AD7771 reference problem

Question

Hi everyone, 
 I'm currently working on a project that involves using the AD7771 delta-sigma ADC. While the SPI interface seems to be functioning properly, I'm encountering an issue with the received data. Upon measuring the voltage output on the REF_OUT pin, I noticed it is at -1.65V, which matches the AVSS voltage level. I have thoroughly checked all the power supplies and couldn't identify any apparent problems. 
 I have an external reference (2.5V), but it's referenced to the GND, so i can't use it. 
 I would greatly appreciate any assistance. 
 Thank you in advance for your support. 
 Best regards, 
 HP

Yugel · Accepted Answer

Hi HPedro , 
 While it is true that the potential at REF_OUT with respect to GND at power-up is -1.65V, this is not the correct connection for the reference. REF_OUT matches AVSS since the internal reference output is disabled at start-up. To enable the output, you can configure the PDB_REFOUT_BUF register to 1. 
 Furthermore, if you want to use this as your reference, you can configure ADC_MUX_CONFIG bits to 01 as per the table below:

Remember to use 100 nF bypass caps. 
 And if you want to further improve its noise performance, you can connect a low-pass filter with a cut-off frequency of 10 Hz, and buffer it up again by: 
 1. Connecting REF_OUT to REFx+ 
 2. Connecting AVSSx to REFx- 
 3. Configuring REF_MUX_CTRL to use external reference i.e. ADC_MUX_CONFIG bits to 00. 
 
 For more details, you can check the Internal Reference and Reference Buffers section in the datasheet. 
 AD7771 (Rev. A) (analog.com) 
 
 Regards, 
 Yugel)

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AD7771 reference problem