configuration for Three Electrode DC Leads Off Detection

The AD8233 indicates when any electrode is disconnected by

setting the LOD pin high. To use this mode, connect the

AC/DC pin to ground.

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