THS4631: Derivation of some formulas and the GBWP for the TIA

Kwok Luo

Part Number: THS4631
Other Parts Discussed in Thread: THS4561

Hello,

I have been trying to design a TIA (Transimpedance amplifier) with a THS4631. According to the THS4631 datasheet, there are formulas which calculates the values of the TIA.

However, in each of the formula, they ask for GBWP. Which GWBP should I use? My TIA consist of a simple feedback resistor and a feedback capacitor. The feedback resistor is 10 kOhm.

I was also wondering how you have derived the formulas, which is noted in the THS4631 datasheet, for calculations of feedback capacitor and the bandwidth. So where does the formulas comes from? I would like to know the derivation

kind regards,

Kwok

over 4 years ago

0 Michael Steffes over 4 years ago

Guru 47485 points

Morning Kwok,

Here is a presentation developing the Zt design equations - not sure what is in the THS4631 - or if they are correct, but these are,

8255.Transimpedance design flow using high speed op amps.pptx

And then I excerpted out the easiest approach in this article, if you need this as a pdf let me know,

https://www.planetanalog.com/applying-high-speed-de-compensated-vfas-hitting-performance-targets-while-tuning-phase-margin-insight-10/

And yes your question on correct GBWP is a good one, kind of confusing some times but the right number to use in Zt design is the Aol =40dB frequency times 100X to project to the crossover ignoring higher order poles in the Aol, discussed here, the THS4561 shows a true GBWP = 210Mhz in the datasheet but has a TINA model GBWP = 140MHz. For simulation work you need to use the TINA model number.

https://www.planetanalog.com/why-is-amplifier-gbp-so-confusing-insight-12/

0 kai klaas69 over 4 years ago

Guru 140200 points

Hi Kwok,

the "210 MHz Gain Bandwidth Product" is mentioned four times in the datasheet of THS4631...

Kai

0 Michael Steffes over 4 years ago in reply to kai klaas69

Guru 47485 points

Maybe, but those folks apparently did not tell the modeling person that is what they needed. I have this large directory of model test files I was doing some years back. _ here is an older THS4631 model test I had done, true GBWP is the 40dB point multiplied by 100X (1pole projection). But here the zero crossing is not off much,

There is a "newer" model on the web, it reversed all the pins (what a pain) but gives the same Aol,

So, if you are doing sim first, you need to use the model GBWP to get sims to match theory. The Part may really be 210MHz, or also possible, the bench data has some errors in it.

0 Kwok Luo over 4 years ago in reply to Michael Steffes

Prodigy 130 points

Thank you for your explanation. This will be clear for some one of us to understand the GBWP of a particular chip.

Kwok

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THS4631: Derivation of some formulas and the GBWP for the TIA