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ADS1274 Noise in High Resolution Mode (ADS1274EVM-PDK)

Other Parts Discussed in Thread: ADS1274, ADS1278, ADCPRO

Hello together,

I recently bought the ADS1274EVM-PDK from Mouser in order to evaluate the ADS1274. With its four simoultanesouly sampling channels, the ADS1274 seems to be the right  ADC for my application in which the phase angle between two AC signals has to be measured with the lowest possible measurement uncertainty.

Since the noise of the ADC and its analog front end is crucial in such an application, my intention is to use the ADS1274 (or in later stage of development the ADS1278) in "High Resolution Mode" with clock frequency of f= 27Mhz (= 6.75 MHz modulator frequency) and a resulting sampling rate of 52.7 kSps.

After installation of the ADCpro software on my PC and setup of the hardware (correct jumper settings etc.), I started to perform noise measurement using the ADCpro Software. Much to my surprise the RMS and peak-to-peak values are substantially higher in "Hi-Resolution"-Mode than in "Hi-Speed"-Mode when measured at 27.0 MHz clock frequency.

The following settings were used for noise evaluation:

- all buffer amps powered down; ADC input switches on "HDR" position (direct input from header); each input  directly shorted using jumpers on the header pins.

- TDM fixed Mode (Frame Sync due to connection to MMB0-board)

- wall plug power supply

- clock generated by PLL

Using the histogram function (block size: 32768 samples, 8 codes/bin = 2.4 mikrovolts) I measured the folowing values at a clock rate of f= 27MHz:

High speed mode:   standard deviation: 26.5 codes = 7.9 mikrovolts RMS , peak-to-peak: approx. 210 codes = 62.6 mikrovolts

High Res. mode: standard deviation: 48 codes =14.3 mikrovolts RMS , peak-to-peak: approx. 550 codes = 164 mikrovolts

While the noise figures for the "high speed mode" are better than specified (< 8.5 mikrovolts), the noise figures for "high resolution mode" are 2.5times the normal specified value of 5.5 mikrovolts.

I tried the following steps to reduce the noise in high resolution mode:

- using the buffer amps

- DC power from a laboratory grade DC supply (linear regulation)

- ferrite beads on the power supply cable and the USB cable

-additional ceramic and tantalum caps between AGND and +5VA

-referencing the ADC inputs directly to AGND

-using the on-board 27MHz XTAL oscillator

None of these measures did really improve the noise figures in "high res. mode". Meanwhile the noise figures in "high speed mode" remained happily as specified.

In high speed mode the shape of the histogram look almost perfectly like a gaussian distribution, but in high resolution mode the the peak-to-peak values become excessive.

I tried the "mulitFFT" to look for particular signals that might interfere with the ADC, but found nothing. However using the ADCpro "multiscope" function I observed unusally large peaks, normally not seen in white or 1/f noise. Again in "high speed mode" these peaks didn't show up.

Finally I reduced the clock frequency to f=22.00 MHz (PLL)  and then the noise figures improved significantly in high resolution mode: 17.4 codes standard deviation = 5.2 mikrovolts RMS,   peak-to-peak: 140 codes.

I measured the noise figures at various clock frequencies. At more than 23 MHz the noise figures more than 7.0 mikrovolts RMS, and above 25 MHz more than 10 mikrovolts RMS.

My question: What is the cause for this strange phenomenon ? Why does the "hi speed mode" deliver noise figures as specified (also using the buffer amps) at various frequencies up to 32.7MHz, but operation in "high resolution mode" show excessive noise values above 22.0MHz  clock ?

I would really appreciate your help. I could use the ADS1274 with high speed mode at 27MHz and 105.5 kSps, but would really prefer running the ADS1274 in high resolution mode at 52.7 kSps and (hopefully) better SNR.

Best regards,

Martin Damev     

  • Hello Martin,

    Welcome to our forums and thank you for your interest in our ADS1274.

    I am fairly certain that the poor noise performance you see in High-Resolution Mode is the result of an EVM PCB issue that our team recently discovered. This is not a device issue and is only found in ADS1274/8 EVM-PDKs that use PCB Rev F.

    An errata will be added to the ADS1274/8 EVM Tools Folder in the next 24 hours to explain the issue and its impact on EVM usability. If you would like, I can email you a draft of the errata now. Please send me a private message with your email address.

    Best Regards,
  • Hello Ryan,

    Thank you very much for the fast reply. I will check the errata in the ADS1274/8 EVM tools folder.

    Best regards,

    Martin