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DAC34SH84: Problem at output with low level I and Q inputs

Juan Salvador Prat
Prodigy 65 points
Part Number: DAC34SH84
Other Parts Discussed in Thread: CDCE62005

Hi there,

We are using the DAC34sh84 in a custom board designed in house. We use the complex mixer to create a RF signal from I and Q components.

I am experimenting some strange behaviour when trying to send small signal values.

As I said I send I and Q values to the DAB interface, I enabled x4 interpolation, configure the NCO and use the complex mixer to have in IOUTA a signal as RF = I*cos()-Q*sin().

When I send a small value with 0º phase I = 0010h and Q = 0000h I can see in the sprectrum analyzer a power of -57dBm

When I send I = FFFFh and Q = 0010h, that is, the same value but with 90º I can see in the spectrum analyzer that the power of the RF signal is -62dBm.

I can not undertand this difference in power. Indeed in the second case, our system is transmiting FFFFh instead of 0000h, so we should expect some little increase in the power but never a reduction.

We operate in twos complementes so FFFFh must be almost as 0 as 0000h.

I can see this behaviour wven with values higher than this but as the value gets higher this effect is less significant. 

Any clue?

Thanks in advanced,

Juan.

 

  • 0
    TI__Intellectual 1400 points

    Hello Juan,

    Your questions have been acknowledged and we are currently working through solutions to provide you.

    Best Regards,

    Kelvin

  • 0
    TI__Intellectual 1400 points

    Hello Juan,

    Can you please send screenshots of the the input, digital, output, advanced and CDCE62005 control pages as outlined by the GUI. 

    Best,

    Kelvin

  • 0 in reply to Kelvin Ayala
    Prodigy 65 points

    Hello Kelvin,

    I do not know what GUI do you refer to. We have built a custom board and we use in it the chip DAC34SH84. We configure the chip by the SPI interface from a FPGA.

    Best, 

    Juan.

  • 0 in reply to Juan Salvador Prat
    TI__Intellectual 1400 points

    Hello Juan,

    Upon review there are several possibilities for the difference in power you are experiencing. To narrow down the search I will be listing action items to be taken to aid in examining for similar changes when testing further in lab.

    • Please provide details on the differences in power when testing at higher codes such as values of above 4096 and 8192. Please document details of the performance when increasing/decreasing codes so that we could better provide assistance if needed

    • Be aware that the DAC34SH84 implements a segmented DAC approach and as such uses a combination of thermometer and binary-weighted circuits. The MSB will be represented by a thermometer coded circuit implementation while the LSB will be binary-weighted. Please reference this in your design implementations in case they should affect the performance of your device.

    • Decrease the NCO frequency and examine if this will resolve the power issues being found. Please document details of the performance when increasing/decreasing codes so that we could better provide assistance if needed.

    Best,

    Kelvin 

  • 0 in reply to Kelvin Ayala
    Prodigy 65 points

    Hello Kelvin,

    About the implementation of the DAC (thermometer and binary weighted circuits), I understand that this is transparent to me, in the datasheet there is no information about this.

    About codes above 4096 and 8192, I use the whole range and with values above 4096 I do not see (or I can not see, maybe the effect is so small) this effect in my spectrum analyzer. 

    About decreasing the NCO frequency I will do this test and I will come back to you with more information.

    Thanks,

    Juan.

  • 0 in reply to Juan Salvador Prat
    TI__Intellectual 1400 points

    Hello Juan,

    I have attached the link " DAC architectures " that will go into further details as to the wide range of possible DAC architectures that could be implemented across our products.

    Best,

    Kelvin