ADS1291_14 Datasheet, PDF (52/75 Pages) Texas Instruments – Power, 2-Channel, 24-Bit Analog Front-End for Biopotential Measurements

ADS1291
ADS1292
ADS1292R
SBAS502B – DECEMBER 2011 – REVISED SEPTEMBER 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 ADS1291, ADS1292, and ADS1292R 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 find out if the electrode is off. As
shown in the lead-off detection functional block diagram in Figure 49, 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_SENS register. Also, the internal
excitation circuitry can be disabled and just the sensing circuitry can be enabled.
Skin,
Patient
Patient Electrode Contact Protection
Model
Resistor
47 nF
51 k
30 k
51 k
30 k
47 nF
EMI
Filter
LOFF1P/
LOFF2P
LOFF1N/
LOFF2N
V_INP
V_INN ^PGA
IN1P_OFF/
IN2P_OFF
To ADC
IN1N_OFF/
IN2N_OFF
47 nF
51 k
30 k
AVDD AVSS
RLD OUT
4-Bit
DAC
COMP_TH[2:0]
NOTE: The R_P value must be selected in order to be below the maximum allowable current flow into a patient (in accordance with the
relevant specification the latest revision of IEC 60601).
Figure 49. Lead-Off Detection
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