ADPD6000 datasheet(19/89 Pages) AD

19 / 89 page

Data Sheet

ADPD6000

THEORY OF OPERATION

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Rev. 0 | 19 of 89

ECG Main Signal Chain

Figure 29 shows the ECG main signal chain diagram.

Figure 29. ECG Main Signal Chain Diagram

DCLO Detection

The DCLO circuit injects a small programmable dc current into each

input electrode and monitors the resulting electrode voltage. Figure

30 shows the DCLO detection diagram.

Figure 30. DCLO Detection Diagram

When both ECGIP and ECGIN, are properly connected, the current

flow from one of the ECG inputs through the patient and into

the other ECG input produces a minimal voltage shift. The fail

current magnitude is common across all the ECG electrodes and

programmable with a step size of 2 nA over a ±16 nA range using

the ECG_DCLO_MAG bit field to change the magnitude.

The polarity of the fail current is independent for each elec-

trode and programmed using ECG_DCLO_POLARITY_IN and

ECG_DCLO_POLARITY_IP. When an electrode connection de-

grades or falls off, the pin voltage of that electrode is pulled high or

low depending on the programmed fail current polarity.

The DCLO detection circuit is based on each individual electrode

input (ECGIP and ECGIN). The detection circuit is comparator

based with independent programmable threshold levels for ECG

inputs.

For each input lead (ECGIP and ECGIN), there is only one compa-

rator with a high-side threshold. For the RLD pin, there are two

comparators with a high-side threshold and low-side threshold.

These threshold limits are chosen such that all the threshold

voltages cover the expected signal range. The window compara-

tor compares the electrode input voltage with the corresponding

threshold voltages. Where the voltage change on a particular

electrode exceeds one of the programmed threshold voltages for

that fail current, the dc resistance of the electrode contact can be

determined to flag a lead off.

ACLO Detection

The ECG path has an ACLO detection circuit that can be used for

lead off detection with the DCLO detection circuit to deal with differ-

ent types of lead. Figure 31 shows the ACLO detection diagram.

Figure 31. ACLO Detection Diagram

The ACLO method of sensing if the electrodes are connected to the

patient is based on injecting ac currents into each ECG electrode

and measuring the amplitudes of the resulting voltages through the

ECG channel. The ECG channel must be enabled to use the ACLO

function.

The magnitude of the ACLO current is programmable through the

ECG_ACLO_MAG bit field. The ac current is driven out onto the

ECG input paths and establishes a voltage between the ECG

channel inputs where it is measured. The measured voltage is

synchronously demodulated and sent through a comparator. The

user can set the threshold though the ECG_ACLO_THRESHOLD

bit field.

ACLO detection functions only on the ECG input pins (ECGIP and

ECGIN) and is not supported for the RLD pin. A properly connected

electrode has a small signal because the drive current flows into the

right leg (RL). An improperly connected or dried out electrode has a

larger signal as determined by a capacitive voltage divider.

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