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LMH1983: LMH1983 startup failure

Part Number: LMH1983
Other Parts Discussed in Thread: LMH1981, LMP7711


I have added the LMH1983 and it fails to startup for some reason. There is also an LMH1981 on the PCB connected and it operates fine feeding back a Vsync & Hsync to the LMH1983.

The VCXO is oscillating @ 27MHz.

I can see the PLL1 output sometimes tries to oscillate @ 27MHz for a few milliseconds after powerup but then dies, other times it doesn't have any output at powerup.

I have the option of fitting the LMP7711 but at present the VC_LPF is fed back directly to the VCXO (CTS 357LB3I027M000).

I erroneously omitted adding the CP, CS & RS components to the PCB layout, will this be the issue? I have strapped on the CP = 1uF but still no joy.

Should have stated, I was trying to access I2C registers and the device does ACK but readback always gives 0x2A, I suspect this is the 0x00 register contents and that the address is not being decoded as the 27MHz PLL1 is not running?



  • Hi Paul,

    I'm not very familiar with device but I will try to help here.

    I think the issue here is the omission of CP, CS, and RS. Without the second order loop filter components, the PLL is apparently not locking. Even with CP installed, you will have no phase margin, and thus the loop will be unstable.

    Special note on I2C communication from datasheet section 8.5.1:
    Please note: The I2C interface of the LMH1983 requires the 27 MHz VCXO clock input to be running in order to read I2C data packets into the 27 MHz clock domain. If the 27 MHz clock is not running, the I2C interface should still respond (ACK), but Write commands may be ignored and Read commands may return invalid data.

    Kind regards,