Datasheet

a)LOFF_FLIP=0 a)LOFF_FLIP=1
PGA
10MW
10MW
AVDD
INP
INN
PGA
10MW
10MW
AVDD
INP
INN
ADS129x ADS129x
a)Pull-Up/Pull-DownResistors b)CurrentSource
PGA
10MW
10MW
AVDD
INP
INN
PGA
AVDD
INP
INN
ADS129xADS129x
ADS1294, ADS1294R
ADS1296, ADS1296R
ADS1298, ADS1298R
SBAS459I –JANUARY 2010–REVISED JANUARY 2012 www.ti.com
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
10MΩ 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
register. The output of the comparators are stored in the LOFF_STATP and LOFF_STATN registers. These two
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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