LMP7702: How to reduce the output noise performance ?

aggil liang

Part Number: LMP7702
Other Parts Discussed in Thread: PCM1803A

We design a circuit for the audio transmitting, We test the input nosie of LMP7702(Pin 2 and Pin 3) is 115dbu, and the output is 108dbu, Do you have any suggestion to improve the output performance ?

　We need to optimize the circuit.

over 4 years ago

0 kai klaas69 over 4 years ago

Guru 140200 points

Hi Aggil,

remove C22 and C12. These caps erode the phase margin and make the LMP7702 instable. This can also result in higher noise.

Mount a 100pF cap in parallel to R24 and an identical cap in parallel to R7.

Kai

0 aggil liang over 4 years ago in reply to kai klaas69

Expert 2660 points

Hi Kai
thanks ,we will try and feedback the result to you, thanks

0 Tamara M Alani over 4 years ago

TI__Genius 10850 points

Hi Aggil,

What package type (size) are you using for the capacitors? Typically, C0G capacitors do not exhibit the same level of signal degradation as high-k capacitors like X7R, for example.
To learn more about how capacitors effect the signal, click here: file:///C:/Users/a0225390/Downloads/Signal-distortion-from-high-K-ceramic-capacitors.pdf

-Tamara

0 aggil liang over 4 years ago in reply to Tamara M Alani

Expert 2660 points

Hi Tamara
thanks for your recommendation, the file your mentioned can't be downloaded. Can you send the link again ?

0 kai klaas69 over 4 years ago in reply to aggil liang

Guru 140200 points

Hi Aggil,

a web search gives this:

Signal-distortion-from-high-K-ceramic-capacitors.pdf

Kai

0 Tamara M Alani over 4 years ago in reply to kai klaas69

TI__Genius 10850 points

Thanks Kai! I don't know why my link broke :)

0 aggil liang over 4 years ago in reply to Tamara M Alani

Expert 2660 points

Hi Kai, Tamara

Thank you

0 Tamara M Alani over 4 years ago in reply to aggil liang

TI__Genius 10850 points

You are welcome Aggil. I'm going to close this thread now. If you have any follow-up questions, reply here to re-open.

Good luck.
-Tamara

0 aggil liang over 4 years ago in reply to Tamara M Alani

Expert 2660 points

Hi Tamara

We try the new parameters, but there is no improvement. Below picture is the 2rd stage. SW2 is the AUDIO OUT of the first stage, and VINR, VINL is the input signal of PCM1803A. How can we optimize the circuit ? thanks

0 kai klaas69 over 4 years ago in reply to aggil liang

Guru 140200 points

Hi Aggil,

first, remove C101. These cap erodes the phase margin and makes the LMP7702 instable. This can also result in higher noise. And replace the supply voltage decoupling caps C95 and C98 by 100nF/X7R caps.

Now, that I know that you try to drive the input of PCM1803A with your circuit, I add the following: Have you seen figure 24 of datasheet of PCM1803A? Please follow the recommendations given there with regard to C1, C2, R1, R2, C7 and C8! In its current form your circuit isn't capable of properly driving the PCM1803A.

Kai

0 aggil liang over 4 years ago in reply to kai klaas69

Expert 2660 points

Hi Kai

thank you, we will try the new parameters and feedback to you.

Amplifiers

Amplifiers forum

LMP7702: How to reduce the output noise performance ?