DAC0808: dac working issue

Hello, 

You should try to isolate the problem in the circuit. Probe the DAC reference and supply while you are seeing the unexpected output noise. Probe A1 through A8 while you are seeing the unexpected output to see if the noise is occurring there. You said they should be high, but are they actually? 

Are you actually measuring the DAC output directly? Or are you measuring the output of the LM358? 

It is a bit hard to see the signals in your screenshots. You can paste the images directly in the post so they don't lose quality, and please explain what is being shown in each image. 

Best,

Katlynne Jones 

Hi,

1.Probe the DAC reference and supply while you are seeing the unexpected output noise.

- Can you cross confirm, dac reference -U5.14 and DAC supply- U5.13 to be probed individually?

2. Probe A1 through A8 while you are seeing the unexpected output to see if the noise is occurring there.

- A1 through A8 are probed there are no difference between the reference Board (expected output board) and the new board (observed output board).

3. You said they should be high, but are they actually?

- They all are high due to PE, but output is toggling.

4. Are you actually measuring the DAC output directly? Or are you measuring the output of the LM358?

- Output of the LM358

5.Images

-reference board

-new board

Hi,

1.Probe the DAC reference and supply while you are seeing the unexpected output noise.

- DAC reference and supply are probed (yellow is DAC voltage voltage),

2. Probe A1 through A8 while you are seeing the unexpected output to see if the noise is occurring there.

-They all are high due to PE, but output is toggling.

Regards,

Mirthunraju M

Hello, 

Yes, probing U5.14 and U5.13 individually during the unexpected output would help confirm if any noise is appearing on the output due to the reference. It would also be helpful to probe the DAC current output directly (U5.4) to see if the issue is actually at the DAC output, or if it is appearing somewhere in the LM358 circuit. 

Are there any differences between the reference board and the new board? Are they the same but manufactured at different times, or were there any modifications to the board itself? It looks like the noise on the output is at the same frequency as the PE signal, so if you are sure that A1-A8 are high during this time, then it is likely that noise from PE is entering the circuit at some point. Probing the signals I mentioned above might help you find the source. 

Best,

Katlynne Jones

Hi,

1. We have probed the U5.14 -> Vref+ , U5.13 -> Vcc , U5.4 -> DAC and and LM358 -> output

-golden board

-New board 

2. Are there any differences between the reference board and the new board?

-only the Vmin and Vmax are varying in both that too 1V difference, but still DAC voltage output is toggling

3. Are they the same but manufactured at different times, or were there any modifications to the board itself?

- reference is old board, no modification we tried to maintain same design

Regards,

Mirthunraju M

Hi Mirthunraju,

It looks like the DAC output is toggling in both the old board and the new board. Am I interpreting the plots correctly? To me it looks like the LM358 output is what is different between the two boards. 

Is the DAC reference turned off during these two scope plots? 

Best,

Katlynne Jones 

Hi Katlynne,

It looks like the DAC output is toggling in both the old board and the new board. Am I interpreting the plots correctly?

- Yes, DAC output is toggling in the both boards. But it is current DAC. I am not sure about current output.

Is the DAC reference turned off during these two scope plots? 

-No, it's ON. It is a very small positive and negative spike. 

Regards,

Mirthunraju M

Hi Mirthunraju,

The DAC could be toggling due to the toggling reference, even if the data bits (A1-A8) are being held high. But if this toggling is consistent on both boards then it might not make sense to focus on the DAC as the issue. If possible, measure the current output of the DAC on both boards.

Do you only have one new board? If there is more then one, are they all showing the same behavior?

Best,

Katlynne 

Hi Katlynne,

Can you please loop Amplifers LM358P team into this Thread?

Regards,

Srikanth Kacchu

Hi Srikanth, 

I believe you created a new thread for the LM358 portion of this circuit? https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1276838/lm358-lm358p-working-operation 

We can continue the conversation in the other thread or I can merge the two if the troubleshooting with the DAC part has been resolved. 

Thank you!
Best Regards,
Ashley

Hi Katlynne,

I couldn't understand the reason for +VREF is Bipolar. Can you pls help to understand it?

Why are we getting positive +VREF pulse during clock rising time and negative +VREF pulse during clock falling time?

Regards,

Srikanth Kacchu

Hi Srikanth, 

I'm not exactly sure. It looks like you have U7 controlling the DAC VREF+ reference. You can try removing R7 and measuring the output of U7 to check if this toggling is coming from U7, or maybe noise from the rest of the circuit is being coupled onto VREF+.

Best,

Katlynne Jones