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.
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?
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 ?
Oh， I see .
Thank you , Hooman.
Really appreciate for your help.
Best regards .
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.
The AD9650 input common mode is 0.9V and the LTC6404-X output common mode is 1.1V for a single 3V or 5V supply. The LTC6404-X must have a -1V VEE to drive the ADC at 0.9V.
Yes , I see .
the LTC6404-1 will be supplied by VSS 3.0V and -2V VEE .
Thank you ,Sir.
Thank you very much , Kirill .