AD8232 Datasheet, PDF (19/28 Pages) Analog Devices – Single-Lead, Heart Rate Monitor Front End

Data Sheet
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.
+V_S
17
2
IA
3
10MΩ 10MΩ
8 REFOUT
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
AD8232
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.
Rev. A | Page 19 of 28