PCM4204 - Input(VinX+, VinX-) with DC-Bias

Hirotaka Matsumoto

Other Parts Discussed in Thread: PCM4204

Hi all,

Would you mind if we ask PCM4204?

Input pins(VinX+, VinX-) are used for Audio input signala like as Fs=48kHz, 96kHz, 192kHz.
In case of under "ABSOLUTE MAXIMUM RATINGS" condition, is it possible to add DC-Bias to Input pins(VinX+, VinX-)?

And we could not find "Recommended Vrms value" at VinX+, VinX-.
So, could you let know us these?

We appreciate your reply.

Kind regards,

Hirotaka Matsumoto

over 10 years ago

0 Justin Bohr over 10 years ago

TI__Mastermind 28020 points

Hi Hirotaka,

The Vcom pins are available to use as a bias signal for the input stages of the system. This bias will be near 0.5*Vcc and should have little to no current draw on the pin. There are examples of input circuits on page 26 and 27 of the PCM4204 data sheet. It states on page 15:

"Each analog input pair accepts a full-scale input voltage of approximately 6.0VPP differential. The analog input should not swing below analog ground or above the VCC1 (pin 5) or VCC2 (pin 44) power supplies by more than 300mV."

Justin

0 Hirotaka Matsumoto over 10 years ago in reply to Justin Bohr

Guru 27585 points

Justin san,

Thank you for your reply.

We'd like to confirm one point.
Our customer would like to get DC-bias value using Analog input.
So, in your case, is it possible to get DC-bias data?
(Otherwise, our customer would like to know that it is possible to read DC data using PCM4204 or not.)

We need your help.

Kind regards,

Hirotaka Matsumoto

0 Hirotaka Matsumoto over 10 years ago in reply to Hirotaka Matsumoto

Guru 27585 points

Justin san

We guess that your days are so busy.
However, we will appreciate for your cooperation, if you reply it.

Kind regards,

Hirotaka Matsumoto

0 Justin Bohr over 10 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 28020 points

HI Hirotaka,

The Vcom pin will output the bias voltage, which will be 0.5*Vcc or near 2.5 V. Is this the data you are looking for?

Justin

0 Hirotaka Matsumoto over 10 years ago in reply to Justin Bohr

Guru 27585 points

Justin san,

No, our customer would like to add DC-bias at Input VinX+, Vin- within ABSOLUTE MAXIMUM RATINGS.
And then, our customer would like to get the data of this added DC-bias value.
So, is it possible or not?

Kind regards,

Hirotaka Matsumoto

0 Justin Bohr over 10 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 28020 points

Hi Hirotaka san,

The PCM4204 is a part meant to take in a signal biased at +2.5V, so your audio signal would need to have a +2.5V DC offset to be input in to the PCM4204, this can be done with the input stage outlined in the data sheet.

Justin

0 Hirotaka Matsumoto over 10 years ago in reply to Justin Bohr

Guru 27585 points

Justin san,

Thank you for your prompt reply!

Ok, so we'd like to confirm one point.
Is it possible to rise +2.5V DC bias?
Is the output from ADC value included with DC bias + input data ?

We're sorry for our poor explanation.

Kind regards,

Hirotaka Matsumoto

0 Justin Bohr over 10 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 28020 points

Hirotaka san,

Yes, the PCM4204 assumes the input signal will be centered around +2.5V. This +2.5V is also biasing the ADC in the part, which is why it is brought out the the Vcom pins in order to bias the input signal.

Justin

0 Hirotaka Matsumoto over 10 years ago in reply to Justin Bohr

Guru 27585 points

Justin san,

A million thank you for your help!

As conclusion, if we need to add over 2.5V(example 3.0V) - DC bias voltage(within ABSOLUTE MAXIMUM RATINGS),
we don't use Vcom in order to bias the input signal.
So, is it possible?

We need your help.

Kind regards,

Hirotaka Matsumoto

0 Hirotaka Matsumoto over 10 years ago in reply to Hirotaka Matsumoto

Guru 27585 points

Justin san,

Could you let us know our last contribution?

Kind regards,

Hirotaka Matsumoto

0 Justin Bohr over 10 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 28020 points

Hi Hirotaka san,

You will not get the correct output if you do not bias the input to Vcom. If you do input a signal biased above the Vcom reference voltage, the ADC will still be biased at Vcom, and therefore the signal will be captured with an offset and therefore the output will be incorrect. You should input the signal through a AC coupling cap and then into a buffer stage that will bias the input signal to Vcom.

Justin

0 Hirotaka Matsumoto over 10 years ago in reply to Justin Bohr

Guru 27585 points

Justin san,

Thank you for your reply.

OK, we got that PCM4204 should not be inserted DC bias voltage to Input directly(except Vcom).

Kind regards,

Hirotaka Matsumoto

0 Hirotaka Matsumoto over 10 years ago in reply to Hirotaka Matsumoto

Guru 27585 points

Justin san,

We are sorry that our explanation made you confuse.

Our customer would like to get the sginal which the signal will be captured with an offset(offset means +input DC-bias, right?)
for some information.
Of cource Input voltage is less than ABSOLUTE MAXIMUM RATINGS

Kind regards,

Hirotaka Matsumoto

0 Roland Bra over 10 years ago in reply to Hirotaka Matsumoto

Prodigy 160 points

Hi Hirotaka,
i came across this post while searching for a similar question...

you have to use the 2.5V because the adc expect that as the avarage voltage. The easiest way is to use the Vcom pins. If you're using an offset provided by an external supply it also has to be 2.5V. I think that your output signal will be not very useful if you rise the offset.

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PCM4204 - Input(VinX+, VinX-) with DC-Bias