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VCA 822 vs. LMH6503

Paul Law49
Intellectual 260 points
Other Parts Discussed in Thread: VCA822, VCA821, LMH5401, LMH3401, OPA847, VCA820

hello i currently need a VGA/PGA that has a slew rate of greater than 300 and a maximum gain of 100.

For VCA 822, what are the needed values for Rf Rg to achieve a gain of 100?

also, is it a good idea to cascade VGAs? what are the precautions that i need to be aware of?

  • TI__Genius 12631 points
    Hello Paul,
    For the VCA822 in a gain of 100V/V, the conditions are on page 18 of the datasheet,
    RF = 845 Ω, RG = 16.9 Ω, VG = +1 V.

    Is the input to the VGA going to be single-ended or differential?

    You could cascade multiple VGAs together, however there are a couple of other options -

    1. Use the VCA821 which has higher bandwidth and slew-rate and lower noise.
    2. Use a high bandwidth amplifier like the LMH3401 or LMH5401 at the input and cascade that with the VCA822. The LMH3401 and LMH5401 have much lower noise so this will improve the overall system SNR. It can take a single-ended input (or differential input) and convert it to a differential output. That differential output can then be applied to the VCA821/VCA822 which converts the signal back to single-ended. The downside of using this method is that the lowest gain configuration possible will be determined by the gain of the LMH3401/LMH5401.

    Anytime we have such high gains it is important to pay attention to the offset voltages as this can eat into the signal headroom if one is not careful. One way to get past this issue is to ac couple the signal chain.

    Regards,
    Samir
  • in reply to Samir Cherian
    Intellectual 260 points
    thank you for the reply Samir.
    follow up question.
    If i'll be needing a total gain of 200-500,
    1. Would cascading VCA822s or VCA821s have a better overall performance?
    2. How do i configure the gain for LMH3401?
    3. What are the possible gains for LMH3401?
    4. How do i apply the differential output of the LMH3401 to the single ended input of the VCA821/VCA822?
  • in reply to Samir Cherian
    Intellectual 260 points

    The input is single ended btw

  • in reply to Paul Law49
    TI__Genius 12631 points
    Hello Paul,
    The VCa82x can take a differential input signal and convert it to single-ended. Since your input is actually single-ended, another option is to use the OPA847 as the 1st stage in a gain of 20V/V. The OPA847 has an input noise of 0.85nV/rTHz and can run on the same +/-5V supplies that the VCA82x uses. You can follow up the OPA847 stage with the VCA821/VCA822 and vary its gain between 10V/V and 25V/V to get your overall 200-500V/V gain range. I believe this will be your most optimal solutions in terms of noise, BW, power and complexity.

    Samir
  • in reply to Samir Cherian
    Prodigy 10 points
    btw my goal is to use these op amps to display the output of a DAC to an oscilloscope. The output of the DAC is around 1mV-100mV and i need to amplify it to 1V and im running it around a frequency of 10MHz.
    1. Is there a way to configure the gains of VCA822? other than having a gain of 2,10,100? Is there a clear formula for this?
    2. What are other equivalents for VCA822? is VCA821 and VCA 820 good enough?
  • in reply to user4785932
    Intellectual 260 points
    btw my goal is to use these op amps to display the output of a DAC to an oscilloscope. The output of the DAC is around 1mV-100mV and i need to amplify it to 1V and im running it around a frequency of 10MHz.
    1. Is there a way to configure the gains of VCA822? other than having a gain of 2,10,100? Is there a clear formula for this?
    2. What are other equivalents for VCA822? is VCA821 and VCA 820 good enough?

    that was my other account
  • in reply to Samir Cherian
    Intellectual 260 points
    1. How will i be able to find 845 Ω and 16.9 Ω?
    2. What type of resistors should i use for these high speed VGAs? presicion resistors or 1/4W resistors will do?
  • in reply to Paul Law49
    TI__Genius 12631 points

    Hello,

    Please see responses below:

    1. Is there a way to configure the gains of VCA822? other than having a gain of 2,10,100? Is there a clear formula for this?

    The equation is 2*Rf/Rg. The equation can be found in section 9.2.If you read that section it tells you that the gain can be controlled by varying the voltage on the Vg pin. Figure 65 gives the relationship between gain and Vg for a configuration of 100V/V @ Vg = 1V. You can use another DAC or  digital potentiometer to control the voltage on Vg. 

    2. What are other equivalents for VCA822? is VCA821 and VCA 820 good enough?

    As the voltage, Vg is changed, the VCA822 gain (expressed in V/V) will vary in a linear fashion while the VCA820 gain (expressed in dB) will vary in a linear fashion. The VCA821 is just a higher bandwidth lower noise version of the VCA820. The choice is yours depending on your system requirements.

    3. How will i be able to find 845 Ω and 16.9 Ω?

    These are standard E96 values and can be purchased on Digikey, Mouser, or your vendor of choice.

    4. What type of resistors should i use for these high speed VGAs? presicion resistors or 1/4W resistors will do?

    1/4W resistors will suffice. For high-speed applications I would use an 0603 or smaller form-factor, surface mount, thick film resistor.