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Noise on ADS1258 inputs due to AD conversion

Eduardo Pellini
Prodigy 140 points
Other Parts Discussed in Thread: ADS1258, OPA27, OPA2727

Hi friends.

We already used the ADS1258 on other projects with great results, but the circunstances now are different.

We are using the ADS1258 on a project to acquire bipolar signals, ranging, per channel, from +2V5 to -2V5 with a bandwith up to 100 Hz.

On this project we need to acquire ONLY 3 signals (S_VALE1, S_VALE2 and S_PICO).And we are interested on the signals S_VALE1 and S_VALE2 to be acquired both as single ended signals and also as the difference between them.

As the ADS1258 have up to 8 differential inputs, we connected our signals on the analog inputs in order to have:

S_VALE1 - AGND, S_VALE2 - AGND, S_PICO - AGND, S_VALE1 - SVALE2,

SVALE2-SVALE1, S_VALE1 - SVALE2, SVALE2-SVALE1, S_PICO - AGND.

This can be seem with more details on Fig1 below.

 

This three signals are buffered, each, with a LowPass filter with OPA27 OPAMPs like the one shown below:

Well the fact is on each analog input we are seeing with an osciloscope a great amount of noise, related with the AD conversion process: spikes of hundreds mV with the frequency of the internal AD mutiplexer (2KHz), tenths of mV with the frequency of the AD sampling (8MHz), and so on.

As seem on the first figure, we are not using a OpAmp on the "multiplexer - adconverter" external loop.

Any help ?

Thank you in advance for your help.

Eduardo Pellini

  • 0
    TI__Intellectual 2970 points

    Hi Eduardo,

    You can see input structure of the ADS1258 in the figure 37 on the page 15 of the data sheet. The multiplexer structure can be seen on the figure 35. The "noise" that you are seeing with scope probe is normal consequence of the sampling process (not in the range of hundreds of mV). More important are results that you are reading from your ADC. Looking your schematics I can see that you are missing RC filter in front of the ADC's analog input. For proper understanding of the sampling process and selection of the filter please take a look at:

    1.      Downs, Rick, and Miro Oljaca, “Designing SAR ADC Drive Circuitry, Part I: A Detailed Look at SAR ADC Operation,” Analog Zone, 2005.

    2.      Downs, Rick, and Miro Oljaca, “Designing SAR ADC Drive Circuitry, Part II: Input Behavior of SAR ADCs,” Analog Zone, 2005.

    3.      Downs, Rick, and Miro Oljaca, “Designing SAR ADC Drive Circuitry, Part III: Designing The Optimal Input Drive Circuit For SAR ADCs,” Analog Zone, 2007.

    The multiplexer on-resistance is in the range of 80 ohm and can limit proper selection of the RC filter. Also I will expect a lot of crosstalk between different channels in configuration where multiplexer is directly conected to ADC. For that reason my sugestion will be to add OpAmp between output of the multiplexer and input of the ADC. Please refere to the figure 67 and 68 of the data sheet. For beter understanding on the OpAmp selection please take a look at:

    1.      Baker, Bonnie, and Miro Oljaca, “External components improve SAR-ADC accuracy,” EDN, June 7, 2007, pg 67.

    2.      Miro Oljaca, and Baker Bonnie, “Start with the right op amp when driving SAR ADCs,” EDN, October 16, 2008.

    These article are treating SAR ADCs but the principal equivalent to ones of the Delta Sigma ADCs.

    Miro Oljaca

     

    Senior Application Engineer

    High Performance Linear

  • 0 in reply to Miro Oljaca
    Prodigy 140 points

    Thank you for the hints Miro.

    When i changed the sampling frequency and other AD register parameters I realised that the noise was generated due to the sampling process.

    As you depicted on the papers I adjusted passive RC filters at the original OPAMP's buffer outputs (at the multiplexer inputs) and created a circuit using a dual OPA2727 as suggested on figure 67 to buffer the mux output (differential) to the AD input.

     We will run some tests on the next week, but we expect a much better result. My guess is that the capacitor between the AD input pins is fundamental for the circuit performance.

    Thank you again for the support and greetings from Brazil.

    Eduardo Pellini