SAADC false reading on RIgado BMD300eval kit

RE: SAADC false reading on RIgado BMD300eval kit

Ankush — Tue, 24 Jan 2017 14:50:53 GMT

Hi Jørgen,

Actually, Gain of the front End is fixed to which I am applying bio potential signal, which is 1100. Hence if input signal amplitude if increases it goes beyond the range that I can apply to ADC. Hence I am getting clipped output. I tried giving proper 3.3 volt so I got 4095 as digital output.

So from this I can summarize following things,

input to the ADC should lie in the range of 0 to VDD (in my case 3.3 volt)
Selection of ADC Gain should be proper to set range for the input voltage.

Thanks for timely support..

RE: SAADC false reading on RIgado BMD300eval kit

Jørgen Holmefjord — Mon, 23 Jan 2017 15:05:41 GMT

I'm not sure I understand what you mean. Can you try to describe in some more detail and give an example? How is the output of the SAADC distorted? Is the SAADC input higher than the supply voltage?

RE: SAADC false reading on RIgado BMD300eval kit

Ankush — Sat, 21 Jan 2017 06:57:16 GMT

Hi Jørgen,

Thanks for your reply. It helped to re-evaluate the things related to ADC.I was wondering about why I am not getting maximum value of 12 bit resolution as 4095(I was getting around 3000), even when I am applying maximum input.

So from the formula if I want to get 4095 as digital output, with VDD=3.3 volts, I have to apply input as 3.3 volts and gain of ADC should be 1/4 (instead of 1/6). Setting gain as 1/6 means I am increasing the dynamic range of the input to SAADC(maximum input to be applied to ADC), but here VDD should also be increased(I was not doing this).

Now I am getting maximum digital output as 4095 for input of 3.3 volts with gain 1/4 and 12bit resolution.

Can you suggest one more thing. My input signal is a bio potential signal with some fixed gain of analog front end. So if signal strength increases suddenly then again it will distort the ADC output. So what is the remedy to avoid this. Can I put one voltage divider for the incoming signal. Does it will increase the noise coming at input to ADC?

Thanks once again!!

RE: SAADC false reading on RIgado BMD300eval kit

Jørgen Holmefjord — Fri, 20 Jan 2017 14:28:59 GMT

Hi,

The digital output is given by this formula. From the settings you posted, you are using 1/6 Gain, VDD/4 as reference, 12-bit resolution, and single-ended mode (PSELN will be ignored).
What is your supply voltage? Notice that the ADC is a full scale input range ADC, it cannot measure voltages outside the area of 0 to VDD.
You can select between VDD/4 and internal reference voltage (0.6V).
You need to periodically run the offset calibration task whenever there is major temperature change. The simple way is to just do this periodicall, e.g. every few seconds. Another way is to measure temperature on the die with the TEMP peripheral sensor and calibrate only if there is major temperature change. Gain error as result of temperature change is given in the SAADC electrical specificaiton as parameter C_EG.

Best regards,

Jørgen