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ADS1294: RLD Setup Example Code

Shayan Raofi
Prodigy 10 points
Part Number: ADS1294

Tool/software:

Hello,

The manual for ADS129xLow-Power,8-Channel,24-BitAnalogFront-EndforBiopotentialMeasurements includes sample code for RLD Measurement/Setup in section 10.1.1.2 Right Leg Drive.

The code is as follows:

WREG RLD_SENSP 0x07 // Select channel 1-3 P-side for RLD sensing

WREG RLD_SENSN 0x07  // Select channel 1-3 N-side for RLD sensing

WREG CONFIG3 b’x1xx 1100 // Turn on RLD amplifier, set internal RLDREF voltage

The CONFIG3 setting in the third line is confusing, as it has bit 1 set to 0. Bit 1 is "RLD_LOFF_SENS", a setting that enables RLD sense (1) or disables it (0). The example code has it disabled. Would that not mean we would get no RLD functionality? Or am I misunderstanding this bit.

As a separate question. CONFIG3 has another setting, "RLD_MEAS" on bit 4 decides whether RLD is on any channel or set to specific ones ("1 = RLD_IN signal is routed to the channel that has the MUX_Setting 010 (VREF)"). If we set Ch2 to be the one that receives RLD_IN signal in the CHnSET register, but have In1P and In1N as the only positive and negative leads connected to the body, would all future registers (ie the Positive/negative RLD Signal Derivation Registers and LOFF sense registers) have to point to ch1 or ch2?

Thank you

  • 0
    TI__Guru 62256 points

    Hello Shayan,

    Thank you for your post and please excuse our delay.

    The RLD_LOFF_SENS function is for monitoring the connection status of the RLD electrode and is not required for configuring the RLD amplifier itself.

    The RLD_MEAS function is used to directly measure the signal present at the output of the RLD amplifier relative to the RLD_REF voltage. This measurement is done by routing the RLDOUT voltage to the RLDIN pin (externally) and connecting it to one of the ADC channels. This feature internally replaces the connection from the INxP and INxN pins. If, for example, you were to route the RLD voltage to Channel 2, IN2P and IN2N would be disconnected from the PGA for that channel. In that case, it would not make sense to enable RLD2P and RLD2N bits.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 10 points

    Our setup is similar to that shown by figure 33 in the datasheet, with the difference being that RLDOUT is attached to the body via a lead (as a reference at a distant point of the body), and only IN1P and IN1N are used (as the positive and negative leads to the body). The other 3 (IN2,3,4P and IN2,3,4N) are tied to the positive supply. RLDOUT is shorted to RLDIN. This is for EMG reading.

    As stated above we have set RLD_MEAS to CH2 in the CONFIG3 register. And in CHnSET: Individual Channel Settings Registers, specified CH2 as the one for RLD_MEAS.

    Since IN1P and IN1N are the ones attached to the body (which I understand is CH1), we have set the registers for RLD_SENSP: RLD Positive Signal Derivation Register, and RLD_SENSN: RLD Negative Signal Derivation Register, to route the CH1 positive and negative signals into RLD derivation, not CH2. To confirm, is this is in line with what you stated in your answer, since CH2 is separated from the PGA due to being taken up by RLD voltage?

    And all of this RLD register setting is divorced from the Lead off registers, correct? They do not necessarily interact? 

    In our case, however, in the Lead-Off Control Register Field Descriptions register we have DC lead off detection turned on. As such , we have set the lead off sensing to be on for RLD (in CONFIG3), IN1P, and IN1N (in LOFF_SENSP and LOFF_SENSN). Is this correct? And should we have LOFF_STATP/N: Lead-Off Positive/Negative Signal Status Register set as 'on' for IN1P and IN1N respectively, or does it not matter?

    Thank you.

  • 0 in reply to Shayan Raofi
    TI__Guru 62256 points

    Hello Shayan,

    Shayan Raofi said:

    Since IN1P and IN1N are the ones attached to the body (which I understand is CH1), we have set the registers for RLD_SENSP: RLD Positive Signal Derivation Register, and RLD_SENSN: RLD Negative Signal Derivation Register, to route the CH1 positive and negative signals into RLD derivation, not CH2. To confirm, is this is in line with what you stated in your answer, since CH2 is separated from the PGA due to being taken up by RLD voltage?

    And all of this RLD register setting is divorced from the Lead off registers, correct? They do not necessarily interact? 

    Everything you mention above is correct. Only the CH1 inputs (IN1P and IN1N) should be used for RLD common-mode derivation in the system as you described it. And the RLD settings are completely separate from the lead-off detection settings.

    Shayan Raofi said:
    In our case, however, in the Lead-Off Control Register Field Descriptions register we have DC lead off detection turned on. As such , we have set the lead off sensing to be on for RLD (in CONFIG3), IN1P, and IN1N (in LOFF_SENSP and LOFF_SENSN). Is this correct? And should we have LOFF_STATP/N: Lead-Off Positive/Negative Signal Status Register set as 'on' for IN1P and IN1N respectively, or does it not matter?

    The lead-off detection feature should be enabled for IN1P and IN1N by setting the relevant LOFF_SENS bits. The LOFF_STAT bits are read-only and are used to indicate electrode connection status. The same LOFF_STAT bits are also contained in the STATUS word which precedes the channel data in each frame. 

    Regards,

    Ryan