HMC681A DC Blocking Capacitor For RFin


Hi, i am trying to figure out how to use HMC681A. In datasheet, it says RFin1,RFin2 pins are DC coupled and an off chip DC blocking capacitor is required. However, i want to use it for low frequencies as much as possible.(for DC if possible). Can i use the amplifier without DC block-cap.

    •  Analog Employees 
    on Jun 29, 2016 1:37 AM

    Hi,

    I assume you'll be using the part configured in the Application Circuit shown on the data sheet? (RFIN goes to pin2)

    I can investigate and get back to you.

    Regards,

    SMcBride

    •  Analog Employees 
    on Aug 1, 2016 7:09 PM

    Hi,

    I just found that I did not follow up on this question...so here is follow up.

    While the HMC681ALP5E could theoretically operate all the way down to DC, operation at DC would be impractical and could cause problem with biasing and/or performance. To help explain the situation, a simplified Interface Schematic of the two contained amplifiers is shown below. The HMC681A RFin1 and RFout1 pins are analogous to IN and OUT below, respectively. (Likewise for the RFin2 and RFout2 pins).

    In the HMC681A Application Circuit shown below (from the data sheet), DC bias for the amplifiers is applied via separate RF choke connections between the supply voltage Vs and the RFout1 and RFout2 pins:

    Considering the above, it can be seen that DC coupling a source to the RF input pins or DC coupling a load to the RF output pins will DC load them, causing disruptions of the RF input and RF output DC bias conditions.

    Rather than DC coupling to the amplifiers, I recommend that you AC couple to the amplifiers using capacitors of adequately large value to reach your lowest non-DC frequency of interest.

    Regards,

    SMcBride