AD7175-2 Unipolar Resolution

Hi,

I am looking to use the AD7175-2 with an external 5V reference to sample a unipolar differential signal (0 to 5V range); however I was concerned with possible resolution loss compared to bipolar operation (-2.5V to 2.5V range), as some of the other ADCs that I have been looking at will simply keep the FSR bipolar, thus leading to a 1 bit reduction in resolution. The datasheet does not mention any NFR performance degradation with respect to this issue, but I would like to confirm this. Many thanks.

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  • +1
    •  Analog Employees 
    on Dec 26, 2020 3:31 AM in reply to owang

    Hi, 

    Apologies for confusion. The above scenario is assuming that the part is operating with a single supply. There are two input voltage range to look at for most of the lower bandwidth Sigma Delta ADCs. First is the absolute input voltage range (per AIN pin) where each pin can go from AVSS to AVDD1 when input buffers are enabled and AVSS-0.05V to AVD11+0.05V when unbuffered. So If you are operating with a single supply then the ADC per input pin cannot accept a negative absolute input voltage. 

    The other specified input voltage range is the differential input, meaning AINP - AINM. The differential input voltage range can go from -VREF to +VREF. So if you have VREF = 5V then the ADC can accept differential input voltage from -5V to 5V. However, the ability for the ADC to accept a negative differential input voltage depends on whether your ADC is configured in unipolar or bipolar mode which is selected at Setup configuration register.  

    Here's an FAQ describing the difference between the two output coding format. 

    AD717x: FAQs - Documents - Precision ADCs - EngineerZone (analog.com) 

    I would recommend to visit our Virtual Eval Tool, this allows you to play around various ADCs including AD7175-2. It will allow you to input different supply, analog and reference input voltage range, and other possible ADC configurations and check the typical performance. The tool will also pop up some errors/warning if the selected configurations is outside the specified range. Hope this helps. 

    Virtual Eval | Analog Devices

    Thanks,

    Jellenie

  • I have tried to use the virtual evaluation tool and using a bipolar power supply. However, I can still cannot achieve the full 10V range (e.g. AIN+ = 5V, AIN- = -5V) as the inputs are limited into the range between AVSS and AVDD1, and as previously mentioned, the maximum specified difference between the two is less than 10V; a similar argument can be made for the VREF+ to VREF- range as the differential input range for those pins is limited to AVDD1, and therefore the inputs differential voltage cannot exceed 5V. This is reflected in my usage of the virtual eval tool. If you are able to configure the ADC to accept a differential voltage of greater than 5V using the eval tool, could you share a screenshot? I have tried without any success.

  • 0
    •  Analog Employees 
    on Dec 28, 2020 12:47 AM in reply to owang

    Hi, 

    I don't think you can achieve 10V differential voltage as that will be outside range on either the absolute AIN pin or differential input pin (AIN+ - AIN-). The -5V to 5V with VREF=5V is a differential input voltage range and not an absolute AIN pin voltage. You are correct the part cannot operate full 10V range (e.g. AIN+ = 5V, AIN- = -5V) as the supply is limited and it is also outside the -5V to 5V differential voltage. Like I said differential voltage is AIN+ - AIN- and must be between -5V to 5V only so a 10V differential voltage is outside range. If you are operating with a split supply then the part cannot handle -5V per AIN pin. 

    What I was saying on the first reply is if you tend to operate on a single supply with AVDD=5V, AVSS=0V and REF+=5V and REF-=0V or split supply with AVDD=2.5V, AVSS=-2.5V and REF+=2.5V, REF-=-2.5V. Then both configuration can have +/-5V differential input voltage range resulting to the same resolution. But I am just taking note that the part should be configured in bipolar mode (coding format) to be able to convert a negative differential voltage (0 to -5V). Please do not confuse with coding format with the actual supply voltage. Please do not also confuse with absolute input pin and differential input range. Have a read through on my previous reply regarding the allowable per input range and differential input range and also the difference between the coding format. The virtual eval tool will let you know if your configuration is outside or within the ADC range. 

    Thanks,

    Jellenie