Negative and positive voltages with LTC1403


Im new here and also electronics ;). I need to measure +/- 3.3V signal. I usually use microcontollers with level shifter but now I need some more precise ADC. I picked LTC1403. How can I measure negative voltages with differential input? Do I need level shifter and IN connect to ground? Or to minus of my of my power supply? Its my first time with external ADC.

Before signal I have some filters.

Parents Reply Children
  • 0
    •  Analog Employees 
    on May 29, 2019 3:25 PM over 1 year ago in reply to Kil307

    The inputs of the LTC1403 will have a small transient on them when the ADC goes into acquire mode. This transient must settle before the start of the conversion. This is the acquisition time of the ADC. For the LTC1403 running at its full speed the acquisition time is only 39ns. If you look at the small signal step response of the LTC1992 you will see that it takes over a 1us to settle. If you look at the settling time of the ADA4945 in low power mode, it settles to a 16-bit level in 150ns. This is to an 8V step. Settling to 12bits from the small transient on the ADC inputs should be possible in 39ns. The supply current of the ADA4945 in low power mode is not that much more than the LTC1992.

  • You really help me understand problem. Thank you

  • Hi

    After some revision I decided that I need bigger ADC. I choose AD7980 or (eventaully) AD7915. A think I can drive AD7915 with ada4945 but what about AD7980? Only chance is to shift level to drive AD7980? You propose me before LTC2328 but it to expensive for this design.

  • 0
    •  Analog Employees 
    on Jun 10, 2019 2:25 PM over 1 year ago in reply to Kil307

    The AD7980 is meant to be driven pseudo-differentially. The -input of the ADC will be grounded while the +input of the ADC is driven from ground to VREF. Using the ADA4945 to drive the AD7980 is possible if only one of the outputs is connected to the +input while the -input is grounded. The AD7915 is meant to be driven fully differentially and can be driven by both ADA4945 outputs which results in better SNR and THD performance. This extra performance is slightly more expensive.