PCM1774: Questions on output level PCM1774

Marion Loessl

Part Number: PCM1774

Hi,

my customer uses two PCM1774 and has the following questions:

Reading the datasheet I was not able to found a register that configure the PCM1774 output as a line output (load > 10kΩ) or a Headphone output (load 16Ω or 32Ω).

Q1: Is there a special register setting to select between LOL/LOR and HPOL/HPOR ?

Reading the datasheet page 3/4 Electrical Characteristic we found: 0dB gives 1Vrms typical at the output with a load of 10kΩ.

But we have done some measurement with all the registers on the PCM1774 set to 0dB, and we have about 750mVrms at the output with a load of 10kΩ.

Q2: why this difference between our measurements and the datasheet ?

Thank you for your help,

kind regards,

Marion

over 4 years ago

0 Paul_Frost over 4 years ago

TI__Guru 53443 points

Hi Marion,

What are you using for the supply voltage? 3.3V?

0 Marion Loessl over 4 years ago in reply to Paul_Frost

TI__Expert 3470 points

Hi Paul,
yes 3.3V is used.

Kind regards,

Marion

0 Paul_Frost over 4 years ago in reply to Marion Loessl

TI__Guru 53443 points

Can you share your schematic?

0 Marion Loessl over 4 years ago in reply to Paul_Frost

TI__Expert 3470 points

Paul,

I sent it to you via Mail.

Kind regards,

Marion

0 Paul_Frost over 4 years ago in reply to Marion Loessl

TI__Guru 53443 points

Hi Marion,

I do not see any issue with the schematic. Can we clarify a few points:

1. In your questions, you said there was a 10kΩ load. Can you confirm there are no other loads on the pins?

2. Does the mono output PCM also have 0.750VRMS output?

3. Can you confirm if the output changes across frequency? I assumed they are using a sinewave tone for the test (otherwise the RMS value is invalid), can they sweep the frequency to ensure we are not seeing some filter attenuation? For example, try 500Hz, 1kHz, 5kHz, 10kHz, 15kHz.

Thanks,

Paul

0 Marion Loessl over 4 years ago in reply to Paul_Frost

TI__Expert 3470 points

Hi Paul,

thank you for your quick reply, that is highly appreciated.

While all of this is to answer question 2, can you in the meantime answer question one?

Kind regards,

Marion

0 Paul_Frost over 4 years ago in reply to Marion Loessl

TI__Guru 53443 points

My understanding is that the device does not have a register setting to determine the mode. The device should always output 1VRMS assuming default configuration.

0 Marion Loessl over 4 years ago in reply to Paul_Frost

TI__Expert 3470 points

Hi Paul,

regarding those question I got already answers:

1 : Yes confirmed is 10kΩ load.

2: When reading the report of the measurement for both stereo and mono output the PCM is about 780mVrms.

3: The signal is a sine wave at 1kHz, no frequency changes.

The signal was taken at the output of the PCM1774, before a serial capacitor.

The team here is reduce to the minimum, so it is not possible for them to do the measurement at different frequency.

This value of 780mVrms, when all the registers are set to 0dB, is OK for them.

But they want to be sure that with another board we will have the same value at the output of the PCM1774.

Kind regards,

Marion

0 Paul_Frost over 4 years ago in reply to Marion Loessl

TI__Guru 53443 points

Hi Marion,

Ultimately, I think there must be some flaw in their design. The output should be 1VRMS, so if they see something different, then it is either a load issue, digital attenuation, or filter behavior. It is difficult to conclude anything without additional testing.

Can you send the complete output schematic as well? Can they detail their measurement setup? Also, did they confirm that their source is really 0dBFS output?

Thanks,

Paul

Edit: Sorry, marked this as "TI Thinks Resolved" on accident.

0 Marion Loessl over 4 years ago in reply to Paul_Frost

TI__Expert 3470 points

Hi Paul,

I try to further investigate.

Kind regards,

Marion

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PCM1774: Questions on output level PCM1774