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ADS1299: Inquiry about setting of [LOFF STAT]

Herry Na
Prodigy 10 points
Part Number: ADS1299

I'm inquiring about ADS1299 because I need some support.

1. There is a phenomenon that the LOFF_STAT value recognition for channel 1 of ADS1299 is not normal.

The register value setting for ADS1299 that we are currently using is as follows.

[

CONFIG1 - 0x96

CONFIG2 - 0xC0

CONFIG3 - 0xEC

LOFF - 0xE0

CH1SET - 0x40

BIAS_SENSP - 0xFF

BIAS_SENSN - 0xFF

LOFF_SESNSP - 0xFF

LOFF_SENSN - 0xFF

LOFF_FLIP - 0x00

MISC1 - 0x00

MISC2 - 0x00

CONFIG4 - 0x02

]

However, the 24 status bits checked in real time are C0 00 00~~, and the values of LOFF_STATP and LOFF_STATN are always output as 0.

However, we can see that CHANNEL1's 24 bits data is output well in real time.

Is there a register value that needs to be changed?



2. Can you explain in detail the meaning of the bit-specific setting register of the LOFF register of Address 0x04? There are some parts that are confusing with the contents on the Data Sheet.



3. Currently, we are constructing a method of reading data through actual skin contact by connecting an electrode to ADS1299.

In order to test it, it is in progress through the process of contacting the actual skin, and can we configure an environment where we can test the change in value separately without such contact? (Ex. Connecting the Ref pin and GND pin with a resistance of 10 ohms, connecting the Ref pin and IN1 pin to check the value, etc.)

  • 0
    TI__Guru 66186 points

    Hello Herry,

    Please excuse our delay. Do you still need help with these questions?

    1. Can you share a schematic to show the connections to the ADS1299? How are you testing the input lead-off detection feature? We need to see what is connected to the inputs to know how the lead-off detection should respond.
    2. The LOFF register (04h) is where the comparator threshold, lead-off current magnitude, and lead-off current frequency are configured. The comparator thresholds are relative to the positive and negative supply (AVDD and AVSS). When the voltage on INxP or INxN surpass the threshold, the comparator will indicate a lead-off event. The current magnitude is self explanatory. For DC lead-off, the current sources are fixed to INxP and INxN (by default, the current source connects to INxP and current sink connects to INxN; use the FLIP bits to swap them). The AC lead-off uses the same current sources and chops them back and forth to produce a square wave. This requires digital post processing to analyze the magnitude of the measured voltage at the lead-off frequency. The higher the impedance between two electrodes, the larger the signal will be.
    3. The ADS1299 can be tested without actual skin contact. For testing consistency, it is helpful to use fixed resistor values as a benchmark to evaluate system accuracy.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 10 points

    Thank you for your response, Ryan Andrews.

    I'd like to inquire because I have additional questions.

    When the register CH1SET is set to PGA 24, the first 7-byte value is output as "C0 00 00 7F FF FF", but when set to PGA1, a value such as "C0 00 00 42 XX XX" is output and the values are confirmed to fluctuate.
    The resistance is infinite by opening IN, IP, and GND completely, but the "7FFFFF" value is not output.

    Do you have any idea why this is happening?

    Can this phenomenon affect LOFF DETECT?
    (e.g. current leaks due to this phenomenon, interfering with LOFF DETECT etc..)

  • 0 in reply to Herry Na
    TI__Guru 66186 points

    Hello Henry,

    If the inputs to Channel 1 are floating, the output of the PGA is undetermined. It may be that the two inputs are floating close to the same potential with some small offer, so Gain = 1 results in a mid-scale positive code with some fluctuation due to noise. When the gain is increased to 24 V/V, the differential output exceeds full-scale, so the device outputs the maximum output code.

    The DC lead-off detection comparators are located at the inputs to the PGA. Therefore, the gain setting of the PGA should not affect this. If there is a change in leakage current, I imagine it is negligible.

    Regards,

    Ryan