ASD1194 datasheet(19/76 Pages) TI

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ADS1198

+1/2V

1/2

REF

Common

Voltage

Single-EndedInput

ADS1198

peak-to-peak

REF

peak-to-peak

REF

Common

Voltage

DifferentialInput

CM+1/2V

REF

+1/2V

REF

1/2

Single-EndedInputs

INP

CMVoltage

REF

1/2

CM+1/2V

REF

DifferentialInputs

INP

INN

CMVoltage

1/2V

REF

Common-ModeVoltage(DifferentialMode)=

(INP)+(INN)

,Common-ModeVoltage(Single-EndedMode)= INN.

INN=CMVoltage

InputRange(DifferentialMode)=(AINP

AINN)=V

( V

)=2V

REF

ADS1194, ADS1196

ADS1198

www.ti.com

SBAS471C

– APRIL 2010 – REVISED NOVEMBER 2011

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 4, (MVDDP – MVDDN) is

DVDD/2. Note that to avoid saturating the PGA while measuring power supplies, the gain must be set to '1'.

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 ADS1198 is fully differential. Assuming PGA = 1, the differential input (INP

– INN) can

span between

–VREF to +VREF. Note that the absolute range for INP and INN must be between AVSS – 0.3 V and

AVDD + 0.3 V. Refer to Table 6 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 ADS1198: single-ended or differential,

as shown in Figure 17 and Figure 18. 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 + 1/2VREF) and the (common-mode – 1/2VREF). When the input is differential,

the common-mode is given by (INP + INN)/2. Both the INP and INN inputs swing from (common-mode + 1/2VREF

to common-mode

– 1/2VREF). For optimal performance, it is recommended that the ADS1198 be used in a

differential configuration.

Figure 17. Methods of Driving the ADS1198: Single-Ended or Differential

Figure 18. Using the ADS1198 in the Single-Ended and Differential Input Modes

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