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AD7177-2 external reference

Category: Hardware
Product Number: AD7177-2

Hello,

In my project, I am using AD7177-2 32-Bit Sigma-Delta ADC. AVDD1 and AVDD2 are connected to +2.5V, and AVSS is connected to -2.5V. Desired to convert analog signal from -2.5V to 2.5V that is inverted and connected to AIN0(inverted signal),AIN1(positive signal) inputs.

I am also using an external reference that is connected to the ADC as +2.5V to REF+ and -2.5V as REF-. The VREF range is then 5V for two differential inputs(AIN0 and AIN1). I have connected the input signal to AIN1 and AIN0 (negative signal) to use the differential input of ADC. I have successfully set the registers of the ADC to use continuous conversion mode, 5SPS, external reference, swapped AIN0 and AIN1 inputs, 32bit output, bipolar coded output and so on. 

When I leave the default settings of internal/external reference and use the internal +2.5V reference, the ADC is fully working in the range -2.5V to +2.5 in the range 0 to 2^32 bits as supposed to. But when I set the register 0x20 to use the external reference the ADC is working only from -1.25V to +1.25V in 0 to 2^32 bits. The full range from -2.5V to +2.5V is not possible to get. I also tried to change the gain of ADC using register 0x38 but this leads to resolution only to 2V, after this value the maximum (or some value between 2^31 and 2^32) value is set and the value is not changed with higher voltage. 

What is the reason for the behavior of the ADC as described? I would assume that the ADC is working in full range with my external voltage reference.

Best regards, Martin Indrych

Top Replies

    •  Analog Employees 
    •  Super User 
    in reply to Nimtram +1 verified

    Hi  ,

    Is it also possible to check the status register if you have a differential input of +1V, +1.25V , -1V and -1.25V and see if the ADC_ERROR bit is set? A 00000000 and FFFFFFFF may indicate an…

  • Hi  ,

    I just want to clarify some things. When using external reference the ADC is working only from -1.25V to + 1.25V, is that the FS value 00000000 and FFFFFFFF in code? Could you share the output code of the ADC at -1.25V to +1.25V? Are there no changes in the output code if an input beyond -1.25V to +1.25V is connected to the ADC? Also, kindly try enabling the reference buffers. 

    Regards,

    JC

  • Hello  ,
    Thank you for your answer. The ADC is wokring normally in the range -1.25V to + 1.25V, the values are as supposed to be between 00000000 and FFFFFFFF. I do not have any measurements on me for that so tomorrow I will measure it and share exact measured values of the ADC output. But when I reach the 1.25V the value of adc is FFFFFFFF and it is not changing with more voltage applied. The same happens for value -1.25, the ADC output is 00000000 and does not change when using for example -2V. I have also tried using the reference buffers, but it did not change anything. 

  • Hi  ,

    Is it also possible to check the status register if you have a differential input of +1V, +1.25V , -1V and -1.25V and see if the ADC_ERROR bit is set? A 00000000 and FFFFFFFF may indicate an underrange or overrange errors. Kindly check also the external reference just so we can be sure that it is really +/-2.5V.

    Regards,

    JC