Datasheet

ADS1298
- V to
+1/2V
REF
REF
1/2
Common
Voltage
Single-EndedInput
ADS1298
V
peak-to-peak
REF
V
peak-to-peak
REF
Common
Voltage
DifferentialInput
ADS1294, ADS1294R
ADS1296, ADS1296R
ADS1298, ADS1298R
SBAS459I –JANUARY 2010–REVISED JANUARY 2012 www.ti.com
Supply Measurements (MVDDP, MVDDN)
Setting CHnSET[2:0] = 011 sets the channel inputs to different supply voltages of the device. For channels 1, 2,
5, 6, 7, and 8, (MVDDP – MVDDN) is [0.5 × (AVDD – AVSS)]; for channel 3 and for channel 4, (MVDDP –
MVDDN) is DVDD/4. Note that to avoid saturating the PGA while measuring power supplies, the gain must be
set to '1'. For example, if AVDD = 2.5V and AVSS = –2.5V, then the measurement result would be 2.5V.
Lead-Off Excitation Signals (LoffP, LoffN)
The lead-off excitation signals are fed into the multiplexer before the switches. The comparators that detect the
lead-off condition are also connected to the multiplexer block before the switches. For a detailed description of
the lead-off block, refer to the Lead-Off Detection subsection in the ECG-Specific Functions section.
Auxiliary Single-Ended Input
The RLD_IN pin is primarily used for routing the right leg drive signal to any of the electrodes in case the right leg
drive electrode falls off. However, the RLD_IN pin can be used as a multiple single-ended input channel. The
signal at the RLD_IN pin can be measured with respect to the voltage at the RLD_REF pin using any of the eight
channels. This measurement is done by setting the channel multiplexer setting to '010' and the RLD_MEAS bit of
the CONFIG3 register to '1'.
ANALOG INPUT
The analog input to the ADS1298 is fully differential. Assuming PGA = 1, the differential input (INP – INN) can
span between –V
REF
to +V
REF
. Note that the the absolute range for INP and INN must be between AVSS – 0.3V
and AVDD + 0.3V. Refer to Table 8 for an explanation of the correlation between the analog input and the digital
codes. There are two general methods of driving the analog input of the ADS1298: single-ended or differential,
as shown in Figure 26 and Figure 27. Note that INP and INN are 180°C out-of-phase in the differential input
method. When the input is single-ended, the INN input is held at the common-mode voltage, preferably at
mid-supply. The INP input swings around the same common voltage and the peak-to-peak amplitude is the
(common-mode + 1/2V
REF
) and the (common-mode – 1/2V
REF
). When the input is differential, the common-mode
is given by (INP + INN)/2. Both the INP and INN inputs swing from (common-mode + 1/2V
REF
to
common-mode – 1/2V
REF
). For optimal performance, it is recommended that the ADS1298 devices be used in a
differential configuration.
Figure 26. Methods of Driving the ADS1298: Single-Ended or Differential
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