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ADS7953 with 3.3V full scale conversion

Paolo Messina
Prodigy 20 points

Other Parts Discussed in Thread: ADS7953, ADS1158

Quick question is: can I use ADS7953 with +VA=3.3V and input channel voltage in range 0-3.3V? What do I need on REFP/REFM pins?

I'm confused by the datasheet and could not find any reference design for my needs.

I see REFP must be in the range 2.0-3.0V, but then I see that conversion uses 2xVref or Vref, whichever is smaller than +VA.

So with +VA=3.3V I can't use Vref=3.3V because it's too high, nor +VA=3.3/2=1.65V because it's too low.

So why does the datasheet specify that +VA can be in between 2.7V and 5.25V, if I can't use 3.3V with full scale conversion?

Can somebody clarify this a bit?

Thanks,
Paolo

  • TI__Mastermind 43915 points

    Hi Paolo, 

    The ADS795x series of parts have certain restrictions which do not allow you to use a full scale range of 0 to 3.3V running at +VA=3.3V and meet datasheet specs. The reference voltage is limited to 2V-3V and the full scale input span is 0 to Vref or 0 to 2*Vref as long as 2Vref<+VA. This part was designed to have the reference set at 2.5V to achieve maximum performance, however, a reference set anywhere from 2V to 3V will still allow you to meet datasheet specs.

    For your application, there are two possible setups, however, they are both out of spec. Setting the range to Range 2 and using a 1.65V reference exceeds the minimum reference we guarantee, and setting the range to Range 1 and the reference to 3.3 V exceeds the maximum reference we guarantee. Now, to meet specs and have a 0 to 3.3V range, you will have to bump the +VA supply voltage up to 4V or 5V and then use a 2 to 2.5V reference using the 0 to 2Vref range. 

    The datasheet specifies that +VA can be between 2.7V and 5.25V because it can, however, the full input range cannot be utilized under some +VA supply voltages. 

    Regards,

    Tony Calabria

  • in reply to Tony Calabria
    Prodigy 20 points

    Hi Tony,

    Thanks for the explanation, it was also my understanding but I wanted to be sure.

    Is there any other TI ADC I could use? I need 16ch (single ended) because I'm space constrained, and have only 3.3V available (derived from 3.6-3.7V battery).

    I saw ADS1158 / 1258 but they have too high AVDD (4.75 - 5.25V).

    Or what should I expect if I use a 3.3V reference with ADS7953? Can it damage the device? Or is it just a performance hit?

    Best regards,
    Paolo

  • in reply to Paolo Messina
    TI__Mastermind 43915 points

    Hi Paola, 

    The ADS795x and ADS1158/1258 are the only 16 channel ADCs we have available in the precision analog catelog. The ADSxx58 parts are delta sigma converters meaning they will give you higher precision (digital filter used to give you higher ENOB) and are used in slower speed applications. The other option would be to use two 8-channel ADCs. Going to two 8-channel ADCs you will have a variety of ADCs to choose from. 

    With the ADS7953 using a 3.3Vref and 3.3V supply. I do not suppose that it will damage the device, it will just hurt the performance. I will have to double check and get back to you. 

    Regards,

    Tony Calabria