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ADS1258 - recommended layout

Tandi Wijaya
Prodigy 90 points
Other Parts Discussed in Thread: ADS1258, REF3040, OPA2365

Hi,

I have just completed the test for the ADS1258EVM, and would like to implement it in our circuit. I am in the pcb design process, and would like to know if you have any recommended layout for this chip, especially grounding layout and other critical component layout. Usually other ICs from different manufacturer would have a design recommendation which I can follow, but i could not find any in the ADS1258 datasheet.

Thank You.

Tandi.

  • 0
    TI__Mastermind 43915 points

    Hi Tandi,

    Give this a read -

    http://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/general-pcb-layout-questions.aspx

    Aside from general layout considerations, make sure that the differential cap between the differential inputs is around 2.2nF and is placed close to the chip inputs, possibly close to the ADCINP an ADCINN pins. Also, make sure you have a cap on the Vref line and it is placed as close to the Vref input as possible. You can look at the EVM schematic in the UG for some design considerations.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 90 points

    Hi Tony,

     

    Thanks for your fast response and for providing me additional design consideration information.

     

    I do have another questions reagarding the output codes of the ADS1258 itself. I have read the datasheet but may be little bit confuse. Here is the question:

     

    First, I would like to design a circuit similat to the one shown in Figure 68. of the ADS1258-RP datasheet, which is the Multichannel +/-10V single ended, bipolar input. I have the following setup:

    * reference circuit built using REF3040, which is a 4.096V reference.

    * MUX out connected to OPA2365 and it has a 2.2nF cap connected very close to ADCINP and ADCINN.

    * ADS1258 is powered with +/- 2.5V

    * Input is with a voltage divider circuit with (8K and 2K), where AINx voltage is set to be +/-2V. Single Ended input, Auto sense.

     

    With the above setting, since my reference is at 4.096V, does it mean the output code would only reflect 50% of full scale resolution? I read somewhere in the schematics that the full scale reference is +/- 1.06 Vref, in this case approx +/- 4.096.

    I was thinking to increase AINx voltage to +/-4V but realized that this is not a good idea as it exceeds +/-2.5V (VDD and VSS).

     

    Please advise.

     

    Tandi.

  • 0 in reply to Tandi Wijaya
    Prodigy 90 points

    Hi TI expert, 

    Please help to comment on my previous post. 

    Thank You.

    Tandi.

  • 0 in reply to Tandi Wijaya
    TI__Mastermind 43915 points

    Hi Tandi,

    Input range is +/-Vref, and since the ADS1258 is a differential part, you can swing the input +4.096V and -4.096V differentially. A couple things to note.

    - Vrefp cannot exceed AVDD per the data sheet, meaning that if you are using a 4.096V reference, you need to be applying it by setting VrefL to -2.5V and VrefP at -2.5V+4.096V. My recommendation is to use a 2.5V ref.

    - Since the range is +/-Vref, if you are using the ADC in a single ended fashion and only swinging the input positive, you will only use half the code range.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 90 points

    Hi Tony,

    Thanks for your feedback. That's what i thought. I will go ahead to change my design to use 2.5V reference.

    Tandi.

  • 0 in reply to Tony Calabria
    Prodigy 90 points

    Hi Tony,

    I would like to clarify the reference again with you. If I were to use the below reference circuit (similar to ADS1258 page 44 but with 4.096V reference used in Electrical characteristics of ADS1258 datasheet), with Input to AINx is set to +/-2V, does it mean:

    1. Single ended input mode will generate 50% of full output code resolution?

    2. Differential input mode will generate 100% of full output code resolution?

    3. Not recommended to use 4.096V Ref, but use 2.5V Ref instead?

    Thanks so much for your expertise.

    Tandi.

  • 0 in reply to Tandi Wijaya
    TI__Mastermind 43915 points

    Hi Tandi,

    Answers below -

    1. Depends on how you configure the inputs. If you use a single ended configuration and connect AINCOM to AVSS (-2.5V), then you can only swing the positive inputs from -2.5V to 0.5V with a 2V reference. If you connect AINCOM to ground, you can swing the positive input from -2V to +2V and have the entire code range to work with since you are using +/-2.5V supplies. If you swing the positive input from 0V to 2V (AINCOM tied to GND), then you will only be using 50% of the full output code resolution.

    2. Differential input allows you to swing the positive and negative inputs so that the differential voltage is +/-2V. This will allow you to use the entire code range.

    3. 2.5V ref may work better. Keep in mind that the full scale is 1.06Vref and not just Vref like typical delta sigmas. Therefore, using a 2.5V ref in scenario 1 (above) will not allow you to use every code within the range. You will probably be utilizing >90% of the codes though.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 90 points

    Hi Tony,

    Thanks for your input. I will discuss this within our design team.

    Tandi.