AD7176-2 reference and AD8475  gain

Hi,

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,

Idan

http://www.analog.com/en/products/analog-to-digital-converters/ad7176-2.html#product-overview

http://www.analog.com/en/products/amplifiers/adc-drivers/fully-differential-amplifiers/ad8475.html

 

Parents
  • Hi,

    In my application I need to sense the load current from isolated 300VDC. In order to do that, I used the TI "AMC1200" with floating supply (to the 300V) and with 5V supply (to logic- analog). I had to use this specific module for the current sensing… The rest of the chain (and a lot of component on the custom card) will implement with Analog Devices products.

    Generally speaking, can would you recommend of using a buffer in both ends (right after the "AMC1200" (output stage of Card 1) and right before the Σ-Δ ADCs (input stage of Card 2). (There is ~70cm between cards, twisted pair and shielded). I think so because with a buffer stage I will have the ability to driver to Σ-Δ ADCs (AD7176-2), to put analog LPF/ antialiasing without damage the ADC input sampling capacitor and even serves as input signal protection. For that purpose I thought to use the AD8476 or AD8656 (from both ends). In addition, if you cloud recommend a low noise, precision, fully differential amplifier with selectable gain (even with external resistors…) I might be able amplify the signal from card 1 (from 0V to ~10V), stretch my signal dynamic range to ADC measurement full scale and improve to the SNR (from noise that might picked up during the 70cm between cards). Card 2 will also have the same driver (with different attenuation gain) or even the AD8475 amplifier. This will make my design a lot more robust. What do you think, can you recommend such amplifier? What about the AD8139 from both ends? Can he drive the Σ-Δ ADCs (AD7176-2)? Can he work with VS = 10V, VOCM = 0V with gain ~2? (The input signal is the differential lines with Voh=3.83V and Vol=1.27V and with Vcm=2.55V so I will get Voh=7.66V and Vol=2.54V and with Vcm=5.1V)

    Maybe I should use the AD8476 in (gain 1) both ends? Would it be better (no external resistance…)? 16 bit is fine for my application and the input voltag protection is more important then 20 or more bit resolution.

    Thanks,

    Idan

Reply
  • Hi,

    In my application I need to sense the load current from isolated 300VDC. In order to do that, I used the TI "AMC1200" with floating supply (to the 300V) and with 5V supply (to logic- analog). I had to use this specific module for the current sensing… The rest of the chain (and a lot of component on the custom card) will implement with Analog Devices products.

    Generally speaking, can would you recommend of using a buffer in both ends (right after the "AMC1200" (output stage of Card 1) and right before the Σ-Δ ADCs (input stage of Card 2). (There is ~70cm between cards, twisted pair and shielded). I think so because with a buffer stage I will have the ability to driver to Σ-Δ ADCs (AD7176-2), to put analog LPF/ antialiasing without damage the ADC input sampling capacitor and even serves as input signal protection. For that purpose I thought to use the AD8476 or AD8656 (from both ends). In addition, if you cloud recommend a low noise, precision, fully differential amplifier with selectable gain (even with external resistors…) I might be able amplify the signal from card 1 (from 0V to ~10V), stretch my signal dynamic range to ADC measurement full scale and improve to the SNR (from noise that might picked up during the 70cm between cards). Card 2 will also have the same driver (with different attenuation gain) or even the AD8475 amplifier. This will make my design a lot more robust. What do you think, can you recommend such amplifier? What about the AD8139 from both ends? Can he drive the Σ-Δ ADCs (AD7176-2)? Can he work with VS = 10V, VOCM = 0V with gain ~2? (The input signal is the differential lines with Voh=3.83V and Vol=1.27V and with Vcm=2.55V so I will get Voh=7.66V and Vol=2.54V and with Vcm=5.1V)

    Maybe I should use the AD8476 in (gain 1) both ends? Would it be better (no external resistance…)? 16 bit is fine for my application and the input voltag protection is more important then 20 or more bit resolution.

    Thanks,

    Idan

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