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ADS1282: Circuit design question

chen zhe
Prodigy 65 points
Part Number: ADS1282
Other Parts Discussed in Thread: ADS1284,

Hi Team,

For below circuit, why must add R5 and R6 on the output of geophone, can I move R5 and R6 to input pin of ADC as below? Is there any risk to do this?

Thanks very much.

  • +1
    TI__Mastermind 18900 points

    Hi Chen,

    There's a really good thread on this topic which can be found here that talks about some pros and cons, and some guidelines for this kind of circuit design: https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/376974/ads1282-input-circuit-for-seismometer 

    In summary, you do not guarantee the lowest noise configuration if you choose to put the common mode setting resistors (R5 and R6) close to the inputs of the device. Really, you at least want the differential capacitor to be last and you can probably slot in the other resistors wherever you'd like as a compromise. If the goal of the filters is to get rid of noise and anti-aliasing, then you should definitely do that last so some of the thermal noise from the resistors can be filtered out.

    That being said, there is no functional risk of doing so, only some performance risk.

    Here is what one of my other team members came up with (which is also on the thread I linked):

    Any reason that I am not considering for why you'd like to move those resistors next to the pins? 

    Best,

    -Cole

  • 0 in reply to Cole Macias
    Prodigy 65 points

    Hi Cole,

    Really thanks for your coments, it`s quite clear, I designed a wrong circuit and that`s the reason why checked with you.

    Thanks again.

    Regards, Zhe

  • 0 in reply to Cole Macias
    Prodigy 65 points

    Hi Cole,

    One further question may still need you help:

    Some of my friends use ADS1284 with single 5V supply, and they do not use these two 20kohm bias resistor and the circuit works very good, they said ADS1284 integrate internal bias, but i don`t found any description on datasheet, so could you help to check if ADS1284 integrate the internal bias? is so, would you help share some detail on the internla bias circuit?

    Thanks again.

    Regards, Zhe

  • +1 in reply to chen zhe
    TI__Mastermind 18900 points

    Hi Zhe,

    The ADS1284 does not have a common mode voltage integration, and neither does the ADS1282. The closest feature I can think of is using the internal MUX to switch to (AVDD-AVSS)/2 through internal resistors, but this is for measuring offset and troubleshooting the inputs. So I’m not sure what they are saying. 

    When using single supply, it’s best not to connect those voltages to GND but (AVDD-AVSS)/2 = (5-0)/2 = 2.5V.

    There’s nothing wrong without using these resistors but there is the possibility of violating input or output common mode range as defined in the data sheet. Establishing a known voltage is always better than leaving it floating. In addition, putting it in the middle of the range makes it the least like to violate those limits. 

    In the ADS1285EVM, which can be found here: https://www.ti.com/tool/ADS1285EVM-PDK

    We establish the Vocm by running a voltage divider from AVDD1 through an active filter circuit to establish the common mode voltage.

    Best,

    -Cole

  • 0 in reply to Cole Macias
    Prodigy 65 points

    Hi cole,

    Got it, thanks.

    Regards, Zhe