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TRF1305B2: How you calculate the Gain, Shunt and Series resistors in table 8-1, page 20 of the datasheet TRF1305B (SBOS971)?

Behzad Boghrati
Prodigy 30 points
Part Number: TRF1305B2
Other Parts Discussed in Thread: TRF1305, LMH5401

Tool/software:

Dear TI support team,

I am trying to understand how you have calculated the shunt and serie resistors in Table 8-1 in the datasheet of the TRF1305B2.

As a reference, I used your document, titled: fully differential amplifier (SLOA054E), to calculate the gain and termination resistance.

For example, in the differential mode, if we assume the Rs = 100 ohm, Rg = 12.5 + 25 ohm then we will end up with the termination resistor of -300 ohm! However, the termination resistor (that you called it shunt resistor) has been specified at 1000 ohm.

Can you please help me understand how you calculate the shunt and series resistors in table 8-1, page 20 of the datasheet TRF1305B (SBOS971)?

How about the Gain? Even with the resistor values you have specified in the table8-, I can't get to the Gains you have mentioned!

I am looking forward to receiving your precious insights and comments on these.

With kind regards,

Behzad.

  • 0
    TI__Prodigy 170 points

    Hi Behzad,

    Thank you for contacting TI. I will get back to you with a response to your question in a day.

    Regards,
    Srinivas

  • 0 in reply to Srinivas Seshadri
    TI__Prodigy 170 points

    Hi Behzad,

    The calculations in the Fully Differential Amplifier (SLOA054E) works for amplifiers at low frequency (say below 1GHz).

    The input resistor and shunt resistor in TRF1305B are chosen to optimize wideband return loss for RF frequencies. They are optimum values to provide specified gain when driven by 100ohm source, driving 100ohm load. 

    Are you looking to configure the amplifier at some other gain (other than 5 to 10dB in 1dB steps provided in the datasheet?)

    Regards,

    Srinivas

  • 0 in reply to Srinivas Seshadri
    Prodigy 30 points

    Dear Srinivas,

    Thanks a lot for your reply. 

    I wanted to use this component as an RF voltage level shifter. 

    In my application, the common input voltage at the amplifier's input is 2.1 V, and the common output voltage is 0.7 V. 

    I also chose Vs+ = 3.3 V and Vs- = -1.7 V to provide enough headroom for the common voltage ranges.  

    As you can imagine, DC current flows from the FDA's input to output pins. If I add a pullup resistor to the input pins and pull-down resistors to the output pins to handle this DC current, these resistors will affect the default external resistor networks suggested by the datasheet. For this reason, I asked how the external resistor network has been calculated.   

    On the other hand, to cross-check the calculation, I adjusted the TI Diff-to-Diff Calculator as follows: Voltage gain = 4 V/V, RF = 161 Ohm and Rs = 50 Ohm, Vs_DC = 2.1 V, Vocm = 0.7 V. Based on their specs, I have selected the LNH5401 since this is the closest FDA to the TRF1305. As you can see, Rshunt has been calculated negative. 

     

    So, my question is still open:

    1) How have the shunt and series resistors been calculated in the TRF1305 datasheet to achieve the gain of 10 dB? 

    2) Is adding any external pullup and pulldown resistor necessary to handle the DC current? 

    3) Do you think using the LMH5401 SPICE model is a good representative of the TRF1305? Or not make sense?

    Thanks and Regards,

    Behzad.