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Recommended SAR ADC input amplifier

Tom Warner1
Intellectual 275 points

Other Parts Discussed in Thread: MSP430FR2422, TLV376, LM7705, INA240, OPA340, INA293

Hi -

My application is for high bandwidth voltage/current measurement of power supplies into an SoC.  I am using an MSP430 to buffer the ADC measurements at ~200 kHz.

I'm looking for 2 different TI input amplifiers to feed input channels into a TI MSP430FR2422 SAR ADC:

  1. A voltage measurement amplifier that can measure the supply voltage which can range from 0V to 0.8V (most likely this supply will always be ~0.65V when supply is active).
    1. My assumption is that I need to amplify the 0.8V supply ~4x to make use of the 3.3V range of the SAR ADC for maximum resolution.
  2. A current sense amplifier that will be across a 40 mΩ resistor with ~40 mV full-scale-range.
    1. I previous had been looking at the TI INA240A2.  Is this current sense amplifier compatible with the TI SAR ADC?

Thanks in advance!

Tom

  • 0
    Guru 48395 points

    Is the SAR input differential or single ended, what is your expected clock rate - I assuming you would like to use 3.3 or 5V single supply?

  • 0 in reply to Michael Steffes
    Intellectual 275 points

    The SAR input will be single-ended.

    I'd like to sample at ~200 kS/s.  I believe the ADC clock is running at ~5 MHz.

    Since I believe the analog input range in the ADC in the MSP430 is capped at 3.6V, I'd prefer 3.3V single supply.  (I'm assuming this may limit my ability to measure voltages close to 0V, but this is okay).

    Thanks,

    Tom

  • 0 in reply to Tom Warner1
    Guru 48395 points

    Well just talking your gain of 4 stage, say we want a solid 1MHz, need about 5MHz GBP min, look at the TLV376 and consider a LM7705 to get true swing to ground range. 

  • +1
    TI__Mastermind 24495 points

    Tom,

    What resolution are you looking to settle to inside this time? A great tool to help you design the front end here is the Analog Engineers Calculator tool, specifically the "ADC SAR Drive" section. This section will help you define needed parameters, as well as help design a charge bucket between the amplifier and ADC inputs:

    I agree with Michael here. You normally need an amplifier bandwidth of about 4x faster than the designed charge bucket cutoff for accurate settling, so the bandwidth of the INA240 is most likely going to fall short here. For a sampling rate of this magnitude, I would either potentially look at the INA293, or buffering the chosen amplifier with something like OPA340. The OPA340 may also serve in the voltage amplifier space you mention as well. 

  • 0 in reply to Carolus Andrews
    Intellectual 275 points

    Thank you Carolus and Michael.