Low Noise Variable Gain AD600 Amplifier

Hello,

I, Satyabrata Sarangi, am using AD 600 in a cascading mode to get 80 dB gain. The problem i am facing is always i am getting an amplified output, with a larger bandwidth. If the original signal is of 1 MHz, i am getting a 25-30 MHz signal as output. Why this is happening? Please help me.

I am implementing Fig. 37 (I am giving 1V directly at VG' instead of controlling VG' through the AD590, 2N3904 and 806ohm resistor) of the attached datasheet.

I have attached the datasheet i am following.

Please help me.

Regards,

Satyabrata

ad600jnz.pdf
  • 0
    •  Analog Employees 
    on Dec 7, 2012 12:29 AM

    Hello Satyabrata

    Thank you for contacting ADI regarding your problem.

    Please explain what you are trying to do. Figure 37 is intended to illustrate an agc circuit. In case you're not familiar with such circuits, the acronym means automatic gain control, and are often used in radio receivers.

    If you juat need a lot of gain you have maximum gain with the 1V applied to VG, and you would not want to use the transistor, current source, etc at all.

    So please fill us in a bit. Is this an experiment for a class? Are you a student?

    Best regards,

    js

  • Hi sir,

    I am a student. This is for my final year project. I need a 60 dB gain amplifier with low noise psd like 1nV/sqrt(Hz). My analog signal input is in micro Volt range and it's bandwidth is 12 MHz. So i chose AD600, as it was mentioned there by cascading the two amplifiers inside that chip, we can get upto 80 dB gain.

    I had also applied 1 V directly to VG, but the thing is.after cascading connection i am not getting 80 dB, and the most interesting thing is that output signal's bandwidth i am getting is 25 times of the input one.

    So please help me where is the fault or should i try some more chips?

    Thank you again.

    Regards,

    Satyabrata

  • 0
    •  Analog Employees 
    on Dec 8, 2012 12:48 AM

    Hello Satyabrata,

    I thought you might be a student and you have a very interesting project.

    It seems you are confusing frequency and bandwidth, they are not the same parameter, even though they can have the same units of measure. From your email question, I cannot tell which you mean. I can assure you that the input and output bandwidth of an amplifier are essentially the same, although BW at an input is not a valid specificaiton. Since you are a student, I would suggest you consult with your educational resources for explanations of these two very different concepts, or refer to Wikipedia or a similar resource site for readily available assistance. It is beyond the scope of this website for me to explain this type of basic concept.

    Best regards,

    JS

  • Hi Sir,

    Actually what i wanted to point out was that after implementing through AD600, i got output signal with frequency 25 MHz where as my input signal had a frequency of 1 MHz and amplitude of 20 mV sine wave supplied from waveform generator. Why did that happen?

    So far as my project is concerned, my input signal for AD600 i am taking from the output of a PZT transducer, having amplitude in micro volt range and maximum frequency content is around 4 MHz. I need a gain around 60 dB with very less noise PSD in system.

    Thanking you.

    Regards,

    Satyabrata

  • 0
    •  Analog Employees 
    on Dec 10, 2012 6:43 AM

    Hello Satyabrata,

    OK thanks for clearing that up, it makes a big difference..

    I believe you also stated that the gain control voltage was 1V. If you are applying 20mV 1MHz at the input, you will be drastically overdriving the part, assuming all your connections are OK. For 1V at the gain pin, the gain will be 80dB so your output swing would theoretically be 200V. Obviously this is impossible, however it is possible to overdrive the part. Instead of 20mV, you should be applying a few tens of microvolts. If you are grossly overdriving an amplifier the results can be anything, there is no more loop control when the amplifier is operating out of its linear range.

    Another point I should make is if you are using figure 37, please note that there are no decoupling capacitors in the circuit. This is because figure 37 is a conceptual circuit, and the original author omitted any power supply decoupling for clarity.

    The last questions I would have is, are you testing a physical circuit, or are you working with a simulation using a SPICE model?

    Once again I would strongly urge you to check with your local resources, such as the professor or course instructor. Because I’m remote, I really have no idea what you have there for your experiments. Please use your professors, as I can unintentionally mislead you by trying to guess what you are actually doing there.

    Good luck,

    js