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PCM1803A VERY NOISY WITHOUT SIGNAL

Gabriel Vendramini
Prodigy 20 points
Other Parts Discussed in Thread: PCM1803A

Dear,

I’m using a microphone + pre-ampl (sens. 3mV/Pa) and an audio ADC converter PCM1803A, controlled by an ARM STM32F4.

This ADC has 2 channel, but I'm using only the channel left. 

 

By the datasheet, the conversion is from 1.0 to 4.0Vcc, with 2.5Vref, so i can input a sin wave with 1.5Vp.

The conversion is: data*1.5/(2^23) [V].

I want measure in [Pa], so using 3m[V/Pa], I need to use a gain of 500. (when I have 1.5 [Vp], I have 500 [Pa]). This is ok.

As you can see, my gain is 500.

The problem is without input signal. In this condition, I measure 0.3 Pa (= 0.6mV os noise.

When I put off the microphone and the signal is below of 0.6mV, I can’t measure. I need to measure 0.006 Pa = 0.018mV (the noise is 0.6mV).

I can see the noise with the osciloscope on analog supply and on input left of adc converter.

As I saw on datadheet, the PCM1803A has an enable pin. (controlled by the MCU).

When it is off, I don’t have this noise on supply and input (and I can’t measure the signal, because it is off ,of course), but when I enabled the adc, I have this noise. So the noise is generated by the ADC converter!! Why?

I'm using the left channel. The other channel I jumped to GND  in one test and put it floating in another test... no difference achieved.

best regards.

  • 0
    Prodigy 20 points
    Only to clarify:

    The normal conversion of the ADC is (24 bits, but 1 of them is for signal - remember the internal medium point of 2.5Vcc of ref): result = (data_left*1.5)/(2^23) [V].
    The microphone sens. is 3mV/Pa, so:
    result = (data_left*1.5)/(2^23) [V] * (1/3m) [Pa/V] = (data_left*1500)/(8388608*3) [Pa] = (data_left*500)/(8388608) [Pa]
    My final result is in Pascal.

    When the circuit is on, the PCM1803A has 5Vc for analogical supply and 3.3Vc for digital supply.
    In this situation, when the pin 7 (PDWN) is off, this supplies is very good, without noise.
    When I put this pin on, the supplies becomes very noisy....

    Please, does anybody worked with this component?

    Maybe the problem is with the pin 2 (right channel)? I'm not using it, so it is floating now.

    Regards.
  • 0 in reply to Gabriel Vendramini
    TI__Mastermind 27980 points

    Hi Gabriel,

    Could you show me a schematic of your design? It would also be helpful if you could post some scope shots of the output when the part is in various states. I would suggest following the data sheet's typical circuit diagram, or the EVM schematic. There are various places noise can be introduced into the signal chain, so getting a better view of the problem could help.

    Justin