OPA381: opa381

Hi Vladimir,

I think this is no mistake but intentional. The transient response of one half wave of a 1MHz square wave with Cf=3pF is shown. So, the 100ns/div scaling is correct. Additionally, the pulse response with Cf=16pF is shown. As the settling time of OPA381 is too long for a 1MHz square wave, a 200kHz square wave is used which - after having changed its time scale for reasons of simplicity - is overlayed on the original curve .

Kai

Hi, Kai. see p,8 of OPA80, please. Transient curves for 380 & 381 look similar at equal speed: 100ns/div. Slew rates of 380 & 381 are differ by more than 6 times: 80 & 12 v/us. It is this contradiction that has caused my doubts. (I already used OPA2381 for Large Area photodiode and now I`m looking faster TI OA).

Vladimir

OPA380, certainly....

Hi Vladimir,

Thanks for your post. Please accept delayed response as many of us are on holiday break. We will support you shortly.

-Tamara

On that web page, under parametric table, the Vos of the OPA380 is given as 25mV which is incorrect. It's 0.025mV.
Do as you will with this info.

You are correct.  There is a typo in parametric search of OPA380 Vos showing 25mV instead of 25uV.

Happy New year, Kai!

Of course, 25mV vs 0.025mV is typo. But my comment was about something else.
I would like to return to the original message (Dec 23, 2019 1:22 PM).
There is a contradiction in the documentation for two similar TIOA: OPA380 & OPA381.
Compare “Large-Signal Step Response” (1v/div) at p.8 (OPA380 datasheet) & p.7 (OPA381).
For OPA380 circuit feedback RC= 10kohm * 2,5 pF = 25 ns or t (0,1) = 55 ns. This corresponds to the speed 100 ns/div.
For OPA381 circuit two feedback RC1= 50kohm * 3 pF = 150 ns & RC2= 50kohm * 16 pF = 800 ns are shown. Time scale is the same (100 ns/div) and transients are finished by 50 and 200 ns. Too fast for RC 150 & 800 ns. This is incorrect. Everything will return to normal if scale will be changed to 1us/div for OPA381 datasheet. Similar changes should be made for Small-Signal Step Response figures close at hand.

Vladimir

I had written my comments for guru kai klaas69.

Vladimir,

You are correct that OPA381 datasheet large-signal horizontal scale should be in 1us/div instead of 100ns/div.  This can be easily seen by measuring the slew rate in the graph below, which would result in ~120V/us instead of 12V/us - changing the scale to 1us results in correct slew rate: SR=4V/300ns=~12V/us 

A small-signal rise time is a function of the effective bandwidth, fc - see below: tr=0.35/fc

If you calculate the expected rise-time for Rf=50k and Cf=3pF, you obtain tr=0.35/(2*Pi*RC)=330ns while for 50k and 16pF the rise-time is ~2us - thus, as you correctly pointed out, the scale in the graph below should be 1us/div instead of 100ns.  I will inform the system engineering team of the errors.  Thank you.

Hi Vladimir,

Thank you for your inputs. Just so you know, we are working on the new precision TIA with integrated switches - see below. 

P.S. We can send you some snow from Arizona... :)

Hi Marek, snow in Arizona is as rare as Moscow without snow (it did fall tonight)...

Very interesting, I would like to work with your new TIOA. If possible, please send samples. A similar amplifier with switches and output buffers ADA4350 recently released Analog Devices.

Vladimir.