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ADS1247 stability at high sps

Other Parts Discussed in Thread: ADS1247

hii I want to use ADS1247 at high speed (1000 sps). but the problem is when i m using ADC on 1000 sps it gives stability of only 12 bits. How can i get stability of 15 or 16 bits at 1000 sps??

  • Pooja,


    When you talk about the stability, are you asking about the noise at 1000 SPS?

    Generally, the noise is both a function of the noise of the input signal and the noise in the reference. You will have to ensure that both are clean and as free of noise as possible. What is the input signal that you are measuring? Do you simply ground the inputs? Or is there a DC source that you are using as an input? For the reference, do you use the internal reference? If you use an external reference, what device are you using?

    On top of that, there may be line cycle noise coupling into your circuit. You may need to isolate your system from any noise that may be coming from 50 or 60Hz EMI.

    Regardless, there may be many sources of noise. A description of your setup and ADC data may help you figure out what is the source of the problem.


    Joseph Wu
  • Thank you joseph,

    I am using ADC internal reference and giving voltage from the other DC source as ADC input.

    Please find attached circuit for ADADS1247.pdfC connection.I am giving voltage input between ANI0 and ANI3.

  • Pooja,


    So what exactly are you measuring? I need more details than just the schematic.


    Joseph Wu
  • joseph,

    i want to use these ADC for RTD calibrator and i require much higher stability. when i give input from DC source to ADC at 20 sps i get the stability of 15 or 16 bits but when i run ADC at 1000sps flutuation comes in reading and i get stability of only 12 bits which is very low.
    which other information do you need?
  • Pooja,


    First, I'd like to know what it is that your are measuring. Do you have a resistor instead of an RTD right now? What are you using as the reference? Do you use a reference resistor to may a ratiometric measurement? Information like that can be used to help identify the sources of noise. So far you haven't told me much about your setup. There's really not much information about your setup in the attached schematic.

    What are you using for your power supplies? Is it a plug-in supply or is it battery powered? My concern is that if you have a plug in supply (or power that comes from the a plug) you will have a large amount of 50Hz noise in your system. If this is the case, you may have a better chance using a battery powered system and putting the circuit in a conductive enclosure.

    That may be why lower data rates are significantly less noisy. If you look in the ADS1247 datasheet on page 27, the frequency response shows that for lower data rates (like 5, 10, and 20 SPS), there is a large amount of 50 Hz rejection. On the next page. you can see that for the 1 kSPS data rate there is very little 50 Hz rejection.

    I don't know enough about your setup to say for sure, but I would concentrate on getting any 50 Hz noise out of the system. The conductive enclosure helps as well as removing any nearby radiative noise sources. Make sure there aren't long wires in the circuit that may pick up and EMI.

    Incidentally, I've looked at your schematic, and have a few comments. I wanted to know what you are using for power. You need to make sure that the -2.5V is a low impedance to an AC ground to makes sure the VREFOUT to VREFCOM capacitance can maintain the reference stability. Also, I noticed that you have two different ground symbols, you should makes sure that they are connected properly in your schematic. Finally, I would say that your supply bypass is quite large, and I'd replace the 22uF with 1uF.

    Again, I've suggested a couple of things to look at. However, I'm still guessing about what you have in your setup. I'd like need to know the values of your RTD and the reference resistor, what excitation current you use, what PGA gain are you using?


    Joseph Wu
  • Hii Joseph,

    Thank you for your reply.

    First thing for power supply i have made different ADC supply from one 5 channel power supply IC which is supplied from battery so there is no plug in power supply.

    Second i am not giving RTD or resistor input i am just giving DC voltage from one calibrator to only see the ADC stability at high speed. I am using ADC at 1000sps with no gain.

    Is there any other information required?

  • Pooja,


    What is the calibrator that you are using? Can you give a model number?

    In my posts, I ask a lot of questions. I need you to read through the posts and answer the questions. You need to be as descriptive as possible. If I don't know what you have in your system, I really can't help much.


    Joseph Wu
  • Joseph,

    I am using battery powered system and also put the circuit in a conductive enclosure. there are no other wires which pick up noise i am using 2mm banana patch cords of 1 m long to give voltage input which is nearly 1 V from beamex calibrator MC-2. i am using ADC internal reference and i generate +2.5V and -2.5V supply from LDO which is stable.
  • Pooja,


    I wasn't able to find much information about this particular calibrator. However, since it is handheld, I would assume that you wouldn't get any 50Hz noise from this.

    I still have other questions for you about the setup.

    What are the other settings for the device? I know the PGA gain = 1, but what about the others settings, VBIAS? IEXC currents? Burnout? How do you set the DC bias of the measurement?

    Do you use any input filtering? If so, what are you using? If you have the opportunity, look at the ADS1248EVM user guide and find the schematic. It has some modest filtering that can be copied and it might help with some of the noise. However, very large series input resistors might have some effect on the noise as the input current reacts to the resistance keep these resistor below 10k.

    If none of this works, let's try to analyze the raw data.

    First, let's makes some measurements for comparison. I would start with an input of VIN=2V and take a 200 point measurement with both 20 SPS and 1000 SPS data rate. Record the raw data. After that set the VIN=1V and take another 200 point measurement for both data rates again. Finally short the inputs and make the measurements again. Put all of this in an excel spreadsheet. Then we can analyze the output by looking at the noise and seeing if there is any periodic noise.


    Joseph Wu