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LMH1983: Free-run Control Voltage through internal DAC

Part Number: LMH1983

Hi all,

We want to manually control the output voltage of VC_LPF pin thanks to internal DAC of LMH1983. To make this, we use the "LOR Control Free-run Control Voltage MSB" register (address 0x18) and the "LOR Control Free-run Control Voltage LSB" register (address 0x19).

We would like to know when the new 10-bit value applies to internal DAC. After the write of MSB Register ? After the write of LSB register ? After the write of any of these two registers ? The datasheet doesn't talk about a locking mechanism (unlike registers x16 and x17). 

My main concern is about the change of value from b1011111111 to b1100000000.

There is a risk that my value going through an unwanted temporary value that may create a large jitter on my output clock.

Many thanks for the help on this subject.

Regards,

Arnaud

  • Dear Arnaud,

    thank you for your interest in LMH1983.

    The mode you are looking to use is described in section 8.4.4. "free run".

    I do not have timing information on register x18 and x19 as to when the DAC is being updated. Section 8.5.2 of the D/s dos not provide that details. You could easily find out with a scope and monitor VC_LPF and the SDA line.

    I have not verified this but I would not worry about transition in the DAC (similar to 0111 to 1000...).

    Side note: Running free on PLL1 will prevent PLL one to do any phase noise reduction inside the loop bandwdith so you rely on the VCXO for all offset.

    Regards, Simon.