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AFE7070: NCO frequency change rate

Part Number: AFE7070

Hello

Does anybody know what would be the fastest frequency change rate of the NCO?

If I change the frequency tune word, will I see a change in frequency on the next  DAC sample or is there any delay I have to consider?

Is there any more detailed documentation about this part? I'm not sure which supply pin has what function. 

Do I need to supply the DACVDD18 and DACVDD33 simultaniously or does it depend if I supply the part with 1.8 or 3.3V?

  • For the power supply, you need to supply both 1.8V and 3.3V to DACVDD18 and DACVDD33 respectively.  Only the Fuse and IO power has the option of choosing 1.8V or 3.3V.

    Refer to the "NUMERICALLY CONTROLLED OSCILLATOR (NCO)" section in the datasheet.  That section describes a couple of options to program the NCO.  In the AFE7070, you have the digital latency (i.e. how many clock cycles to get sample data through the device) and analog delay with the LPF and modulator.  The digital latency has some dependency on the clock mode and FIFO.  The datasheet outlines the options there.

    In short, if programming the NCO non-synchronously, you will only have the normal latency of the DAC in its existing mode.  In other words, the NCO updating does not introduce a delay/latency of its own.  After that, there will be the normal analog delay of the BB filter and modulator.  There is no supporting data for the analog delay.  There will be some dependence on the programmable BB filter setting and BB frequency of operation.

    -RJH

  • Thanks for the clarification, a new question has arrived

    I have read that it is neccesary to pull the SDENB low for 2 to 5 bytes in order to start a serial transfer.

    Is this correct are 2 to 5 bits meant? Otherwise you could use the serial interface only for half of the time.

  • No, not exactly.  Please refer to section on Serial Interface on page 15 of the datasheet.  The conventional SPI programming approach would be to write the address followed by the data.  Each are 1 byte, so total of 2 bytes.  In the address byte there are two bits to designate how much data is being sent.  Normally set those bits to b00 which corresponds to the aforementioned 2-byte transfer of 1 address byte followed by 1 data byte.  The SDENB bit is pulled low to facilitate that communication to the device. 

    You may want to use the multi-byte transfer on occasion, but it is not necessary.  Multi-byte transfer can come in handy when programming something like the NCO that spans across several registers.  The SDENB is maintained low across all the bytes being transferred and returns high once the operation is completed to load the data to the device.

    --RJH