Datasheet

51kW
51kW
100kW
100kW
47nF
51kW
100kW
47nF
47nF
4-Bit
DAC
Skin,
ElectrodeContact
Model
Patient
Protection
Resistor
AVDD
LOFF_SENSP
VLEAD_OFF_EN
AND
AVSS
LOFF_SENSN
VLEAD_OFF_EN
AND
LOFF_SENSP
VLEAD_OFF_EN
AND LOFF_SENSN
VLEAD_OFF_EN
AND
RLDOUT
FLEAD_OFF[1:0]
Vx
3.3MW
3.3MW
3.3MW
3.3MW
3.3MW
3.3MW
Anti-AliasingFilter
<512kHz
FLEAD_OFF[0:1]
AVDD AVSS
ToADC
LOFF_STATP
LOFF_STATN
COMP_TH[2:0]
V
INP
V
INN
(AVDD+AVSS)/2
7MW 7MW
10pF 10pF
PGA
EMI
Filter
12pF
12pF
Patient
ADS1294, ADS1294R
ADS1296, ADS1296R
ADS1298, ADS1298R
www.ti.com SBAS459I –JANUARY 2010–REVISED JANUARY 2012
AC Lead-Off
This method uses an out-of-band ac signal for excitation. The ac signal is generated by alternatively providing
pull-up resistors and pull-down resistors at the input with a fixed frequency. The ac signal is passed through an
anti-aliasing filter to avoid aliasing. The frequency can be chosen by the FLEAD_OFF[1:0] bits in the LOFF
register. 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.
Figure 59. Lead-Off Detection
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