Noise from speakers when digital input is on dc level

[jackyhsu]

noise in speaker

Hi :

My design is 10bit current mode dac (1LSB:3uA) of consumer audio appllication.
But there is noise from speaker when digital input is on dc level (such as some code input,no transition).The audio appllication circuit is bjt .(dac output is the "base" node and shut a resistance;speaker connects between collecter and vcc ).

I'v change another PCB boad and another application circuit.The " sa sa " noise still exist. does anyone have the same experiment? or give me some suggestion.

thanks.

 

[moda]

Re: noise in speaker

What is your Clk source? you need "jitter free" source. (one possibility)
Regards
Moda

 

[barrybear]

Re: noise in speaker

I would split the sauce ADC from the output speaker amp . Determin if the noise is coming from the DAC or from the speacker circuit. Procead from there.

Barrybear

 

[Vamsi Mocherla]

Re: noise in speaker

Can you split the supplies to your ADC DAC and the speaker. It could be due to the digital noise at supplies. Put a huge capacitor at the VCC and at the bias terminal and run the test.

 

[andy2000a]

noise in speaker

do you probe BJT base ?? if you said noise
you will find D/A output have noise source ..
if D/A output keep DC , maybe your power source have noise by digital block

 

[jackyhsu]

Re: noise in speaker

do you probe BJT base ?? if you said noise
you will find D/A output have noise source ..
if D/A output keep DC , maybe your power source have noise by digital block

Thanks for "andy2000a"
I've probe the base of BJT and compare with another 8bit DAC of no noise.
The result of 10 bit dac output is a little large than 8 bit .
I also guess the noise from digital block . my ation is to cut the power line of digital block .I not yet do it . do you have other suggestion?

Best regards.

 

[andy2000a]

noise in speaker

if your 10bit & 8bit full scale output is equal or not,
if you drive much more signal on BJT base maybe BJT work in "saturation" not cloass_A ampl .
you can try reduce 10bit D/A level .. and make sure
why 10bit D/A have noise but 8bit not ..
I think your D/A 8bit 10bit have the same clock source . if digital clock have noise , 8bit DA also have noise source .