ADS129X datasheet(34/83 Pages) TI2

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a)ExternalPull-Up/Pull-DownResistors

b)InputCurrentSource

PGA

10MW

AVDD

INP

INN

Device

PGA

AVDD

INP

INN

Device

ADS1291, ADS1292, ADS1292R

SBAS502C – DECEMBER 2011 – REVISED APRIL 2020

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Product Folder Links: ADS1291 ADS1292 ADS1292R

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8.3.10.2.1

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 38. 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-M

Ω pull-up or pull-down resistor.

Figure 38. 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 section.

8.3.10.2.2

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 fDR / 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.

ADS129X Datasheet(PDF) 34 Page - Texas Instruments

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