ADS1294 datasheet(63/88 Pages) TI

63 / 88 page

a)LOFF_FLIP=0

a)LOFF_FLIP=1

PGA

10M

AVDD

INP

INN

PGA

10M

AVDD

INP

INN

ADS129x

a)Pull-Up/Pull-DownResistors

b)CurrentSource

PGA

10M

AVDD

INP

INN

PGA

AVDD

INP

INN

ADS129x

ADS1294, ADS1294R

ADS1296, ADS1296R

ADS1298, ADS1298R

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SBAS459H

– JANUARY 2010 – REVISED OCTOBER 2011

LEAD-OFF DETECTION

Patient electrode impedances are known to decay over time. It is necessary to continuously monitor these

electrode connections to verify a suitable connection is present. The ADS129x lead-off detection functional block

provides significant flexibility to the user to choose from various lead-off detection strategies. Though called

lead-off detection, this is in fact an electrode-off detection.

The basic principle is to inject an excitation signal and measure the response to determine if the electrode is off.

As shown in the lead-off detection functional block diagram in Figure 59, this circuit provides two different

methods of determining the state of the patient electrode. The methods differ in the frequency content of the

excitation signal. Lead-off can be selectively done on a per channel basis using the LOFF_SENSP and

LOFF_SENSN registers. Also, the internal excitation circuitry can be disabled and just the sensing circuitry can

be enabled.

DC Lead-Off

In this approach, the lead-off excitation is with a dc signal. The dc excitation signal can be chosen from either a

pull-up/pull-down resistor or a current source/sink, shown in Figure 57. The selection is done by setting the

VLEAD_OFF_EN bit in the LOFF register. One side of the channel is pulled to supply and the other side is pulled

to ground. The pull-up resistor and pull-down resistor can be swapped (as shown in Figure 58) by setting the bits

in the LOFF_FLIP register. In case of current source/sink, the magnitude of the current can be set by using the

ILEAD_OFF[1:0] bits in the LOFF register. The current source/sink gives larger input impedance compared to the

10M

Ω pull-up/pull-down resistor.

Figure 57. DC Lead-Off Excitation Options

Figure 58. LOFF_FLIP Usage

Sensing of the response can be done either by looking at the digital output code from the device or by monitoring

the input voltages with an on-chip comparator. If either of the electrodes is off, the pull-up resistors and/or the

pull-down resistors saturate the channel. By looking at the output code it can be determined that either the P-side

or the N-side is off. To pinpoint which one is off, the comparators must be used. The input voltage is also

monitored using a comparator and a 4-bit DAC whose levels are set by the COMP_TH[2:0] bits in the LOFF

registers are available as a part of the output data stream. (See the Data Output Protocol (DOUT) subsection of

the SPI Interface section.) If dc lead-off is not used, the lead-off comparators can be powered down by setting

the PD_LOFF_COMP bit in the CONFIG4 register.

An example procedure to turn on dc lead-off is given in the Lead-Off subsection of the Guide to Get Up and

Running section.

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ADS1294 Datasheet(PDF) 63 Page - Texas Instruments

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