AD7091R-5 using internal reference


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 :
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?



  • 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.



  • 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


    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


  • 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 !