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TPA2012D2 - Driving differential inputs from single ended DAC

stomp
Genius 4675 points
Other Parts Discussed in Thread: TPA2012D2

Hi,

I apologize if this is a duplicate of http://e2e.ti.com/support/amplifiers/audio_amplifiers/f/6/t/15.aspx

 

We have designed the TPA2012D2 (stereo class D amplifier) into a board.  It is driven from a CS4334 audio DAC.

We have the positive inputs of the TPA2012D2 connected to ground via 0.1uF capacitors.  We have the negative inputs connected via series 0.1uF capacitors to the outputs of the DAC (there is also a filter between the DAC and the input capacitor).  The 8ohm/1watt speakers are connected directly to the TPA2012.  This is exactly as the datasheet details.

The forum post (above) details the negative input connected to ground via a cap, not the positive input!!

We are experiencing some heating with the speakers, and associated current draw.

It is not clear to me exactly what the input voltage range should be on the DAC side of the 0.1uf input capacitor into the positive input of the TPA2012D2.

Can anyone assist me?  Should this voltage be 0V when no sound is required, or should it be at VCC/2 (2.5V?).  Should the negative or positive input be connected to ground via a cap?

Also do we require any DC blocking capacitors on the output of the speakers?

 

Thanks

Stomp.

 

  • 0
    TI__Guru**** 189401 points

    Hi, Stomp,

    I believe the post you referenced doesn't make reference to positive or negative (Steve's response, not the original question. Notice Steve doesn't differentiate between positive and negative). Our class-D amps with differential inputs use a true fully differential input stage, so it makes absolutely NO difference which input you use. I personally prefer to use the negative input, but some of my colleagues like to use the positive input. I guess, use whichever makes your layout easier...

    The voltage on the DAC side of the capacitor should be whatever the DC bias of your DAC is. On the TPA2012D2 side of the caps, it should be VDD/2 (where VDD is the power supply applied to the TPA2012D2). Any mismatch in impedance between + and - will result in turn-on and -off pop, so make sure they are matched.

    You do have the speakers connected in BTL (bridge-tied load), right? One side of the speaker connected to VO+ and the other connected to VO-, right?

    Otherwise, you may need to connect an RC LPF on the outputs so you can make audio-bandwidth measurements to see what is going on. This is shown in the schematic right after the tabular data in the d/s, just before the graphs. It depicts the circuit we use when generating the d/s graphs. The RC is just needed for measurement purposes, you don't need it in your end equipment.

    Let me know if this fixes the problem for you.

    -d2

  • 0 in reply to Don Dapkus
    Genius 4675 points

    Hi Don,

    Firstly thanks for taking the time to respond.

    I didn't suspect there would be any issue with the differential inputs, but I just needed to get clarification, thanks...

    The DAC works fine, and yes the TPA2012D2 side of the input capacitors is at VDD/2, so no problems there either.  We are experiencing some popping and clicking and now that you have mentioned it, we think it is the filter on the output of the DAC.

     

    You have me worried now about the bridge-tied load scenario.

    I am using the TPA2012 to drive two speakers, in stereo mode.   Each speaker is connected to its respective output on the TPA2012.  I hope this is correct?

     

    I will try and get a RC filter made up and verify whats going on, however I simply suspect it is just a mismatch between the DAC and the TPA2012.

     

    Thanks again for your time.

    Stomp.

     

  • 0 in reply to stomp
    TI__Guru**** 189401 points

    Stomp,

    If you have your output configured as shown in Fig 33 (ferrite beads and caps on the output are optional, only needed to pass EMC requirements), you should be golden.

    As I previously mentioned, it doesn't matter if you drive IN- or IN+, they are identical in this device.

    -d2

  • 0 in reply to Don Dapkus
    Genius 4675 points

    Thanks again Don,

     

    Looks like the problem may have been with the filter on the output of our DAC.  By removing the filter, we have fixed the problems related to popping also.

     

    The TPA2012D2 is connected exactly as figure 33. 

     

    Kind Regards

    Stomp.