HMC-980 and AN-1363 again

Hi Folks!

I've found similar questions asked but it's still not clear to me and my colleagues so here we go.

Could you please clarify the difference in purpose of the VNEG and VGATE feedback resistors as it is described in AN-1363?

I guess VNEG is clear since that's the output of the internal negative voltage generator defaulting to -2.5V. Since at turn on VGATE drops close to VNEG I might want to adjust the VNEG feedback not to violate the AMR of the amp's gate.

What's not clear is how the VGATE feedback affects behavior. The app note shows that the typical default VGATE starting voltage is -2V when VNEG is -2.5V. Where does that 0.5V difference coming from? Can I adjust how close VGATE gets to VNEG? If not this, what is the purpose of the VGATE feedback resistor in limiting VGATE to amp AMR (if any)?

Thank you!

  • +1
    •  Analog Employees 
    on Feb 18, 2020 3:39 PM

    Hi Marsupilami,

    We are working on a datasheet revision which will hopefully clear up some of this, since it is quite confusing as is.

    Essentially, the feedback resistors are used as below:

    - VNEG FB resistors are used to set VNEG higher or lower than the default of -2.46V. The equations in AN1363 allow you to calculate the resistor value. Note the placement depends on whether you want VNEG to be higher or lower than default.

    - VGATE FB resistors are used to set the threshold where the HMC980 will start the control loop to maintain the desired current in your amplifier. Once enabled, the HMC980 won't turn on until this threshold is met. This is why it's so important to set the VGATE threshold HIGHER than VNEG. If the threshold is below VNEG, the HMC980 will never turn on. The recommended buffer is about 0.3V to 0.4V above VNEG. Note that this value is the lowest VGATE will ever go when the HMC980 is enabled. VNEG is the lowest VGATE will go when the HMC980 is disabled. Therefore, the VNEG setting is used to meet the amplifier's AMR table.

    I hope this answers your question,