Datasheet

51kW
100kW
47nF
Skin,
ElectrodeContact
Model
Patient
Protection
Resistor
AVSS
RLD_SENS
VLEAD_OFF_EN
AND
AVSS
RLD_STAT
Patient
RLD_SENS
VLEAD_OFF_EN
AND
ToADCinput(throughV
connectiontoanyofthechannels).
REF
ILGND_OFF[1:0]
ADS1194, ADS1196
ADS1198
www.ti.com SBAS471C –APRIL 2010– REVISED NOVEMBER 2011
AC Lead-Off
In this method, an out-of-band ac signal is used 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 can be chosen to be either f
DR
/2
or 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.
RLD LEAD-OFF
The ADS1194/6/8 provide two modes for determining whether the RLD is correctly connected:
• RLD lead-off detection during normal operation
• RLD lead-off detection during power-up
The following sections provide details of the two modes of operation.
RLD Lead-Off Detection During Normal Operation
During normal operation, the ADS1194/6/8 RLD lead-off at power-up function cannot be used because it is
necessary to power off the RLD amplifier.
RLD Lead Off Detection At Power-Up
This feature is included in the ADS1194/6/8 for use in determining whether the right leg electrode is suitably
connected. At power-up, the ADS1194/6/8 provide two measurement procedures to determine the RLD electrode
connection status using either a current or a voltage pull-down resistor, as shown in Figure 49. The reference
level of the comparator is set to determine the acceptable RLD impedance threshold.
Figure 49. RLD Lead-Off Detection at Power-Up
When the RLD amplifier is powered on, the current source has no function. Only the comparator can be used to
sense the voltage at the output of the RLD amplifier. The comparator thresholds are set by the same LOFF[7:5]
bits used to set the thresholds for other negative inputs.
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