AD8232-EVALZ Datasheet AD8232-EVALZ | P7-P9

Rev 03 Jan 2013 19:42 | Page 7

Additional Low-Pass Filters

The components R14, C13, R19 and C15 offer additional filtering options. The pin labeled VFILT is

located after the first filter (R14 and C13). The pin labeled VFILT2 is located after both filters.

LEADS-OFF DETECTION

The AD8232 includes leads off detection features. It provides modes optimized for either two or

three-electrode configurations.

DC Leads-Off Detection

To use this mode, the switch position must match the text labeled as “DC” on the silkscreen. This

mode works by sensing when either input goes high. Note that the system must be a three-electrode

configuration to operate properly. Therefore, the RLD output terminal must be connected to a driven

electrode.

In DC leads-off mode, the AD8232 checks each input individually, and is therefore able to indicate

which electrode is disconnected. The AD8232 indicates which electrode is disconnected by setting the

corresponding LO- or LO+ pin high.

AC Leads-Off Detection

To use this mode, the switch position must match the text labeled as “AC” on the silkscreen. To

maintain the inputs inside the common-mode range of the amplifier, provide a bias return path for

each input with a 10MΩ to REFOUT (through J1) or PRLD (through J3).

This mode is useful when using only two electrodes (no separate driven electrode). The AD8232

detects when an electrode is disconnected by sourcing a small 100 kHz current into the electrodes. As

opposed to the DC detection mode, the ADC8232 is only able to determine that an electrode has lost

its connection – not which one. During such event, the LO+ pin goes high. In this mode, the LO- pin is

not used.

FAST RESTORE

The fast restore function (FR) reduces the duration of otherwise long settling tails of the high-pass

filters. After an abrupt change that rails the amplifier (such as a leads-off condition), the AD8232

automatically adjusts to higher filter cutoff. This functionality allows the AD8232 to recover quickly,

Rev 03 Jan 2013 19:42 | Page 8

and therefore take valid measurements soon after connecting the electrodes to the subject. To access

this function, set the jumper P2 connection between pins 1 and 2. To disable, connect pins 2 and 3.

BOARD SETTINGS FOR TWO ELECTRODE CONFIGURATION

The AD8232-CHAR-Z can be configured to suit two-electrode applications, as shown in Table 2.

TABLE 2. Two-electrode configuration

FUNCTION LABEL CONFIGURATION

AC/DC Leads Detection P5 AC method enabled

Input pull-up voltage J3 Pull up inputs to PRLD or place jumper on J1 to connect to VREF

Keep in mind that the gain and filtering may need to be adjusted depending on the location of the

electrodes. Note that narrow bandwidths offer the highest rejection to motion artifacts and other

external interferences, but introduce distortion into the ECG signal.

EVALUATION BOARD COMPONENT EXPLANATION

Input Bias Circuit: R4, R5, J1, J2, J3, J4

In three-electrode configurations, pull-up resistors are necessary for the leads-off functionality to work

correctly. In two-electrode configurations, biasing the inputs within the input range of AD8232 is

necessary for the instrumentation amplifier to work. R4 and R5 are 10 MΩ on the evaluation board.

The resistors may be pulled up to four different values, depending on which jumper is set. Only one

jumper should be connected at a time.

Two-electrode configuration:

J1: Connects pull up resistors to VREF, which is typically at mid-supply.

J3: Connects pull up resistors to the RLD (right-leg drive) amplifier output. This is the recommended

configuration for two electrode configurations

Three-electrode configuration:

J2: Connects pull up resistors to +VS. This is the standard configuration for three-electrode

configurations.

J4: This jumper position allows connecting the pull-up resistors to a voltage provided by the user. This

is useful in systems that expect high power-line noise (50Hz or 60Hz). A higher voltage than the

supply, such as 5V, can help in maintaining the inputs within the common-mode range of the amplifier.

Apply this voltage at the VSETIN pin.

Thoroughly clean the board after performing any solder work around this area, so that residual flux

Rev 03 Jan 2013 19:42 | Page 9

does not create resistive parasitic paths

In-Amp High Pass Filter: C6, R9

The cutoff frequency is set to 7.2Hz. Refer to the High-Pass Filters section for more information

AC-Coupling: C9, R11

The cutoff frequency of this high-pass filter is set to 7.2Hz. Because this is a DC-blocking circuit,

connecting R11 to the REFOUT voltage is necessary to allow negative signal swings. Refer to the

High-Pass Filters section for more information

Low Pass Filter (Sallen-Key): R12, R13, R17, R18, C7, C14, C11

This circuit provides two functions: a dual pole low pass filter and gain. On the evaluation board, the

cutoff of this filter is set at 25 Hz, and the gain to 11. This brings the total gain of the AD8232 to

11×100 = 1100. Refer to the Low-Pass Filter section for more information

Additional low pass filters: R14, C13, R19, C15

Cutoff frequency for both filters on the evaluation board is 23.4Hz. Refer to the Additional Low-Pass

Filters section for more information

Mid-supply level: R8, R10, C8

R8 and R10 form a ratiometric reference voltage for the system. The AD8232 buffers this voltage and

makes it available at the VREFOUT pin. Select high resistor values for R8 and R10 to reduce power

consumption. The evaluation board uses 10MΩ for both. C8 is required for filtering and stability.

Power line noise will be considerably worse without C8.

The user may bring an external reference level through the VREFSET pin. Removing R8 and R10,

adding series resistance between the external source and VREFTSET, and adjusting C8 may become

necessary if the reference voltage becomes unstable.

Right leg drive: C1, R1

C1 is 1nF, which together with an internal 150kΩ resistor, results in a loop gain of 20 around the

typical line frequencies and a crossover frequency of 1kHz. The value of R1 is set to 499kΩ to avoid

stability problems. For more information, refer to the Right-Leg Drive Amplifier section.

Optional RFI Filter: R6, R7, C3, C4, C5

Pads are provided for capacitors to implement additional RFI filtering. C3 and C5 should be the same

value and well matched. C4 should be a larger value than C3 and C5. This minimizes degradation of

CMRR at higher frequencies due to tolerance mismatch of C3 and C5.

If C3 = C5 = C and R6 = R7 = R then

Common mode signal cutoff frequency: 1/ (2πRC)

P1-P3 P4-P6 P7-P9 P10-P10

AD8232-EVALZ

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