UG-1016

AD8233CB-EBZ User Guide

Rev. 0 | Page 6 of 12

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 RLD amplifier.

To create this connection, place jumpers at P4, P6, and P7. RLD

must also be enabled by putting the S4 switch in the RLD_EN

position.

RLD

10MΩ

10MΩ

TO DRIVEN

ELECTRODE

IA

B5

C5

D5

Figure 7. Circuit Configuration for Three-Electrode DC Leads Off Detection

The AD8233 indicates when any electrode is disconnected by

setting the LOD pin high.

AC Leads On/Off Detection

The ac leads on/off detection mode is useful when using two-

electrodes. To use the ac leads on/off mode, switch the S2 switch

to the AC position. In this case, a conduction path must exist

between the two-electrodes, which is usually formed by two

resistors, shown in Figure 8.

These resistors also provide a path for bias return on each input.

Connect each resistor (place jumpers at P6 and P7) to the

REFOUT pin (place a jumper at P3) or RLD pin (place a jumper

at P5) to maintain the inputs within the common-mode range of

the instrumentation amplifier. If the resistors are tied to the

RLD pin, the S4 switch must be in the RLD_EN position.

REFOUT

10MΩ

10MΩ

IA

B5

C5

A2

D3

Figure 8. Circuit Configuration for Two-Electrode AC Leads Off Detection

The AD8233 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 the +IN pin to the

−IN pin 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 must be less than 3 MΩ to maintain the drop below the

comparator threshold.

While the REFOUT pin is at a constant voltage value, using the

RLD output as the input bias may be more effective in rejecting

common-mode interference at the expense of additional power.

In three-electrode ac leads off detection mode, shown in Figure 9,

pull-up resistors are not required, which imporves the input

impedance of the circuit. This mode is beneficial for dry

electrode applications. The ac mode currents do contribute 1/f

noise to the system; therefore, depending on the application, it

can be advantageous to utilize ac leads off as a spot check and

switches to dc mode for improved ECG acquisition. To create

this connection, remove the Input Bias Selection jumpers and

the Electrode Terminals jumpers.

RLD

IA

B5

C5

A2

D5

TO DRIVEN

ELECTRODE

Figure 9. Circuit Configuration for Three-Electrode AC Leads Off Detection

The ac leads off detection mode continues to function in shutdown

mode. To keep power under 1 µA, the clock is disabled and the

ac currents become dc currents. The current source on the +IN pin

is 250 nA, while the current sink on the –IN pin is –300 nA. The

stronger pull-down current on –IN acts as a wake-up function,

pulling the LOD pin low when the electrodes are reconnected.