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ADS8345 Single-ended measurements datasheet confusion

Other Parts Discussed in Thread: ADS8345, INA333

In my application, I am using an ADS8345 16-bit, 8-channel ADC in single-ended measurements mode to measure a 1-5V signal from INA333. The conversion is set up using external clock from my Atmega325, which I interface to using the MCU's built-in spi. 

My question stems from observing 2 figures in the ADS8345 datasheet - Figure 3 (Single-ended input-common voltage range vs Vref) and the data format table (Table VIII). Namely, in the Data format table, the full-scale range is described as 2xVref, ranging from +full-scale to -full-scale (0x7FFF - 0 - 0x8000), with midscale being 0V. However, at 2.5V, this would mean the full-scale is 5V. Could I get some clarification on the actual usable ranges, or is my application (1-5V measurements, with 0-1V range not being counted [essentially throwing away 20% of the measurement range] )

What am I missing here? Is it even possible to use this ADC for inputs that aren't  +/- around some range (0V midscale in this case)?

  • Hi Rainer,
    The analog input for ADS8345 is the difference between CHx input and COM input. For your single-ended input on CHx, the 0V in table VIII is not your signal's 0V, it means the 0V difference between your signal on CHx input and the common-mode voltage on COM pin. For example, if you make +5V as the power supply on Vcc of ADS8345, and connect +2.5V to the COM pin(your common-mode voltage Vcmv is 2.5V) and your Vref is +2.5V on Vref pin:

    when your single-ended input signal on CHx is +2.5V, the difference between CHx and COM will be 0V, so the conversion code will be all zero(Midscale).
    when your single-ended input signal on CHx is +5V, the difference between CHx and COM will be +2.5V, so the conversion code will be 0x7FFF(+Full-Scale).
    when your single-ended input signal on CHx is 0V, the difference between CHx and COM will be -2.5V, so the conversion code will be 0x8000(-Full-Scale).
    Your +1V minimum signal voltage is covered in the negative range in table VIII.

    In general, the selected input channel (CH0~CH7) will properly digitize a signal in the range of (Vcmv – Vref) to (Vcmv + Vref).
    Please let me know if you have more questions. Thanks.

    Best regards
    Dale Li

  • Thanks!
    In my panic of having done something wrong in my design, I literally forgot about COM input.