AD8232ACPZ-R7 datasheet(18/28 Pages) AD

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AD8232

Data Sheet

Rev. A | Page 18 of 28

Figure 49. 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 49 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 AD8232 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 AD8232 returns to its normal

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

cating that an electrode has been 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 OFF DETECTION

The AD8232 includes leads off detection. It features ac and dc

detection modes optimized for either two- or three-electrode

configurations, respectively.

DC Leads Off Detection

The dc leads off detection mode is used in three-electrode con-

figurations only. It works by sensing when either instrumentation

amplifier input voltage is within 0.5 V from the positive rail. In

this case, each input must have a pull-up resistor connected to the

positive supply. During normal operation, the subject’s potential

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.

Figure 50. Circuit Configuration for DC Leads Off Detection

Because in dc leads off mode the AD8232 checks each input

individually, it is possible to indicate which electrode is discon-

nected. The AD8232 indicates which electrode is disconnected by

setting the corresponding LOD− or LOD+ pin high. To use this

mode, connect the AC/DC pin to ground.

LEADS OFF

LEADS ON

SATURATION DETECTED

NO SATURATION

tS1

tS2

tRST

RLD

10MΩ

TO DRIVEN

ELECTRODE

+VS

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