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Differential and Single ended impedances and return loss

Luis M Brugarolas
Intellectual 295 points
Other Parts Discussed in Thread: PGA870

These are general purpose questions arisen from studying PGA870 (High bandwidth Variable Gain Amplifier) data sheet:

Q1.- Return loss specifications and plots for differential input (fig 36) are impressive: better that 40 dB for 300 MHz and up un a Zsys of 150 Ohm. However, differencial input impedance may vary from 139 to 173 Ohms. With such a dispersion in input impedance, return loss even at at low frequency should suffer. There is something I am missing?

Q2.- Simmilar question. Single ended impedance typical value is specified to be 141 Ohm. In figure 37, Single Ended Input Return Loss is specified for Zsys of 150 Ohm.

Best regards

Luis Miguel

  • 0
    TI__Genius 17905 points

    Hi Luis,

    Thanks for your interest in the PGA870.

    Regarding Q1:  The potential variation of the input impedance is typically not a cause of too much concern. Using either of the two boundary values, the return loss will be about -22dB, or less than 1% reflected power. I believe the min and max numbers are based off of a six sigma worst-case calculation - the vast majority of parts will have input impedance close to 150ohms.

    Regarding Q2:  I am not sure what the question is, but I will explain the figure. Figure 37 implies that the source/system impedance (Zo) used in calculating the return loss is 150ohms. Plugging in the typical single-ended input impedance of 141ohm into the reflection coefficient equation, we find that RL = (ZL- Zo)/(ZL+Zo) = (141-150)/(141+15) = -0.031, which is  -30.2dB. This is the return loss shown at low frequencies in Figure 37.

  • 0 in reply to Kris F
    Intellectual 295 points

    Thank you Kristoffer. Your answer has been very helpfull.

    Best regards

    Luis Miguel