ADA4870 - composite amplifier


Hi,

I am trying to make a project based od ADA4870 figured as unity gain current buffer. Looking on it's 23th page of datasheet i found it connected with composite amplifier and i have some questions now:

1. Is the ADA4637-1 absolutely best amplifier in Analog Device catalogue in term of small value analog signal parameters (error, noise, etc. in aplication like DAC output stage). If not what else should I look at?

2. Refering to the 23th page of ADA4870 datasheet if i understand correct resistors 1k + 110R sets the gain? ADA4637 is stable at gain more than 5, so can i change those resistors to 1k2 + 200R to set ADA4637 at gain = 6 without further troubles?

3. Same page of datasheet as above - is the capacitor connected pararell with 1k resistor needed to achieve stability? I need only audible frequency response, but i am not sure if it will be fully stable without this capacitor (i guess yes, but it's better to ask anyway).

4. Last one - in such a configuration can i bias output stage of ADA4870 (mean connecting output also to a V- via resistor to make output stage active all the time) as i did with "alone" operational amplifiers, without risk of damage ADA4870 or ADA4637? (again i guess yes)

  • 0
    •  Analog Employees 
    on May 23, 2016 9:46 PM

    Hi.

    1. ADA4637-1 is an excellent amplifier due to its precision at higher Gain Bandwidth Product. We have a lot of high performance amplifiers in our portfolio that will cater various application requirements needed by our customers. Anyway, you can look between ADA4637-1 and ADA4084-1 for your application (~20 kHz signal).

                            

    GBP Vnoise Vos Ibias
    ADA4084-1 15.9 MHz 3.9 nV/rtHz 100 uV 400 nA
    ADA4637-1 79.9 MHz 4.8 nV/rtHz 200 uV 5 pA

    2. Yes you can change the value of the resistors to achieve your desired gain. For a gain of 6, use 1k0 and 200 in the feedback loop.

    3. Even if the signal frequency is way below the corner frequency, the amplifier itself will generate instability. It is highly recommended to put that compensation capacitor to avoid such instability. Take note that the values are chosen based on the layout, which means you must carefully select components to match the pcb for optimum performance.

    4. If you connect the output to V- thru a resistor, it will be pulling current from the output that will heat up the part.

    Regards,

    Jino