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PCM6260 Input Attenuator always active?

David Dubrow
Prodigy 30 points

Part Number: PCM6260-Q1

Tool/software:

I am curious about the specifics of how low-swing and high-swing mode setting works in the PCM6260. 


A coworker of mine said that the input attenuator on the front end is always active, with 14dB of attenuation.  
Of course it's active for low-swing mode, since 2Vrms (low swing) is 14dB less than 10Vrms (high swing). 

The question is, if the input attenuator is always active, then where is the 14dB of gain getting added back in for high swing mode?   

I'd think that it would be applied in the PGA stage. 

But if I test high swing mode with +14dB gain vs low swing mode with no additional gain, I do not get the same result when using a DC differential input.  
Actually, with the +14dB PGA gain in high swing mode, the input is clipping on one side of the waveform before the other!  I'm applying this gain in CFG1 register, "Channel Gain", which I assume is PGA gain. 

It is worth noting that I only see this behavior on a DC differential input.  It does seem to work as I'd expect on an AC single-ended input (Low-swing gives the same value as high swing with +14dB in the channel gain).

Also, in DC differential mode, if I apply the +14dB of gain in the digital volume control, it works as expected. 

So I'm just trying to figure out what's going on with the Analog gain section (PGA).  Why the behavior is different for DC differential mode than it is for AC single-ended mode?

  • 0
    TI__Mastermind 29210 points

    What is the application? What are we trying to achieve?

  • 0 in reply to Daveon Douglas
    Prodigy 30 points

    The application is just a mic input circuit.  We're taking in some 2-wire differential mics and a 3-wire single ended mic.  The 2-wire mics are using the 8V mic bias from the PCM.  

    I got the application handled, using the digital volume control instead of the PGA analog gain.  SNR seems fine.  So I'm not blocked.  

    I was just hoping to better understand the nuance of how the input attenuator and PGA gain stage work with respect to low-swing and high-swing mode.  Since my assumption of how it must work (based off my co-worker's assertion that the input attenuator is always active), doesn't quite track.  So I'm just looking for some education here.  

  • +1 in reply to David Dubrow
    TI__Mastermind 29210 points

    Hi David, 

    The input attenuator is always active and the 14dB gain will either be applied in the PGA or in DVOL depending on the input configuration of the input. The  14dB cannot always be applied in the PGA because if there is a voltage difference at the input pins, the PGA is amplifying this and noise associated as well. So when the input configuration is DC coupled, single-ended for example, the 14dB will be applied in the digital signal chain following the digital high pass filter.

    When the input is AC coupled, differential the IC is realizing the same voltage across both pins and can apply the gain in the PGA.

    This is why you see different behavior depending on the input circuit configuration.

  • 0 in reply to Daveon Douglas
    Prodigy 30 points

    Great explanation, thanks!