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TMS320F28377S: Connect output of AMC1350 to ADC Input of TMS320F28377S

Michael Jeong1
Expert 2630 points
Part Number: TMS320F28377S
Other Parts Discussed in Thread: AMC1350, , TMS320F28384S

Hello Expert,

I would like to ask if it is possible to directly connect the output of the AMC1350 after setting the ADC of TMS32028384S to 16bit differential mode and Vref=3.0V.

Internal review results:
Output Vcmout of AMC1350 : 1.34V ~ 1.49V ( normal 1.44V )
The Vcm input range in 16bit Differential mode of TMS320F28377S is as shown in the datasheet below.

Question:
1) In this case, is it impossible to connect directly to the AMC1350 and TMS320F28384S?
2) If no, Is there any other way to directly connect AMC1350 and TMS320F28384S while using them in 16BIT Differential mode?

Best Regards,

Michael

  • 0
    TI__Mastermind 36526 points

    Hi Michael,

    You will have decreased performance if your ADC input common mode voltage is not within the range specificed by the datasheet. If you are using 3V reference, then your input Vcm needs to be between 1.45V and 1.55V . 

    I cannot think of any way to directly connect your AMC1350  to TMS320F28384S while using them in 16BIT Differential mode. I think you would need an amplifier with a small offset and gain less than 1. You could then remove the offset with the ADC post processing block. 

    Best Regards,

    Ben Collier

  • 0 in reply to Benjamin Collier
    Expert 2630 points

    Hi Benjamin,

    Thanks for your reply.

    Can I get your guide (device information, schematic, etc for design) like your below comment?

    Your comment : think you would need an amplifier with a small offset and gain less than 1

    Best Regards,

    Michael

  • 0 in reply to Michael Jeong1
    TI__Mastermind 36526 points

    Michael,

    I will try to put something together today.

    Best Regards,

    Ben Collier

  • 0 in reply to Benjamin Collier
    TI__Mastermind 36526 points

    Michael,

    Actually could you give me some more context about how the signal will be used? I am thinking of the possible amplifier circuits that would help solve this issue, but some of them will reduce the ENOB that you will get from the ADC, and I am not sure if that is acceptable.

    Best Regards,

    Ben Collier

  • 0 in reply to Benjamin Collier
    Expert 2630 points

    Hello Ben,

    I have an another inquiry like below.

    Could you please check and send feedback?

    1. Do you have a reference schematic of amplifier circuit to solve your commented issue?

    2.  About ENOB decreases, but if you can accept this, you can just connect it directly

         I have a question here.

        1) For example

            When AMC1350's Vcm=1.44V and AMC1350's input InP=5.0v, InN=0.0V, the output becomes Voutp= 2.44v, Voutn= 0.44v.

            At this time, the tms320f28384s

            ADC Result_x= 65536*( AdcInyP – AdcInyN + VrefH)/(2*VrefHi)

                                   = 65536*( 2.44v – 0.44v + 3.0v)/(2*3.0v) = 54613

         2) But is the above analog to 16bit digital formula wrong?

             I would like to know why ENOB is decreasing and by how much.

             If possible I would like to connect directly without a separate amplifier.

    Best Regards,

    Michael

  • 0 in reply to Michael Jeong1
    TI__Mastermind 36526 points

    Michael,

    That formula is only guaranteed for when Vcm is inside of the allowable range. 

    A couple suggestions to work around this:

        1. Use a difference amplifier circuit then use single-ended ADC :https://www.ti.com/lit/sboa274

              I think this would work, but then you could lose ENOB if the op-amp does not have the right parameters.

        2. Use differential amplifier to reduce the window for Vcm. I think this could help you understand how to do that: https://www.tij.co.jp/jp/lit/pdf/slyt119

    Best Regards,

    Ben Collier