Post Go back to editing

AD7124-8 Decouplig voltage pin 24

Hi,

We are using the AD7124-8 with ± 1.8V power. On pin 24 I find a voltage of 0.105 V and as I see it must be 1.8 V aproximately. I have changed the decoupling capacitor and the voltage does not change. What could be the problem? The readings on the SPI are erratic, changing from positive to negative full scale. IOVDD is +3.3 V. At pin 22 there is 0.700V. Vontage at  pin 1 is 1.8 V .

Thanks.

Parents
  • Hi, 

    Does "We are using the AD7124-8 with ± 1.8V power" mean that your AVDD to AVSS is ± 1.8V? if that's so, the AD7124-8 will not work properly and will not provide the right voltages from mentioned pins above. 

    These are the specified values for the AVDD to AVSS of the AD7124-8:

    Regards,

    Andrei

  • Thanks for your answer. Maybe I didn't express it correctly, I'm sorry. The ADC is connected in the dual power supply mode, with 3.6 V between AVdd and AVss and ±1.8 V between ground and AVdd and AVss. 

  • Hi, 

    Are you using our evaluation board or your own customize board? 

    Can you ensure that the voltage at AVSS is -1.8V and not directly connected to GND? And have you tried to operate in single supply for example are you using the same issue? And what cap value did you used?

    Thanks,

    Jellenie

  • Hi,

    Thanks for your answer. I'm sure that both lines, DGND and AVss are different. The board is the 6th version of a prototype (with little differences between every version). I can't easily change to a simple supply power. I've changed the capacitors at pin 1 and pin 24 to ensure they were fine, both 0.1 uF, but the problem persists with 0.1 V al pin 24 (referred to DGND).

    Thanks a lot.

  • Hi, 

    Can you probe/measure the following? 

    What are the voltages (relative to 0V / GND) of AVDD, AVSS, REGCAPA, IOVDD, DGND, REGCAPD ?

    The decoupling cap at REGCAPA I’m curious why you reffered it to DGNF? Can you referenced it to AVSS instead?

    Can you also share your schematic? I would suggest to check our EVB schematic and use it as your reference. 

    Thanks,

    Jellenie

  •  Hola Jellenie,

    Los voltajes referidos a DGND (pin 3, 0 Voltios) son: AVdd 1.800 V, AVss: -1.800, REGCAPA (Pin 24): 0.102 V, IOVdd 3.3 V, DGND es la referencia de 0 V, REGCAPD (pin 1) 1.800V. El voltaje entre los pines del REGCAPA (0.1 uF) es 1.902 V. Les muestro la parte ADC del esquema.

    Thanks

  • Hi, 

    Apologies. Can you speak english please?

    EngineerZone (ez.analog.com) offers support for English language inquiries only. Please post your question in English so that all members can read it. 

    However, looking at the numbers I think what you are seeing is normal 1.9V with respect to AVSS. The REFOUT will also be 2.5V with respect to AVSS so 0.7V with respect to GND is normal if AVSS is -1.8V from GND. 

    What do you mean exactly by erratic readings?

    Thanks,

    Jellenie

  • Hi,

    Sorry because my english translator play a joke on me...  I think all voltages are right so the problems must be from other part of the circuit, because teh ADC works fine with a external linear power supply, and not with the LTC3260 we are using. When I said erratic it means that in every reading it's different, passing from the upper to the lower full scale and intermediate values without doing nothing at the input. So maybe it's a problem of compatibility between both circuits.

    Thanks a lot   

  • Hi, 

    So you meant that the issue is only present when you are using LTC3260? Can you confirm that you are getting valid results from the ADC when an external supply is used? 

    I think the power product has high switching speed and high transient so careful consideration especially in the component placement and layout is needed. We do not also usually recommend to use switching regulator to supply directly our precision ADCs as it is really subject to high noise due to its switching frequency. 

    I’d suggest to ask someone from the power by linear community to help you with the power solution. 

    Thanks,

    Jellenie

  •  an LMThat's it. When I use the LTC3260 appear the problems but not with a supply using two linear regulators (LM 317 / 337). When supply is linear all works fine.I will ask for a power supply solution.

    Thanks a lot.

Reply Children
No Data