OPA3692: have output ringing issue

user5195153

Part Number: OPA3692
Other Parts Discussed in Thread: TINA-TI

Dears,

We use OPA3692 for VGA signal single end input,then a gain of 1 voltage buffer output,but the outpu have ring or noise,Pls check below shot, CH1 is input,CH2 is output. Vcc power supply is +5V and -5V.

Pls help to check and give us some suggest,thanks!

over 5 years ago

0 Rohit Bhat over 5 years ago

TI__Genius 15975 points

Hi,

Would it be possible for you to attach the schematic in-order to help debug further? Do you proper 50 or 75-ohm termination at the input as well as the output?

Best Regards,
Rohit

0 user5195153 over 5 years ago in reply to Rohit Bhat

Intellectual 450 points

Dear Rohit,

thanks for your help,schematic as below:

0 kai klaas69 over 5 years ago in reply to user5195153

Guru 140200 points

Hi,

the filter chokes in the supply lines are at the wrong place. The decoupling caps must sit directly at the supply pins of chip.

Kai

0 Rohit Bhat over 5 years ago in reply to kai klaas69

TI__Genius 15975 points

Hi,

I think Kai is right that the decoupling caps must be close to the supply pins of the chip after the filter choke.

Another thing I noticed is that the input filter 3dB cut-off frequency is set to 440MHz, whereas the OPA3692 3-dB BW is rolling off at 225MHz. So, it is possible to improve your design by limiting the input filter cut-off frequency lower than the OPA3692 BW in-order to avoid any higher order distortion from the input cables corrupting the signal waveform.

Also, do you have a voltage source or current source such as a DAC at the input (RED)? From the shunt 75-ohms, it seems that you should be using a current source. If not, then it is possible to end up with the input filter frequency response as not being proper which again could affect your signal waveform.

Best Regards,
Rohit

0 Willing over 5 years ago in reply to Rohit Bhat

Prodigy 50 points

Thanks for your reply.

We have a voltage source ibput.Thre is a problem whcithe outputsigal amplitude comes dwn when one vga load is connected. It seems that the op drive strength is not enough, or somelse parameter setting not correct？

0 kai klaas69 over 5 years ago in reply to Willing

Guru 140200 points

Hi Willing,

what load do you connect?

Kai

0 Willing over 5 years ago in reply to kai klaas69

Prodigy 50 points

Hi kai, we connect a standard vga display to run the test.

0 kai klaas69 over 5 years ago in reply to Willing

Guru 140200 points

Hi Willing.

why have you two 75R rsistors in parallel at the input? What is the source impedance of the driver of OPA3692? Also 75R? Then you get an increased dampening, of course.

About the outputs: Have you connected three lines to one output of OPA3692? Why are there three 75R resistors?

Kai

0 Willing over 5 years ago in reply to kai klaas69

Prodigy 50 points

Hi kai,

the first 75R is the ouput matching reg of vga source, the second 75R is the input reg of opa3692,they are the character impedance.

About output:we use opa3692 to drive 3 way vga, but in this situration only one vga is connectted with othertwo ways not connected.(Only one R/G/B for each channel)

0 kai klaas69 over 5 years ago in reply to Willing

Guru 140200 points

Hi Willing,

I think there's a mistake at the input. The two 75R are in parallel which massively reduces the input impedance. What happens if you omit the 47n choke, the 3p3 cap and the second 75R resistor? Is everything correct then?

Kai

0 Willing over 5 years ago in reply to kai klaas69

Prodigy 50 points

Hi kai,

if only leave one 75R in parallel with the vga source output, then it regard as no load exists and will not output rgb signal.Removing 47n choke and 3p3 cap does not affect the signal amplitude.

0 Willing over 5 years ago in reply to Rohit Bhat

Prodigy 50 points

Hi, Rohit

are there any more suggestions?

0 kai klaas69 over 5 years ago in reply to Willing

Guru 140200 points

0 kai klaas69 over 5 years ago in reply to kai klaas69

Guru 140200 points

0 kai klaas69 over 5 years ago in reply to kai klaas69

Guru 140200 points

0 kai klaas69 over 5 years ago in reply to kai klaas69

Guru 140200 points

0 kai klaas69 over 5 years ago in reply to kai klaas69

Guru 140200 points

0 kai klaas69 over 5 years ago in reply to kai klaas69

Guru 140200 points

And here's still the TINA-TI file:

willing.TSC

Also keep in mind that all 75R conenctions have to be made with individual 75R cables or 75R transmission lines! Ringing can only be prevented by using characteristic impedance matching!!!

Kai

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OPA3692: have output ringing issue