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Datalogger with ADS1282

Ivo Bernardo
Prodigy 150 points
Other Parts Discussed in Thread: ADS1282, THS4522, ADCPRO
I want to make a datalogger to measure signals from -0.5Vdc to 0.5Vdc with a sampling rate of one minute, ie every minute obtain one sample, one-shot!
The battery that I will to use is 3.7Vdc and the ADC is ADS1282. 
Should I put VREFN =- 2.5V, VREFP = 2.5V or less?? and Vin input connect directly with what is going to measure? 
Or should I use a Fully Differential Amplifier THS4522 in ADC input? Or should I just use a passive filter?
What tensions do I have to put in VREFN, VREFP, AVDD and  AVss? And which components should I use?
Regards,
Ivo Bernardo
  • 0
    TI__Guru* 97835 points

    Ivo,

    I understand that you'd like to create a datalogger. However, I think the best way to go through this is to find out more about the application and what you need from it.

    First, do you really need to use the ADS1282? What sort of data rate do you need? What kind of noise or resolution are you looking for? Do you need two or more channels?

    The ADS1282 requires AVDD-AVSS to be 4.75 and 5.25V. With a battery of 3.7V, you'll need some sort of power management to bring that up and minimize noise on the supply. Some other devices require less voltage to operate and plenty can operate with less than 3.7V.

    In the datasheet, the specification table lists the supplies at AVDD=+2.5V, AVSS=-2.5V, VREFP=+2.5V, and the VREFN=-2.5V. This may be a good place to start, but the device can certainly be run with a unipolar supply of AVDD=+5V, AVSS=0V, VREFP=5V, and the VREFN=0V. I think either would work fine.

    The decision to use an input buffer or filter may depend on what you are measuring. If element you are measuring has a high impedance, then you should consider some sort of buffer. In the ADS1282, the standard clock is 4.096MHz, and the modulator runs off 1/4 that. Since the THS4522 has a GBW of about 95MHz, it might be more than needed. You may not need more than a passive filter, it will depend on what you're measuring.

    I think your question needs much more detail about the application for a complete answer.

    Joseph Wu

    If you are interested in the ADS1282, we do make an ADS1282EVM. It is an evaluation module that can be used to take data with a program called ADCPro. You should be able to find details, a user guide, and the schematic on the TI website.

  • 0 in reply to Joseph Wu
    Prodigy 150 points

    Hello Joseph,

    Thank you very much for your insight on this matter. I will try to make myself clearer by providing some added information:

    I want to make a datalogger to measure geophysical signals, small in amplitude.

    It basically consists of two dipolos both being separared by 10 metres, one North-South while the other one is East-West.

    These, in turn, will be placed under water, in a river or sea.

    I would like to be able to use the ADS1282 with the PGA 4 so that i might be able to obtain a nanovolt resolution.

    1.Do you believe this would be a useful solution in order to measure the nanovolts without getting any noise regarding the signals?

    2. Do you think the ADC would eventually cause drifts, offsets or noise in the signals that i intend to measure?

     3. Do you think it will be necessary to develop a low-noise preamplifier in order to measure the signal more accurately? 

    By the way, the intended measurements are second-by-second or minute-by-minute.

    Thanks in advance for your attention

    Best regards,

    Ivo

  • 0 in reply to Ivo Bernardo
    TI__Guru* 97835 points

    Ivo,

    Thanks, I have a clearer understanding of what you'd like to do.

    First, I think the ADS1282 is the part that you should use. It was designed for seismic applications, and I think it would have the AC performance you're probably looking for. However, you will still need to consider the power source because the battery is much lower than the needed supply.

    1. Lets consider the noise for the ADS1282 with a PGA of 4. It will vary by data rate, but let's use 1000SPS for now. According to Table 1 of the ADS1282 datasheet, the SNR is 123dB.

    The FSRrms = Vref/(2*sqrt2*PGA) so working to the NOISErms, this would be:

    123dB = (2.5/(2*sqrt2*PGA))/NOISErms

    NOISErms = (0.221)/1.41e6 = 156nVrms,

    which may or may not be close to what you want for your noise. You would be able to get 78nVrms of noise for 250SPS.

    2. I'm not sure what you are referring to on this question. I don't expect the noise to change through the course of time, and I think offset and gain are something that typically are constant for the ADC. Besides, gain and offset are things can can be calibrated. However, you should consider the reference and it's performance carefully. It will determine alot of how the system performs.

    3. As for the low-noise preamplifier, that may be determined what you are trying to measure and the level of noise you are after. In most hydrophone and geophone applications, I don't think a preamplifier is necessary.  Again, the ADS1282 has a low-noise PGA (5nV/rtHz) integrated into the device. However, if the above noise level outlined above is too high, then this may be solved with some additional gain.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 150 points

     

    Hello Joseph,

    This datogger is not meant for seismic purposes, but rather to measure potential differences between two electrodes, with an electrode separation of ten meters at the bottom of the sea. I would like to be able to measure ten nanovolts. Basically, I want to make a nanovoltmeter. What do you think it would be the best solution in my case?

    If I use the ADS1282 with PGA16, 250SPS I will get a Nrms of 31nVrms, so does this means that I won´t be able to measure signals below 31 nV?

    How can I amplify the input ADC signal without having to apply offset to the signal?

    Thank you very much for your attention once again.

    Best regards,

    Ivo

     

     

  • 0 in reply to Ivo Bernardo
    TI__Guru* 97835 points

    Ivo,

    First, I think I'd made a mistake on the last calculation. The reference on the ADS1282 is 5V, not 2.5V. So the noise is 2x worse.

    Now in your given example of the ADS1282, PGA=16, 250SPS, the noise will then be 62nVrms. This means when you get out the samples, the standard deviation of all the samples read will be 62nV. In this case it will be unlikely to measure signals smaller than this without averaging. With averaging, you could better the noise by sqrt(2) for every factor of 2 averaging. You could get much better performance by averaging samples, but you will slow the data rate that you get from the ADC.

    At the best case, when the input amplifier is in gain of 64, the ADS1282 still has 55nVrms noise at 250SPS.

    As for amplification, You might be able to get a gain stage before the ADS1282 to help. However if you do use something like two OPA211s for gain, you need to worry about several things. First, since the ADS1282 input amplifer is chopped, there is little effect from offset drift and 1/f noise.  If you build some gain stage in front, you need to consider the amount of noise that the op-amp and gain setting creates and you also need to consider the effect of offset drift.

    In my mind, the averaging is the better option if you can get away with the noise performance and the reduction of speed.

    Joseph Wu.