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THS3491: THS3491 Stability at low gain levels

Intellectual 510 points

Replies: 4

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Part Number: THS3491

Hi I'm trying to use the THS3491 and minimize the signal chain noise in my application. Using the TI evaluation board I notice that for a non-inverting amplifier with a gain of x2 the part is stable if RF = Rg = 499 Ohms but is unstable if Rf = Rg = 249 Ohms (+/-9V supply).  Assuming we want a 10dB gain margin and 30 degree phase margin can you possibly define the optimal resistor values for gains of say +2, +5, +10, -2, -5, -10 ? I would do this in simulation but the TI spice model is broken and doesn't run in LTSpice ! Thanks Steve

  • Hi Steve,

    Current feedback stability is proportional to Rf, not gain as in voltage feedback amplifiers. We have Table 2 on page 25 of Rf and Rg values for these gains that will exceed your stability criteria. Increasing these will trade bandwidth for stability, and vice versa. Maybe you could use all TI parts and simulate in TINA-TI? It runs there. Here is an app note explaining current feedback amplifiers, like the THS3491, that you may find helpful cas.ee.ic.ac.uk/.../OA-13.pdf.

    Best regards,

    Sean
  • In reply to Sean Cashin:

    Thanks Sean, I didn't see this table - useful for general applications. Unfortunately 976/976 is going to generate a lot more noise than I'd like. are a lot higher than I'd like. Currently I'm using a THS3001 with x2 gain and 500 Ohm feedback resistors which seems to be stable. I'd ideally like to do the same or go even lower on Rf for this part. What are the lowest values that I could reasonably use for a non-inverting x2 gain assuming a gain margin of 10dB and phase margin of 30 degrees ? Thanks Steve
  • In reply to steve smith17:

    Hi Steve,

    it's not wise at all to move too far away from the recommended feedback resistance when using a current feedback OPAmp. Even if the simulation shows a stable performance with a phase margin of 30°, the OPAmp can become instable with a real world load or with real world parasitic layout inductances and stray capacitances.

    Why not running a TINA-TI simulation with the THS3491 TINA-TI reference design and doing a phase stability analysis? See the following training videos:

    A rough estimation shows that in your case a phase margin of 30° will occur with feedback resistances of about 487R (gain = +2, THS3491). But again, I would not recommend to lower the reommended feedback resistances from 976R fully down to 487R. 750R should be ok, though.

    By the way, there's a very simple method to very effectively decrease the noise: Put several identical OPAmp circuits in parallel, like shown in figure 66 and 69 of datasheet. Provided the noise is uncorrelated, the noise will decrease by 3dB when having two OPAmp circuits in parallel and by 6dB when having four OPAmp circuits in parallel, and so on. In the datasheet of OPA27 you will find a brief explanation of this technique:

    Kai

  • In reply to kai klaas69:

    Thanks Kai, I very much appreciate your answers on this. If I don't ave any luck with your simulation group giving me a P-spice model that works I'll probably try the part alone in Tina as you suggest.   Very good to know where the boundaries are. I did do some bench measurements using your evaluation board in this configuration and mine was stable at 430 Ohms so not far from  your 487 Ohm number. I think I'll try a different topology that will reduce the noise impact of this stage that way I should be able to get away with higher resistor values. Thanks again, Steve

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