Datasheet

51 k
51 k
47 nF
51 k
47 nF
47 nF
4-Bit
DAC
Skin,
Electrode Contact
Model
Patient
Protection
Resistor
AVDD
LOFF1P/
LOFF2P
AVSS
LOFF1N/
LOFF2N
RLD OUT
To ADC
IN1P_OFF/
IN2P_OFF
IN1N_OFF/
IN2N_OFF
COMP_TH[2:0]
V
INP
V
INN
PGA
EMI
Filter
Patient
30 k
30 k
30 k
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.
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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