Datasheet

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
S1
S2
SATURATION DETECTED NO SATURATION
t
S1
t
S2
t
RST
10866-048
RLD
10M
Ω
10MΩ
TO DRIVEN
ELECTRODE
IA
2
3
5
+
V
S
10866-049