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ADS1293: Lead off Detection Without RLD

Jon Ciccozzi17
Prodigy 140 points
Part Number: ADS1293

We are using the ADS1293 in a battery powered ECG monitor.  

We  opted not to use the Right Leg Drive.  So we are only using IN2, IN3, IN4 inputs.  

The DataSheet says the following:

It is important to note that the lead-off detection circuit requires a low impedance return path from the right-leg

electrode-to-ground, such as a voltage reference or the RLD amplifier output. Without a proper low impedance

return path for the LOD currents, all enabled LOD pins will report a lead disconnected.

 

So Is it possible to get Lead detection without a RLD?  How to connect the current path to ground without RLD amplifier?

 

I am unable to get a stable lead off detection.  When I try DC lead off detection setup, the ads1293 is always flaggin Lead off detected in LOD_ERROR register. 

DC Lead off detect  config:

LOD_CN: 0x00

LOD_CURRENT: 0x0F

LOD_END: 0x0E

with Analog AC Lead off detect is not working at all for us with the following setting

LOD_CN: 0x14

LOD_EN: 0x0E

LOD_AC_CN: 0x00

LOD_Current: 0x00

  • 0
    TI__Guru 66346 points

    Hi Jon,

    Thank you for your post.

    The only way for the lead-off current to return to the negative supply voltage is through the RLD amplifier or some other path to a common-mode voltage, typically mid-supply. You could try a resistor divider between the supplies on each input, but the resistance will need to be rather large in order to limit the leakage current through those components. Without biasing the common-mode voltage of the body, the input electrodes are essentially floating relative to the electronics on your circuit board.

    What's the reason you opted to not use RLD? Keep in mind that one of the other benefits to using RLD is to help suppress common-mode noise, namely power line interference at 50 Hz or 60 Hz.

    Best regards,

  • 0 in reply to Ryan Andrews
    Prodigy 140 points

    We decided to use 3 leads instead of 4 leads and filter out the noise in with a DSP as we are doing other analysis.

    The circuit does have 4 leads with one of them being a RLD and port for it. However the product casing only allows for 3 leads. Is it possible to still use only 3 leads to get 3 channels while still using one of the 3 leads as RLD?

    Also, is it possible to use the Digital AC lead off detect without the RLD drive?

    Jon

  • 0 in reply to Jon Ciccozzi17
    TI__Guru 66346 points

    Hi Jon,

    By "3 channels" I assume you're trying to measure the three primary leads (I, II, and III). You do not need to directly measure all three since any two can be used to calculate the third. However, you will need a dedicated electrode for RA, LA, and LL. Since you only have three available, you cannot use RLD and will need to AC couple the electrode inputs.

    Generally, AC-coupled inputs are biased to mid-supply after the DC-blocking capacitor. This can be achieved through either a resistor divider between the analog supplies or by a pull-up resistor to a near mid-supply voltage on the board (this could be the RLD amplifier output configured for mid-supply). Digital AC lead-off could then be used to converted the AC lead-off waveform into a digital output, which you can then analyze in an FFT to observe the magnitude at the lead-off frequency.

    Regards,