DAC7822: Noise in the VREF input

Joaquin Lopez contreras

Part Number: DAC7822
Other Parts Discussed in Thread: OPA2822

Dear all,

I have the following design (see picture attached) using DAC7822.

I need a voltage output of the DAC in the range 0 to +1,2V, so according to the datasheet of DAC7822, the VREF should be -1,2V.

This negative reference is generated using a ADR3412 and a OPA2822. What I see in TP38 is a oscillation as shown in the picture. This oscillation stay even when I am not powering the DAC (3V3D off), that is, stay when all the digital electronic of the board is off.

In the the picture below, the +5V_DAC signal is shown.

Any advise to improve this? What could be the cause? Could be the DAC causing this?

Any comment will be welcome.

Maybe this is not the forum to place this, in that case, please, let me know, and I will forward to the right place.

Thanks in advance,

Joaquin.

over 3 years ago

0 Joaquin Lopez contreras over 3 years ago

Intellectual 440 points

I forgot to upload the +5V_DAC signal picture. Here it is.

0 Paul_Frost over 3 years ago in reply to Joaquin Lopez contreras

TI__Guru 53163 points

Hi Joaquin,

This seems like a stability issue with the large cap load on the output of the of the amplifier. I will move this thread to the PRAMPS team.

0 Joaquin Lopez contreras over 3 years ago in reply to Paul_Frost

Intellectual 440 points

Thank you Paul for you support.

I understand that you have already move this thread to the PRAMS so I dont need to do anything, am I correct?

On the other side, just to describe more in detail the situation, in the schematic, signal -1V2_REF goes in total to 3 DAC7822 ICs, that is goes to 6 DACs. Maybe this can help to clarify the problem.

Regards,

Joaquin.

0 SimaJalaleddine over 3 years ago in reply to Joaquin Lopez contreras

TI__Mastermind 23445 points

Hello Joaquin,

Paul moved this thread to the High speed amplifier group. Thank you for sharing the schematic and scope shots with us. I agree with Paul, it is most likely caused by the cap load at the output of the amplifier. Would you be able to replace C145 with a capacitor between 10pF to 30pF and retake the scope shot at the same testpoint (TP38)?

Thank you,

Sima

0 Joaquin Lopez contreras over 3 years ago in reply to SimaJalaleddine

Intellectual 440 points

Hello Sima,

Thank you very much for your support. I have changed C145 for 10pF and this resolve the problem. Sorry I am not able to upload right now the new shot, dont know if it is a problem of the application or my computer.

Before close this thread, just to try to learn something, could you give a small explanation of this? C145 was 100nF as recomended by ADR3412 datsheet.

Regards,

Joaquin.

0 SimaJalaleddine over 3 years ago in reply to Joaquin Lopez contreras

TI__Mastermind 23445 points

Hello Joaquin,

No worries on scope shot it was incase it didn't solve the problem; I am glad it did. For the external capacitors, these are used as a low-pass filter to lower the amount of noise and are used for helping with output switching transients. In generally, it optimizes DC performance at the reference. Here is a link that has more information on this concept. The reason the amplifier can become unstable with high capacitive loads because it introduces a pole in the frequency response. Depending on where the noise gain intersects AOL, this solution can become unstable which leads to oscillations. Most of our amplifier datasheets will have frequency response graphs with various capacitive loads. Here are some more references on this topic: link1 and link2.

Thank you,
Sima

+1 Joaquin Lopez contreras over 3 years ago in reply to SimaJalaleddine

Intellectual 440 points

Thank you very much!!!

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DAC7822: Noise in the VREF input