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THS4552: FDA feedback resistors values

David Green103753
Prodigy 150 points
Part Number: THS4552
Other Parts Discussed in Thread: TINA-TI

Hello all,

In my project, I'm using a IQ Modulator (LTC5599) and a IQ modem (CMX7164), and I have two "problems" interfacing the IQ outputs of the modem to the IQ modulator inputs.

1. The DC common mode voltage of the modem's I or Q outputs is 1.6V, while the DC level required for the modulator is 1.4Vdc

2. The maximum load of the modem's I or Q outputs is 20kohm, while the input impedance of the modulator is about 1kohm

Here is the IQ Input, and outputs specs:

Modulator's inputs:

Modem's Outputs:

I thought about using a dual fully differential amp (such as the THS4552), because the IQ interface is differential, and I want to use a single chip for the solution. By using this solution, I can interface the modem to the modulator using a single chip, and the 1.4V common mode DC level can be adjusted using the VCOM pin of the FDA

I do not have much of experience in using FDAs...by looking at application notes of different FDAs, I have noticed that all of them have a relatively low  feedback resistors (1-2kohms), which create a input impedance of a several kohm, while I need to keep the input impedance above 20kohm...at least.

Question 1:

Can I use a larger values of feedback resistors? (20-100kohm) - what would be the consequences on the FDA performance?

Question 2:

Can someone offer a different/better solution for this matter?

One more thing worth to mention - My baseband bandwidth is relatively low, only a few KHz wide.

Thank you all!

David

  • 0
    TI__Genius 15975 points

    Hi David,

    To answer your questions:

    1. Yes, it is possible to use a larger value of feedback resistor for the THS4552. The trade-off is going to be some roll-off in the frequency response and increase in 4kTRf noise at the FDA output. If the base-band bandwidth is relatively low on the order of few kHz wide, then I think increasing the Rf value to 20kohms (or higher for G>1V/V) should not be of significant impact in-terms of bandwidth.

    2. I think using the THS4552 FDA for level-shifting and buffering operation is the best approach for I/Q modulation. You could use the below THS4552 TINA-TI circuit that is optimized for your application in-between the modem and the modulator. The THS4552 operates on single 5V supply with an output CM of 1.4V because the device has a rail-to-rail output. If the desired output swing is higher, then it might be better to center the THS4552 supplies (say +3.9/-1.1V) around the output CM for maximum output voltage swing.  

    Here is the TINA-TI circuit for the same: THS4552_IQMod.TSC

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 150 points

    Hi Rohit,
    Thank you very much for the detailed answer, and for the simulation!
    The output swing that required for the modulator is 1Vp-p differential (or 500mV single-ended) so working with a single supply of 3.3V over 1.4Vdc offset should be just fine...
    Thanks again, You've been a great help.
    Best regards,
    David

  • 0 in reply to David Green103753
    Prodigy 150 points

    Hello again,

    Following my last post regarding using FDA to interface between a IQ modem to a IQ Modulator:
    I need to adjust my output common voltage around 1.4V, while the IQ outputs of the FDA are centered around 3.3V/2 = ~1.65V.
    I was wondering if I can use a simple resistor in parallel to the voltage divider bottom resistor, instead of connecting an external DC source of 1.4V to Vocm pin?
    /cfs-file/__key/communityserver-discussions-components-files/14/5633.3365.1.pngThanks,
    David
  • 0 in reply to David Green103753
    TI__Genius 15975 points

    Hi David,

    You could probably set the Vocm input voltage to 1.4V using only a 180k resistor to GND. However, I would not rely on the accuracy of the internal 60k ohm resistor to set the Vocm input voltage. Instead, I would recommend to use the below voltage divider circuit in-order to decouple or minimize the impact of the internal 60k resistor while setting an accurate 1.4V Vocm.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 150 points

    Hello again Rohit,

    I will try this solution on the EVB.

    Thank you for your kind help!

    David