Digital noise when using ADCINC

[Anonymous]

Hi all,

   

I'm trying to add an analog input to my cy824794 project, which is a basically an SPI based digital sensor.

   

I added an 10bit ADCINC as follows:
<img src="http://i.imgur.com/W8hFCmJ.png" width="870" alt="" />
<img src="http://i.imgur.com/Y7CcQQV.png" width="578" alt="" />

   

But I''m getting some digital noise on the readings. Here's the relevant part of the schematics.
<img src="http://i.imgur.com/9XFoN6p.png" width="841" alt="" />

   

I call these at initialization: <code>ADCINC_Start(ADCINC_HIGHPOWER);
ADCINC_GetSamples(0);</code>

   

And this on the main loop:
<code>    if (ADCINC_fIsDataAvailable() != 0)
    {
        unsigned int ui = ADCINC_wClearFlagGetData();
        ...
    }</code>

   

Can anyone help?

[SampathS_11 · Accepted solution]

Hello David,

   

These are the items which can be done to reduce digital noise on ADC readings

   

- Set OpAmp Bias to high

   

- Enable AGndBypass in the Global resources. AGND is connected to pin Port2[4]. Capacitor values range between 100nF to 10μF. You can find a discussion of this on AN2219 - PSoC® 1 Selecting Analog Ground and Reference, on page 3 and 4

   

- Use low ESR / ESL bypass capacitors

   

- Add a capacitor to the motion input. This value should be such that it should not limited the bandwidth of interest.You can use 10nF as a starting point. This input comes through a connector shared with SPI.

   

- Is there any cross-talk on the board which connected to the other end? What happens to the noise if you disconnect the SPI lines? Does it reduce?

   

- Is the noise random or has any specific frequency. If it has a specific frequency, does it correspond to any periodic tasks being done in firmware?

   

Thanks,

   

Sampath Selvaraj