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ADC12020: Peak to peak input voltage swing in Differential input operation vs single ended input operation

Mohandas Amarnathan
Prodigy 50 points
Part Number: ADC12020

Hello,

We are evaluating ADC12020 for a new product development. The characteristics of the input to the ADC is as follows:

1- Single Ended
2- 0 to 4.15 volts p-p

In order to avoid saturation of the ADC12020 can TI confirm if the following is only configuration:

1- Single Ended Input
2- Vref = 2.075 (we prefer to keep this to 2.4 volts, the max recommended)
3  Vcm = Vref

Is this correct that with Differential input, the maximum swing is Vref? And to achieve the 2Vref swing we have to use the ADC in single ended mode of input.

  • 0
    TI__Guru* 91947 points

    Mohandas,

    We are looking into this.

    Regards,

    Jim

     

  • 0
    TI__Genius 12780 points

    Hi Mohandas,

    If you would like to use the single-ended input mode, the VIN(-) must be connected to the same common mode voltage that VIN(+) is using.

    From page 17 (Single Ended Operation) of the datasheet...

    "The peak-to-peak differential input signal should be twice the reference voltage to maximize SNR and SINAD performance (Figure 38b). For example, set VREF to 1.0V, bias VIN− to 1.0V and drive VIN+ with a signal range of 0V to 2.0V ".

    According to the datasheet, your assumptions are correct.

    Best Regards,

    Dan

  • 0 in reply to dBrock
    Prodigy 55 points

    Hello Dan,

    Myself is Vinod, colleague of Mohandas.

    Couple of concerns about this ADC in single ended application

    Are you saying that by giving VREF=2.4 and VIN- to 2.4 we can feed VIN+ with a signal of 0V to 4.8V?

    In that case what is meant by saying "The input signals should remain between ground and 4V." in the datasheet?

    and later it says " The input common mode voltage, VCM, should be in the range of 0.5V to 4.0V and be of a value such that the peak excursions of the analog signal does not go more negative than ground or more positive than 1.2 Volts below the VA supply voltage."

    If VA is 5V then this means peak input cant be >3.8V. Considering both this case how we can apply a signal swinging 0V to 4.8V to VIN+ ?

    Secondly single ended operation is not recommended by datasheet beyond 0 to 2V swing. And further the datasheet says "Because very large input signal swings can degrade distortion performance, better performance with a single ended input can be obtained by reducing the reference voltage when maintaining a full-range output". How much degradation are we talking about here?

    Regards

    Vinod

  • 0 in reply to vinod kunnambath
    TI__Genius 12780 points

    Hi Vinod,

    I appreciate your patience as I have been looking into this further.

    On page 18 of the datasheet, it says (what I believe is a typo is crossed out)...

    "The peak-to-peak differential input signal should be twice the reference voltage to maximize SNR and SINAD
    performance (Figure 38b). For example, set VREF to 1.0V, bias VIN- to 1.0V and drive VIN+ with a signal range of
    0V to 2.0V."

    "Are you saying that by giving VREF=2.4 and VIN- to 2.4 we can feed VIN+ with a signal of 0V to 4.8V?

    In that case what is meant by saying "The input signals should remain between ground and 4V." in the datasheet?

    and later it says " The input common mode voltage, VCM, should be in the range of 0.5V to 4.0V and be of a value such that the peak excursions of the analog signal does not go more negative than ground or more positive than 1.2 Volts below the VA supply voltage."

    If VA is 5V then this means peak input cant be >3.8V. Considering both this case how we can apply a signal swinging 0V to 4.8V to VIN+ ?"

    According to the datasheet, yes (the reference will be 2*VREF applied), but the 4.8V exceeds the 3.8V limit that you pointed out. I believe that 1.9V is the maximum VREF that can be used in single-ended mode if you would like to exercise the entire signal swing (0V to 3.8V).

    "Secondly single ended operation is not recommended by datasheet beyond 0 to 2V swing. And further the datasheet says "Because very large input signal swings can degrade distortion performance, better performance with a single ended input can be obtained by reducing the reference voltage when maintaining a full-range output". How much degradation are we talking about here?"

    I don't know how much degradation will be seen. I would follow the recommendation of the datasheet, and keep the VREF and input signal swing to what is recommended.

    Best Regards,

    Dan

  • 0 in reply to dBrock
    Prodigy 50 points

    Thanks Dan. Based on your response can we conclude that if we want the 2*Vref swing we should operate the ADC in differential mode. For example if we keep Vref to 2.4 volts and Vcm to 1.2 volts, Vin+ and Vin- will swing Vref w.r.t to Vcm. And Vin+ minus Vin- will be 2*vref. Can you please confirm this. We really are looking into this ADC because of 2*Vref swing it gives us.

  • 0 in reply to Mohandas Amarnathan
    TI__Genius 12780 points

    Hi Mohandas,

    Yes, this is correct. With the differential input, VIN(+) - VIN(-) will give you 2x the signal swing/VREF. I think it is a better use of the part to use differential inputs, if your application allows.

    Best Regards,

    Dan