opa2277u

Mario,

First, please, I'll ask a basic troubleshooting question relating to offset voltage:  Have you tried another unit? Has the offset you are measuring only been seen on one device?  You may be working with a device that has been damaged.

We have additional information (a scope photo showing the sawtooth behavior) from Mohammad on an internal forum posting. We will check into this behavior, also, but it is first important to understand whether we could be dealing with a damaged device.

I will delete your previous posting that did not include the schematic.

Regards, Bruce.

Hi Bruce.

I solved the offet problem: It was a soldering problem.

Now i must solve the  problem about the mux.

With an another amplifier (ada4000) it works.

Regards.Mario

Hi Bruce.

is there any news about the mux-problem with opa2277u ?

It's very urgent for us; we must give an answer to our customer.

With an another amplifier (ada4000) it works.

Thank you very much for your support.

Regards.Mario

Mario,

I have spent considerable time puzzling over the waveform shown in Mohammad's picture (not shown in this thread). My best troubleshooting information has come from his posting, not in this thread. I was hoping that perhaps you would provide further information that might give a clue.

The ADA4000 is a faster op amp and this may help. Based on the scope photo (not too clear) I would have guessed some problem with the 10uF coupling capacitor. Mohammad indicated that this had been removed at one point without change.

Do you have any further troubleshooting information?

Regards, Bruce.

Bruce.

I tried to remove the capacitor  (short circuit) without any change.

Cannot  opa2277u  drive the adg1608 mux ?

I need opa2277u at first stadium for the low offset; maybe I should put a buffer between opa2277u and mux  like ada4000 ?

or to use a low offset amplifier  that can dive the mux directy.

Thanks.

Regards.Mario.

Mario,

In my simplified modeling of the application, I would believe the OPA2277 could drive the mux. The polarity and slope of these glitches make little sense to me.

Have you tried more than one unit to be sure it is not defective or damaged?

Regards, Bruce.

Bruce.

I tested  two boards,  each board has 8 channels.

I have always  seen the same behaviour.

If  i set  a fixed channel the signal is ok.

Only when i change the channel i have the problem: the signal is shifted  down and  goes slowly to the right point of signal (sine wave).

Regards.

Mario.

Mario,

Does the voltage on the previously active channel affect the waveform? Of particular interest is whether the direction or slope of the triangle ramps changes with changing voltage on the previous active channel.

More scope pictures showing any variation in this behavior would help. I will discuss with other experts later today.

Regards, Bruce.

Bruce.

About the channel with ADA4000 (the first 3 channel of mux: see the schematic) i don't see any problems.

The sine waves are clean and there are not the sawtooh.

While on the other channels with opa2277u i see the problems.

During the sampling the value of sine is shifted...

Regards.

Mario.

Mario,

Perhaps I didn't state my question clearly. I am not asking about the appearance of other channels. I want to know whether voltage present on other channels influences the shape of the waveform on the offending channel. Please provide scope pictures of the offending channel with different voltages present on the other inputs.

Regards, Bruce.

Bruce.

Yes it seems there is an influence: the channel after  the sine wave that should be 0V has the shape in attached.

This also if i remove the 10uf capacitor before the mux.

Also if i change the  the value before the channel with sine wave the sawtooh change

No influence between  the first 3 channel with ada4000.

Regards.

Mario.

Hi  Bruce.

Have you managed to have a look my last night mail ?

I'm at  stand still, i really don't know what to do.

Do you have other suggestions ?

Regards and thanks a lot.

Mario.

Hi Mario,

I have looked and looked at the two scope photos. And looked again this morning. It is a puzzle that makes little sense to me.

I believe there may be something in the multiplexing that is disturbing the op amp. But I think the OPA2277 should perform okay in this application. I don't doubt that the ADA6000 could perform better when disturbed by the multiplexer. It is a significantly faster device.I wonder whether it shows some residual disturbance. I am suspicious of some type of timing issue in the multiplexing that would create this behavior.

Not sure where to go from here, Mario. I wish I could be there to probe it myself.

Regards, Bruce.

Dear Bruce.

I have run out of ideas.I was wondering if woud be possible to discuss the problem via skype tomorrow (afternoon for me) (contact: mario soccini).

Thanks a lot.

Regards.

Mario.

Mario,

Good idea. Unfortunately, I'm in Arizona, MST time zone. Same time as California right now. I can call you at 6:00 p.m.  Is that too late?

Bruce

Mario,

I mean 6:00 p.m. Germany. That's my impression of your location. That is 9:00 a.m. California time.

Bruce

Bruce.

6 pm for me and for you 9 am, right ?  tomorrow is ok.

Thanks a lot.

Regards.

Mario.

 Bruce.

ok. I'm in italy same time of germany.

Talk to you tomorrow.

Thanks.

Mario.