Please inform the impedance of the signal source, signal amplitude, load impedance (resistance and capacitance). It would not be bad to see your layout in the photo.

 

Please inform the impedance of the signal source, signal amplitude, load impedance (resistance and capacitance). It would not be bad to see your layout in the photo.

Hello. I'm now at the lab again.
There are some weird things going on I cannot explain.

Anyway I'm using the lab signal generator which I configure with 5Mhz, sine wave, 2Vpp just for a test before connecting things, but the scope sees a 5Mhz sine wave with about 1.5Vpp. The scope probe is set to 10x and also the scope setting matches the same 10x. The signal generator probe is set to 1x. So, there is a 0.5V drop I can't see where.

Then another weird thing is happening.
The DC power supply we have at the lab, has 2 channels like the following picture:

So, to be able to have -5V from it, we need to shunt left black jack with right red jack. Then +5v comes from left red jack and -5v comes from right black jack, ok? Like this:

The problem is that when I connect the amplifier to the power supply, the signal generator signal gets completely messed.

This is the signal generator signal without the amplifier connected to it:

This is the same signal (input signal for the amplifier) after I connect it to the amplifier input pin:

For now I still haven't attached any load impedance, but it will be a resistive bridge so it will be very close to 50 ohm.

 

hi Jose,
It appears the OPA circuit is oscillating, try it with a dummy 50R load.
E

 

hi Jose,
It appears the OPA circuit is oscillating, try it with a dummy 50R load.
E

It's a bit better, but my concern is that this is just a test with 2Vpp but with the real circuit I'll have just about 400mV of input signal to this amplifier.

I mean, the whole circuit in a block like way is something like this:

Block 2 is the OPA691 circuit proposed by Mr. @Bordodynov.

At this point I'm just testing Block 2 by feeding it a known signal from the signal generator and also the +5V and -5V from the described power supply.

At this point I set up a signal of 2Vpp in sginal generator for testing purposes. Later, the input signal coming from Block 1 will be much smaller. And if I add -20dB of attenunation directly from the signal generator, well, the signal can't even be triggered. I just can't describe this.

 

Hi,
OK,
To check the circuit stability, have you checked the Output, with no input signal, but with Vin connected to ov via a 50R resistor, also a 50R on the output.?

 

Hi,
OK,
To check the circuit stability, have you checked the Output, with no input signal, but with Vin connected to ov via a 50R resistor, also a 50R on the output.?

I will ask you to be very accurate with the wording please. I mean, specify output of what, Vin of what, etc, etc, please! This is to help me clearing out questions, doubts I might have.

You're talking about Block 2 circuit stability, right?
When you say 'Output' it's this Block 2 output, right?
Vin, you mean the +V the Block 2 OpAmp needs, right?

If so, I'll do that next. Then, if the circuit is stable, what should I observe?

 

hi,
Look at this marked up image.
The Vout should be 0V and free of any oscillation.
E

 

Hi,
OK,
To check the circuit stability, have you checked the Output, with no input signal, but with Vin connected to ov via a 50R resistor, also a 50R on the output.?

Signal looks the same on that particular situation. No input signal at the amplifier. I just connected the ground leads of scope probe and signal generator probe tho the ground of the circuit!

 

hi,
Look at this marked up image.
The Vout should be 0V and free of any oscillation.
E

The output signal is not 0V,. It's about -3.7V.

 

Signal looks the same on that particular situation.

Hi J,
Do you mean it still showing that very high frequency oscillation on the output.??
Do not have any test input leads connected to the Vinput.
Post a photo of the Vout signal
E

 

Hi J,
Do you mean it still showing that very high frequency oscillation on the output.??
Do not have any test input leads connected to the Vinput.
Post a photo of the Vout signal
E

Wait, no!
That picture I showed was of the input signal before and after I turn on the power supply that feed the amplifier circuit OpAmp. It was not the output signal.
What I was trying to say was that the input signal (coming from the lab signal generator) was getting messy after I turn on the (split) power supply that is feeding the amplifier circuit's OpAmp (the OPA691).

 

Ok, wait. My mistake when I said -3.7V. I had a bad conenction.
It's close to 0V the output voltage. Ignore that.
I get about 0.09V at the amplifier output!

 

hi,
I would suggest you try a different 'cleaner' noise free power supply as the next step.
E

 

I will give advice to check the amplifier without connecting the signal source. Instead of a signal source, use a 51 ohm resistor. Look at what you have on the way out. If you see a parasitic signal, then load the output with a 51 ohm resistor. I asked for a photograph of your board, to see how you split the power, how the filter capacitors are soldered. If you do not connect the ceramic capacitors to the chip and the length of the capacitor leads is large, then this can cause oscillation.

 

hi Bordodynov,
He did that test in post #67, result in post #72.
E

 

I am sorry.
I missed these posts.
But I have a question to the ratings of the feedback resistors.Indeed, in post # 45 with smaller values of resistors, the gain was greater!And this variation is closer to the datasheet.

 

I'm now heading. I bought 2 7905 because my home power supply don't allow - 5V. So, with the 7905 I'll be able to run more tests. Sorry for not posting pictures of the pcb. I'll do it when I get home

Thanks

 

Ok, I have some pictures of the small pcb before and after soldering components.
Let me say I'm just a beginner and obviously I make a lot of mistakes, so what you are about to see is due to my lack of experience. When soldering the pcb components I accidentally touched the green film caps. I hope they are still ok. Also, there was a pad on the back side of the pcb that I partially damaged but I checked continuity and it was ok. If you think this might be an issue, I'll remove both green caps and as a last resort, if needed, I'll try to remove all components and solder them into another pcb ( i have 5, minimum quantity allowed to manufacture).

https://photos.app.goo.gl/4EUnQHyezF1CeZ9Q8


Edited;
Ok, I tried to measure that damaged capacitor with multimeter and it was not able to measure. Is it supposed to measure the correct cap value while it's in the circuit?
Maybe it's better I remove it and use another one, no?


Edited 1;
One other detail. When I sent the gerber files to the site that manufactured the pcb, I accidentally flipped polarised capacitors polarity so when I soldered the capacitors I flipped them and now the drawing on the pcb that says '-' has connected the '+' lead of the actual capacitor. This happened for both polarised capacitors!

Edited 2;
Ok, after removing the capacitor, I measured it and I got 12.54nF. It's a 10nF, so will it be damaged?

 

What happened when measuring a large value is not terrible.But I have doubts about the high-frequency parameters of capacitors.I usually use ceramic capacitors to filter high frequencies.Maybe someone has more experience in applying film capacitors to HF.
Ceramic capacitors are much smaller in size.In this case, the parasitic inductance of the capacitors is important.Less is better.

 

What happened when measuring a large value is not terrible.But I have doubts about the high-frequency parameters of capacitors.I usually use ceramic capacitors to filter high frequencies.Maybe someone has more experience in applying film capacitors to HF.
Ceramic capacitors are much smaller in size.In this case, the parasitic inductance of the capacitors is important.Less is better.

I can replace the film capacitors but probably their value matters, no? I only have ceramic caps of 1.5pF, , 2.2pF, 12pF, 22pF and 33pF. Then I only have film caps of 1nF, 10nF and 100nF. That's all I have here at home!