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PCM5242: About output current limited and the value of resisitor.

Alex Lee
Prodigy 160 points
Part Number: PCM5242

Hello
There are two questions would like to know.

1. Is it safe to connect the output to ground(maybe caused by mistake such as connecting the output to single ended amplifier, the negative signal will be connected to ground.)
The calculation of the current would be 2.1Vrms/470R = 4.47ma? If so, what is the safe current limited of PCM5242?(not getting damaged)

2. Is there any reference for choosing the value of resistor?(ignore the cutoff frequency becasue it can be set by capacitor) How does it affect the distortion and harmonics? Can it be 100R or even 10R? 

Thank you very much!

  • 0
    TI__Mastermind 20830 points

    Hello Alex,

    PCM5242 is a differential DAC , meaning each output (P & N)  carries half the signal , usually you have to convert it to a SE signal , then along with the other pair of the output ( after converting those  to SE) you have a 2-channel stereo signal (Left and Right,) . In the above schematic you are sending half the signal to the LP filter and then to the amplifier input. The minimum load resistance for PCM5242 is 5k , it can not drive 470 Ohm as a standalone load. I don't have visibility into its interior design but I don't think it would damage it as the current limit in the chip should kick in.

    In general since the cutoff frequency is calculated based on R and C,  fc=1/2*Pi *R*C , as you choose much smaller R, you have to increase the size of Cap  for a given Fc and gain , which might not be so desirable to increase the cap size. Also for larger cap,  the impedance of the cap (Xc)  is smaller and at power up you might pull the device into current limit. As frequency goes higher,  the input impedance of the RC circuit gets closer to the value of the resistor and if this value  is too small , it might load the signal source.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 160 points

    Thanks Arash.
    Could you explain more about load the signal source? What data can be observed if the resistor used is too small? We are using 250R and it seems to be fine, but not sure if long-term use will damage the chip or performance.

  • 0 in reply to Alex Lee
    TI__Mastermind 26125 points

    Hi Alex,

    Arash is out of office today and will follow up with you tomorrow.

    Thanks for your patience,
    Jeff McPherson

  • +1 in reply to Jeff McPherson
    TI__Mastermind 20830 points

    Alex,  250R  is not loading the DAC  so you should be ok and I don't see any long term issue. ( if it was a problem, the output of the dac would have collapsed) You have also  a load connected to this network anyway. so it is not a problem to use 250R if you want.

    Regards,

    Arash 

  • 0 in reply to Arash Loloee
    Prodigy 160 points

    Arash, it's very clear! thanks so so much!