Datasheet

500

10k

0.01

0.1

Threshold Voltage (V)

Excitation Frequency (Hz)

C01

Level 4

Level 3

Level 2

Level 1

ADS1293

www.ti.com

SNAS602B –FEBRUARY 2013–REVISED MARCH 2013

Figure 24. Analog AC Lead-Off Reference Levels

The comparator outputs can be accessed at the OUT_LOD[x] bit of the ERROR_LOD register. A high

comparator output signal indicates that the AC voltage at the excitation frequency is larger than the programmed

threshold, which indicates that the lead is not well connected.

The lead-off detection circuit requires a low impedance return path from the right leg electrode to ground, such

as a voltage reference or the RLD amplifier output. Without a proper low impedance return path for the LOD

currents, all enabled LOD pins will report a lead disconnected.

Figure 25. Simplified Analog AC Lead-off Detect Block Diagram

Digital AC Lead-Off Detect

The digital AC lead-off detect (LOD) allows for measurement of the impedance of the two electrodes connected

to an AFE channel by measuring a signal through the AFE. In this mode, the lead-off detect current is injected in

a balanced manner at the inputs of the AFE behind the flex routing switch. The AC test current is injected into

the positive input of the AFE behind the flex routing switch; while at the same time, a similar test current with

opposite sign is injected into the negative input of the AFE. Since the AFE has a very high input impedance, the

current injected into the positive input pin cannot flow into the AFE. Instead, it will flow through the flex routing

switch, via the positive input electrode into the patient, and then back through the negative input electrode and

via another path in the flex routing switch towards the negative input of the AFE, where it is cancelled by the

current injected at that point. As a result of this test current, an additional AC voltage input will occur at the input

of the AFE with a frequency equal to the frequency of the AC LOD test signal frequency. The magnitude of this

voltage equals the magnitude of the AC LOD test current (programmed into the CUR_LOD bit in the

LOD_CURRENT register) multiplied by the impedances of the two electrodes routed to the AFE input in series.

Product Folder Links: ADS1293