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AD7176-2 reference and AD8475  gain

Question asked by iozana on Jul 22, 2017
Latest reply on Aug 3, 2017 by JinoL


I my application I need at least 22 bits to measured 1.5V to 4V input voltage, with Output common mode voltage of ~2.55V that source from Fully-Differential Isolation Amplifier (current shunt). This amplifier is located on one custom card (card 1). For that application I thought to use the AD7176-2 cascading to AD8475 (on another card, card 2, located ~70cm from card 1) component but in order to do so, I need you to answer the following questions:  

  1. According to Figure 47 in the AD7176-2 device datasheet, you used 5V external reference (ADR445). Why is that? Is it because the signal that comes from the AD8475 component is between 4.5V to 0.5V (In+/ In-)?
  2. According to Figure 55, In the AD8475 device datasheet, it seems that the gain is 0.2 and not 0.4. I understand it connected somehow to that the differential gain is double the single ended gain… what will be the output voltage in my application and 0.4 gain (1.5V to 4V instead +-10V as showed in Figure 55)
  3. Can you recommend another Precision, Selectable Gain, Fully Differential Funnel Amplifier but with gain 1? (I only need the drives switched capacitor and Σ-Δ ADCs capability with precision to ~22bits).
  4. If the answer to question 3 is "no" (don’t have recommended component), can you recommend another Precision, Fully Differential Amplifier but with gain 2 (I'll amplifier the signal right after the current shunt amplifier (Card 1) just before the AD8475 (Card 2) in that case I might reduce the noises that will probably added to the 70cm between cards)

Thanks a lot,