Hi,
I am using the ADS1248 to measure a DC line which has a 50Hz AC noise.
The DC element is ~1.2V (Measured by Fluke)
The AC element is ~1.3V RMS (Measured by Fluke).
I didn't have a scope with me so I couldn't see the actual signal.
I am using the ADS1248 with an internal clock i.e. no external clock is available.
How should I configure the ADS1248 to get read of the AC element and measure just the DC?
Thanks
Noam
Noam,
Normally, the ADS1248 will filter out line noise (50 or 60Hz) with the digital filter at low data rates (5, 10, and 20 SPS). Every data rate will have a frequency response shaped by the digital filter. As an example, below is the frequency response of the ADS1248 when the data rate is 10 SPS (taken from figure 58 of the ADS1248 datasheet).
The frequency response is also dependent on clock frequency. If the external clock frequency is not 4.096MHz, then the filter profile scales with the clock frequency. Also, if the internal oscillator is used, there will be some variation in internal oscillator frequency (roughly ±5%)
The datasheet also gives Table 10, which shows the attenuation of line cycle frequencies for different data rates. The table gives attenuation with a variance band for the frequency. In your case, the internal clock wlll give a larger frequency error range than shown in the table.
However, in your case there are two things that concern me. First, you have a 1.3VRMS signal on a 1.2VDC signal. If the RMS signal is really 1.3V, then the peak-to-peak signal is ±1.8V. That means the input is moving from -0.6V to 3.0V. If you are using a unipolar supply of +5V, then the input is outside the range of operation. The ADC input for Gain=1 is AVSS+0.1V to AVDD-0.1V. If the input is outside the ADC input range, then the digital filter will be unable to fully reject the line noise. Even if you are using a bipolar supply of ±2.5V, the max input is at +3.0V, which is also outside the input range.
Do you know for sure that the AC noise is 50Hz? At this point, I assume you've measured this with a Fluke multimeter. If the AC noise isn't 50Hz, then the ADC may not reject the noise signal.
First, I highly recommend getting an oscilloscope. You can use it to guarantee your input signal and the nature of the noise. Also if there are communication problems, you can easily view all your SPI signals. Second, if the input signal is really going from -0.6V to 3.0V (and the ADC is using a unipolar supply), you will need to level shift this signal with some differential amplifier to measure the signal. The input will need to be within the range of the ADC's PGA.
Joseph Wu
