• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 237 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

THS4500: Using TI differential amplifiers and buffers to sum together two baseband signals for sampling from a single ADC

Giovanni Peroni
Intellectual 695 points
Part Number: THS4500
Other Parts Discussed in Thread: THS3215

Tool/software:

I have two, differential, baseband signals, coming from the output of two different down-conversion mixers. They are 1Vpp, 30 Ohm source impedance, and they go through a set of lumped-element filters. They then become:

  • First Signal: 10-60 MHz (useful frequency content would be 30 MHz +- 1 MHz)
  • Second Signal: 60-110 MHz (useful frequency content would be 80 MHz +- 1 MHz)

As my ADC does 250 MSPS, I would like to sample them both using a single ADC (and then process them in the DSP). To do that, I think I need to:

  1. Buffer the output of the differential signals (2x TI buffers)
  2. Use a non-inverting, summing op-amp to merge the two signals together, as they have different frequency contents (1x TI differential amplifier)
  3. Probably use the same component as (2) to drive the ADC.

As I can vary the amplitude of the baseband signals, I don't think I need variable gain at the second stage (non-inverting sum). I think this is what I need to achieve:

This is not the smartest question, but can this be done with any differential amplifier/adc driver TI offers?

What about the buffer, any suggestion for the part number?

Thank you!

  • 0
    TI__Genius 16905 points

    Hi Giovanni,

    I have a couple questions about the signals you are trying to buffer. At the buffer stage are the two signals already single-ended therefore you need to buffer two signals or is it still two differential signals that would require four buffers? If it is a set of differential signals and you require a high input impedance, you could implement two INA circuits to convert your signals to single ended and then use a traditional summing amplifier configuration for the two single ended signals coming from the INAs. You could also implement an integrated device like our THS3215 to convert your differential signals to one single-ended signal. If you do not require a high input impedance, I believe the circuit you shared could work for you as a difference + summing amplifier.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Intellectual 695 points

    Hello Ignacio,

    Those are two differential signals that I'm trying to sum together. The output should also be differential (as the ADC input is differential). I'm looking for a low p/n and low cost solution if possible as this circuit is repeated 10+ times per board. If the cost add goes above ~$10 per combination/chain, I'm better off adding a second ADC (the THS3215 used for the differential to S/E conversion would be that $10 already). 

  • 0 in reply to Giovanni Peroni
    TI__Genius 16905 points

    Hi Giovanni,

    Do you require a high input impedance therefore requiring four buffers for the four individual signals? If this is not required, you could implement a stage like the one you shared followed by an FDA to convert that single-ended output back to a differential signal to go into your ADC.

    Best Regards,

    Ignacio