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THS7530-Q1: Is there a simulation / PSPICE model available for this device?

Peter Roos
Prodigy 90 points
Part Number: THS7530-Q1
Other Parts Discussed in Thread: THS7530, LMH2832, OPA855, OPA818

Hello TI experts!

We are considering using the THS7530-Q1 in our application and have a few questions that I hope you can help clarify.


Question 1: I do not see a SPICE model listed on the TI website.

  • Does a PSPICE model exist so I can model the behavior for our intended purposes in TINA?


Question 2: In the datasheet on page 1, under Description it states: "The THS7530-Q1 device is a DC-coupled, wide, bandwidth amplifier".

  • We would like to use this VGA in a cascaded (two-stage) setup in order to boost the overall gain range to suit our needs. At the same time our application requires the ability to allow 0dB gain so I'm planning on attenuating the signal such that we have 0-34.9dB gain range for each stage as opposed to 11.6 to 46.5dB. In our application we would use the first stage with single ended input, differential output and the second stage as differential input and differential output. Will this device support such a cascaded setup? The input signal varies between ~1MHz - 75MHz over an anticipated ~56dB overall gain envelope.

  • We would like to be able to use the first stage in a DC-coupled setup (scenario 1) and also another instance where we use the first stage in an AC-coupled setup (scenario 2).
    The output of the first and second stage would be differential and DC-coupled as shown in the image below.
    Section 9.1 Application Information of the datasheet suggests that this *can* be accomplished. However I believe there may be an error in Figure 21 "DC-coupled single-ended input with DC-coupled differential output". The figure shows an AC-coupling capacitor at the single-ended input. I'm assuming that the description should state: "AC-coupled single-ended input...."?
    I just to confirm that the two scenarios shown in the image below are valid applications for this device. 


Thank you for your feedback,

Peter

  • 0
    Prodigy 90 points

    I just noticed that there is a PSPICE model listed under THS7530 while I was looking under THS7530-Q1 so this part of my question is already resolved!

  • 0 in reply to Peter Roos
    TI__Expert 6050 points

    Hi Peter,

    We will look into your question and will get back to you.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    TI__Genius 15541 points

    Hello Peter,

    1. I am glad you found the SPICE models Slight smile.

    2. I am confused about your attenuation comments; are you intending to attenuate the signal prior to the first THS7530?  Would this attenuation be relative to the system level, but the THS7530 still applies 11.6dB of gain?  I am not sure how you intend to use the THS7530 in a gain lower than 11.6dB, unless it is by attenuating the signal prior to the VGA.

    3. I do not see an issue with your proposed coupling scheme.

    4. Your comment on the datasheet labelling of Figure 9-4 is a good spot.  There is either a need to correct the caption/label or to remove the capacitor from the diagram; the coupling pattern appears to be showing AC-inputs with AC-outputs and DC-inputs with DC-outputs.  You may submit feedback to either or both datasheets so this can be officially reviewed.  The submission link is on the bottom right of page 15.  Please include relevant details.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 90 points

    Hi Alec,


    1. The SPICE model is not behaving in TINA as advertised in the datasheet which states "Gain control is linear in dB with 0V to 0.9V varying the gain from 11.6dB to 46.5dB with 38.8dB/V gain slope". I downloaded the SPICE model and example from TI and the Vgain control vs actual VGA gain does not appear to match up. For example: Vgain = 0V results in an actual gain of 41.9dB. Vgain = 0.5V results in an actual gain of 41.9dB. I have to increase Vgain to 2V or higher in order to reduce the actual gain. I was hoping to run circuit analysis based on several examples in the datasheet but it appears that the SPICE model will not work in those setups? I would like to run a full circuit simulation of our analog front-end to model behavior prior to fabricating a PCB.

    2. For our purpose, it would be great if the VGA gain would start at 0dB as opposed to 11.6dB due to the fact that we are using a photo-diode with transimpedance amp at the front-end, where we have observed the best small signal behavior when increasing the transimpedance gain as much as possible for the scenario where the photo-diode output is the largest.
    It would be **nice** for our application if the THS7530 gain range would be 0dB to 35dB as opposed to 11.6dB to 46.5dB as this would be a better match our system, but we can make this work as well by lowering the transimpedance gain. It was more intended to describe our specific use-case. If TI has a VGA with a 0dB to 25..35dB 'ish gain range, that would work better for our specific case, but I did not see such a VGA while searching for products. We also require the analog gain control as opposed to digital control which is too course for our application. 

    3. Great, many thanks for confirming!

    4. I'll submit datasheet feedback to have the labeling for Figure 9-4 correct so it matches with the actual figure. 

  • 0 in reply to Peter Roos
    TI__Genius 15541 points

    Hello Peter,

    Thank you for getting back to me on all points discussed.  This week had included a lot of lab work for me, so I will take a closer look at your follow-up questions tomorrow & Monday.

    In the meantime feel free to ask additional questions.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 15541 points

    Hello Peter,

    My responses & updates are as follows:

    1. I am looking into this model behavior; I would agree the gain should be variable in the model, and the issue of the model being 'stuck' at maximum gain is odd indeed.  I am looking to provide a circuit example soon when I have it working.

    2. I am sharing your feedback and use with our systems team; as of right now the THS7530 is a good choice.  Devices such as the LMH2832 have a -9dB to 30dB gain range, but are digitally controlled.  As you need the fine control of analog, the THS7530 should work well.

    3. No worries!  Glad to help!

    4. I still have not seen this come through, but I also know the form asks many questions and can be a bore.  Let me know if you have issues submitting feedback.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 90 points

    Hi Alec,

    Sorry for the delay in responding, I have been setting up a simulation in Tina with the OPA855 and two cascaded THS7530 VGA's and I'm now getting some results that are more in line with how the VGA is supposed to behave per datasheet, however I had to AC-couple between VGA1 and VGA2 in order to get it to work. I'm not sure if that is due to some issue with the simulation environment (THS7530) or not.
    But at least I can get some basic functionality this way, AND the gain setting seems to work from 0V to 1V with correct min/max gain of around 11dB, up to 46dB. 

    I'm now trying to replace the OPA855 by the OPA818, which has slightly lower BW but still more than enough for our purpose, but the added advantage of a wider bipolar supply range, which would allow a larger output voltage. I imported the SPICE model but when I replace OPA855 by OPA818, the THS7530 no longer works, even though the output signal at the OPA818 is identical to that of the OPA855 which is puzzling. 
    I just want to check if I'm chasing my tail due to simulation bugs, rather than issues that I would encounter in real life as well. 

    I could share the Tina setup that I have but there does not appear to be a way to attach it here. so I hope the image below helps to show the setup that is working for OPA855, but not at all for OPA818, despite these being very similar OpAmps, used in a transimpedance setup. 






    I have submitted the datasheet correction as well.

    Best,

    Peter

  • 0 in reply to Peter Roos
    TI__Genius 15541 points

    Hello Peter,

    Absolutely no worries on response time.  I can help you attach your file here: 1. go to "Insert", 2. go to "Image/Video/File", 3. click on "Upload", 4. Browse for your file, 5. click "Ok".

    I can look into this, I agree the OPA818 should replace the OPA855 easily enough in your circuit.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 15541 points

    Hello Peter,

    I am working on simulating your circuit.  Could you confirm your current source configurations for a working circuit?  I would also like to see your output waveforms, as I am not sure how to proceed with repeating your work.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 90 points

    Hi Alec, 

    Can I send the files to your e-mail address in order to keep them confidential?

    Best,

    Peter

  • 0 in reply to Peter Roos
    TI__Genius 15541 points

    Hello Peter,

    Yes, we can move the conversation offline.  Please check our e2e inbox for a friend request/message.

    Best,

    Alec