AD7091R-5 using internal reference

Hello,

until now I'm not able to use the AD7091R-5 with internal reference of 2,5V.
I'm sending this bytes to the AD7091-R to configure it :
\x02\x06\xC1\x01\x0F\x00
meaning config reg 0x02 is set to 0x06C1
-> CMD Mode = 1,
-> Software Reset = 1,
-> Cycletimer to default,
-> P_DOWN = 1, which should enable the internal reference.
config reg 0x01 is set to 0x0F -> All 4 channels on
then set config Reg 0x00

After this I do 8 byte reads where I can see that the conversion results are comming,
the channel index in each word counts also up, but the read values are wrong because
the internal reference is not 2,5 V (I measured around 3,2V at the REFIN/OUT pin)
Related to this "reference" the values are correct, so the AD Converter itself is ok,


But what's going wrong setting the INTERNAL REFERENCE ?

Please help!

  • 0
    •  Analog Employees 
    on Aug 19, 2018 5:47 PM

    Hi Wollli,

    AD7091R uses internal reference as default unless external voltage is applied on the REFin/REFout. For me to help you, can I have your schematic?

    Regards,

    Daryl

  • 0
    •  Analog Employees 
    on Aug 19, 2018 11:02 PM

    Hi Wollli,

    Apologize, I was referring on the wrong device (AD7091R). From the Table 3 of the AD7091R-5, Analog input voltage should be from -0.3V to Vref + 0.3 V only with respect to ground. Having input voltage greater than the maximum voltage will cause the diode (D3 from Figure 36) to conduct affecting the operation of the reference voltage.

    Regards,

    Daryl

  • Hey Daryl,

    I think the internal reference is default OFF, this is also what the datasheet says:

    Datasheet Page 15 : of AD7091R-5

    "When the P_DOWN LSB bit is set to 0, supply an external
    reference in the range of 2.5 V to VDD through the REFIN
    / REFOUT pin. At power-up, the internal reference disables by
    default."


    ????

    The schematic I can post later. Is it possible that this behavour can be caused by overvoltage
    at the one of the analog Input pins? (3,5V) This is more than 2,5V+0,3...

  • Hey Daryl,

    No problem, but your idea with the diode is also my thinking. yesterday I saw later that one of the channels
    was always in overdrive. I'll correct that and then the 2,5V internal ref. hopefully works :-)

    Thank you !

    Best regards

      Wolli

  • Hello Daryl,

    Everything is running fine now, the overload at the analog input made the 2,5V internal reference 
    to rise to Uin-0,3V !

    bye