ADS1291_14 Datasheet, PDF (53/75 Pages) Texas Instruments – Power, 2-Channel, 24-Bit Analog Front-End for Biopotential Measurements

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ADS1291
ADS1292
ADS1292R
SBAS502B – DECEMBER 2011 – REVISED SEPTEMBER 2012
DC Lead-Off
In this method, the lead-off excitation is with a dc signal. The dc excitation signal can be chosen from either an
external pull-up or pull-down resistor or a current source or sink, as shown in Figure 50. One side of the channel
is pulled to supply and the other side is pulled to ground. The internal current source and current sink can be
swapped by setting the FLIP1 and FLIP2 bits in the LOFF_SENS register. In case of current source or sink, the
magnitude of the current can be set by using the ILEAD_OFF[1:0] bits in the LOFF register. The current source
or sink gives larger input impedance compared to the 10-MΩ pull-up or pull-down resistor.
AVDD
AVDD
10 MW
Device
INP
INP
PGA
INN
INN
10 MW
Device
PGA
a) External Pull-Up/Pull-Down Resistors
b) Input Current Source
Figure 50. DC Lead-Off Excitation Options
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 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 digital-to-analog converter (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_STAT register. 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 CONFIG2 register.
An example procedure to turn on dc lead-off is given in the Lead-Off subsection of the Quick-Start Guide section.
AC Lead-Off
In this method, an out-of-band ac signal is used for excitation. The ac signal is generated by alternatively
providing an internal current source and current sink at the input with a fixed frequency. The excitation frequency
is a function of the output data rate and is f_DR / 4. This out-of-band excitation signal is passed through the
channel and measured at the output.
Sensing of the ac signal is done by passing the signal through the channel to digitize it and measure at the
output. The ac excitation signals are introduced at a frequency that is above the band of interest, generating an
out-of-band differential signal that can be filtered out separately and processed. By measuring the magnitude of
the excitation signal at the output spectrum, the lead-off status can be calculated. Therefore, the ac lead-off
detection can be accomplished simultaneously with the ECG signal acquisition.
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