Part Number: THS4541
How to get the eq.1 in TIDA-00294? Can you provide the detailed process? What's the target of this equation?
Thanks and best regards, Charles Chen Applications Engineer | LED Drivers
Check out these helpful resources
LED Drivers ICs | LED Drivers Reference designs | LED Driver Basics Training series
yes, good question. Shouldn't be this the correct assumption?
(RS // RT) + Rg+ = Rg-
(RT // Rg+) = RS
with Av = RF / Rg-
" // " means parallel impedance.
I guess something important is missing in the appnote and in the datasheet. Has it to do with the dynamic input impedance of THS4541?
With the above assumption I get:
RT = SQRT ( (RF / Av x RS^2) / (RF / Av - 2 x RS) )
We are glad that we were able to resolve this issue, and will now proceed to close this thread.
If you have further questions related to this thread, you may click "Ask a related question" below. The newly created question will be automatically linked to this question.
In reply to kai klaas69:
Hardly anyone ever asks that - oddly enough. In the early FDA days there was a problem with resolving the R values to get both the matched input impedance and gain as the single to differential configuration has an active input impedance. This arises as the input CM voltages move with the input signal. Early solutions by NSM and ADI executed an iterative solution that does work. That seemed odd to me, so back around April 2013 I pounded out the closed form solution you are asking about and it also works very nicely. This solution starts by assuming you want to pick the Rf value for other reasons. However, the nice thing here is if you can let it be anything, solving the denominator for zero will eliminate the Rt element to ground at the input and give you the perfect active match solution for Rg and Rf - this is the lowest noise matched input solution that I also call the active balun.
Anyway, I don't send out the 4 page algebraic derivation anymore, but here is the 1st publication.
And then I did some videos on this as well that are still out there on the Renesas website. These are all about 4minutes so pretty quick. This is in fact a new op amp function oddly enough - I was pretty excited about it at the time. You would think with all this public info ADI would migrate to using this equation in their FDA data sheets (like the new ADA4945). Not so for the citadel of NIH.
In reply to Michael Steffes:
Thanks for your answer. It is helpful to me.
In reply to Charles-Chen:
Hi Michael, hi Charles,
I found this in the www:
Yes Kai, that is just an earlier version of what I did for EDN.
Here is one the national guys did showing the iterative approach
ADI also uses an iterative approach that is hiding behind the DiffAmp Calculator tool. One of the things I was doing validating the solution I had developed was to test it against the ADI reported R values, they match.
This app note tells you how to use the ADI tool (which is very nice) but not how they are executing the solutions. I of course have built an FDA design tool that does everything and more than the ADI tool - it is populated with the required parameters for all industrywide FDA's. ADI's most recent FDA is the ADA4945 - they describe their iterative approach on page 39. Kind of silly really to stick with that with the closed form solutions (there are no estimates in those equations, they are exact) available.
And the TI guys are doing some more recent things - this one is pretty straightforward when you are coming from a low impedance op amp output and not trying to achieve a matched input impedance - that simplifies things considerably,
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.