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Using ADS 1298 for non ECG purposes (using it as a A/d converter)

Niranjan Madhavan
Intellectual 605 points
Other Parts Discussed in Thread: ADS131E08, ADS1298

I am in a project to develop a data acquisition system. ADS 1298 seems perfect for our application. So, I bough the evaluation board and found that it is mostly used in ECG purpose and in fact the forums, user manual and every thing has been explained in the view point of an ECG application. I used a d15 connector to give a 100m vPP, sine wave @ 10 hz to the evaluation kit. But, when I am giving an input all the negative electrode is going off. I am a new bee to the instrumentation designing industry. Could you please help me with a test circuit and the result to start  with. I also found difficulty in going through the pin out of d15 port on users manual. If you could give me a better pin out of the d15 port. Screen shorts are attached. Please help me to begin with

The result on vpp is much lower than the input of 100mv at 10 hz frequency. Please help me to begin. If I am in a wrong path please let me know

Regards

niranjan

  • 0
    TI__Mastermind 23235 points

    Niranjan -

    The device is capable of being used as a generic ADC, you just need to change some of the default settings on the EVM.

    Take a look at the schematic in the user guide (end of http://www.ti.com/lit/ug/sbau171c/sbau171c.pdf).  Using the D15 connector, you probably won't be able to achieve what you want since it is setup with more ECG in mind (negative inputs are common on all but 2 input signals).  You will need to remove JP26-33 jumpers and then using the right side (pins 2 & 4) for the signal inputs to the device.

    Additionally, you can look at the ADS131E08 for an alternative.  This device says it is for power monitoring, but it also works well as a generic ADC.

  • 0 in reply to Greg Hupp
    Intellectual 605 points

    Thank you for the quick reply.  \

    What would be the reason of I am getting my negative electrode floating? I have connected the pin 6 and 9 to the ground according to the pin diagram (that is the reason I asked for another pinout on the previous post). Do I need  to change any jumper setting of the mmb0 mother board for any chance? 

  • 0 in reply to Niranjan Madhavan
    TI__Mastermind 43915 points

    Hi Niranjan,

    I see what the problem is here.

    The way that the lead off design works is it uses an internal comparator to compare the input voltage to a ratio of the power supply. The negative electrode is set to 5% by default (it is adjustable) of the power supply which would trigger a lead off. The internal design has pull up resistor to AVDD and pull down resistor to AVSS which pull the inputs respectively in a lead off event. If I was to guess, I would think you have the ADS1298 configured for single supply (3V AVDD and 0V AVSS). See JP2 and JP20 on the EVM board to see how they are positioned. If they are set for single supply, when you set the INxN pin to ground, you are triggering the internal comparator making the device think that there is a lead off event when the channel is actually just tied to ground.

    Let me know if that makes sense or if you need further explanation.

    This app note talks about lead off a little more - http://www.ti.com/lit/an/sbaa196/sbaa196.pdf

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 90 points

    Hello Tony,

    I designed my own ADS1298-based board for acquiring signals from some magnetic sensors. So far I've played around with the ADC and have managed to configure it, run noise measurements and get the internal test signals. However, now that I´m trying to acquire real signals from the analog inputs I can´t seem to get any response at all, just some noise-like behavior which doesn't change when I put my fingers on the inputs (See attached image). Is there any special configuration for the CONFIG3, 4 LEAD-OFF, RLD and WCT  to use the ADS1298 for non-ECG purposes, that I might have overlooked??

    Could you please shine some light on me regarding this topic? It's driving me crazy so far.

     

    Thanks in Advance,

    Gilbert