LTC6404 input circuit

Dear Sir or Madam 

I am implementing my design with LTC6404-2 , and not sure if the  circuits  will  work well , especially about the input part , where switches are inserted  between  the input resistors (R1,R2)  and feedback resistors(R5 , R6) ,  Besides , are there any issues if I place the R1 ,R2 far away from LTC6404-2 input pins respectively  in my layout design ?

Is the circuit OK?

Thank you in advance  for your advice.

sincerely

Best regards

Anglic

  • Hi Anglic,

    Please clarify exactly what problems you expect to get and what are you afraid of?

    Yes, this amplifier is optimized for high-speed operation and does not tolerate unity gain, this is stated in the specification. But I see resistors R3 and R4 that can provide increased noise gain when S1 and S2 are open.

    I'm confused about capacitors in the feedback circuit, why do you use them?

    Regards,

    Kirill

  • Oops ,  My bad.

    It was LTC6404-1 , however , being used as a half unity gain (0.5V/V) . Is it OK? 

    The problems are ,  Doesn't it matter if  R1 R2 are placing far away from LTC6404-1 on PCB board ?

    According to the datasheet P25 , feedback capacitance can help to mitigate peaking in frequency response . 

    And , here is the modified circuit , where the input resistor is 130ohm , the feedback resistor is 68ohm .

    Another problem is , can LTC6404-1 be used to drive AD9650 ?

    Best regards!

    Anglic

  • +1
    •  Analog Employees 
    •  Super User 
    on Feb 27, 2020 1:59 AM in reply to angelic

    Hi Anglic,

    I posted a response to your other post about using LTC6404-1 at lower than 1V/V gain here:

    https://ez.analog.com/amplifiers/f/q-a/122606/questions-about-ltc6404-1/364681#364681

    In general you'd want to minimize all trace lengths with a device as fast as LTC6404-1. You probably have to experiment with it on a prototype board to make sure though.

    I see the plot you've noted about cap across RF to mitigate peaking. That's certainly a valid technique:

    You had asked if LTC6404-1 can be used to drive AD9650 ADC.

    From AD9650 datasheet:

    • Analog input: 2.7Vpp (Vref = 1.35V)
    • Input CM voltage: 0.9V

    From LTspice, these conditions look doable as shown below:

    With some drivers, approaching the ~0.2V from ground may be troublesome. I've seen designs where the FDA is used with its V- supply pin powered from a slightly negative supply such as -0.7V to alleviate the swing so close to the V- rail. But, if LTspice can be trusted (which I tend to do), first order seems like a doable proposition (unless distortion terms become too large with swing so close to the V- rail).

    Here is the LTspice file for your review:

    LTC6404-1 AD9650 driver EZ 2_26_20.asc

    Regards,

    Hooman

  • Oh, I see .

    Thank you , Hooman.

    Really appreciate for your help.

    Best regards .

    Anglic

  • Hi Anglic,

    The main feature of a fully differential amplifier is that it always works in inverting mode. Therefore, the noise gain is always greater than one. Even when it works with a gain of 1 it corresponds to a noise gain of 2. Most very high-speed amplifiers are decompensated and do not allow operation with a noise gain close to or equal to 1.

    Since you want a fully differential amplifier to work with a gain less than one, this corresponds to working with a noise gain close to 1 (specifically in your case, 1.5). It is necessary to check the behavior of the amplifier at the extreme value of the noise gain - 1 to find out about potential stability problems. I suggest this model:

    The noise gain here is strictly equal to 1

    You can see a peak that indicates a small phase margin. In principle this should not lead to self excitation but will cause overshot an impulse response. You can try using a gain 0.5, I think.

    Regards,

    Kirill