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ADS1115: ADS1115 input impedance matching

as
Intellectual 625 points
Part Number: ADS1115
Other Parts Discussed in Thread: OPA2314, ADS1248, LMP7721, ADC122S021

Hello Im reading buffered (zero impedance theoretically) output signal coming from an op-amp with ADS1115. But do I have to put series resistor or capacitor to match ADS1115 input impedance or sampling capacitor?

  • 0
    TI__Guru* 93465 points
    ism,


    No, a series resistor or capacitor for impedance matching is not necessary for measurement with the ADS1115.

    However, ADCs often have some amount of input filtering for anti-aliasing and noise rejection. A brief discussion on filtering can be found here:

    e2e.ti.com/.../three-guidelines-for-designing-anti-aliasing-filters


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 625 points
    so ADS1115 works well with impedances near by zero. I already have active low pass filter before ADC in my setup, I think it's will do the job you have said.
  • 0 in reply to Joseph Wu
    Intellectual 625 points
    I still don't get the point why there is"a resistor/capacitor pair that drives the ADC’s input" www.ti.com/.../slyt626.pdf althought they already used the filter before it?
  • 0 in reply to as
    TI__Guru* 93465 points
    ism,


    In most systems, we'll put in some sort of basic RC filtering at the input of the ADC. You're correct that theres already an LPF at the front end of the source. It might seem that the RC filtering directly at the input of the ADC is redundant. However, it still reduces any additional high-frequency noise from the buffer circuit and it also gives a high frequency path to remove any EMI and RFI from the circuit.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 625 points
    I understand the part of filter function in here but Im still curious about the input impedance . The latest LPF can be buffered or not?
  • 0 in reply to as
    TI__Guru* 93465 points
    ism,


    Are you talking about the RC filtering at the front end? There's no need to buffer it as long as keep the series R to below 10kΩ. In general, the input impedance to the device is rather high, however, the series R should still be kept low to maintain the sampling integrity.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 625 points
    Hi Joseph, Im asking about this sentence still ""a resistor/capacitor pair that drives the ADC’s input". It pointing out driving the ADC not filtering. Not written as a low pass filter, written as resistor/capacitor pair. And it comes after the op-amp's output which outputs very low impedance.
  • 0 in reply to as
    TI__Guru* 93465 points

    ism,


    The RC at the front end of the ADC has two main functions. First, it acts as input filtering. It is used both for anti aliasing and for high frequency noise rejection. Second the capacitance at the input is also used as a source of charge. The input of the ADC is capacitively sampled, and the external capacitor at the front end acts as a storage element from which the sampling capacitor can draw charge.

    I think the way the application note is worded is a bit awkward. It doesn't really explain much about the op-amp and the RC at the front end. The op-amp is used as a buffer, but with the Butterworth filter it already has an OPA2314 as the output, and it doesn't require a buffer. Normally you would need a buffer if the output impedance is too high. Additionally, with a low output impedance, you don't need to do any kind of


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 93465 points

    ism,


    I just noticed that my response was cut off.

    The point I was making at the end was that with low output impedance, you should need to do any buffering or impedance matching.

    Regardless, normally we do have some sort of RC filtering on the front end for the reasons I explained at the beginning of the last post. It might not be as useful for 16-bit converters, but for our 24-bit converters, it is common to see them in applications. If you look at something like the ADS1248 Evaluation module, there are differential and common-mode input filtering at the front end.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 625 points
    another question in here, www.ti.com/.../sbaa266.pdf , they added same RC filter before ADC.

    "The output of the LMP7721 is connected to one of the inputs of the ADC122S021 SAR ADC. The
    sampling capacitor of the ADC is 33pF and the external capacitor placed next to the pin of the ADC
    should be 10 times larger, or 330pF. A small resistor of 20Ω is added in series to isolate the capacitor
    from the LMP7721."

    The ADS1115 haven't got sampling capacitor because its sigma delta ADC, right?
  • 0 in reply to as
    TI__Guru* 93465 points
    ism,


    Actually, the ADS1115 also uses a sampling capacitor. In our implementation, the modulator of the delta sigma ADC uses a sampling capacitor to sample the input and then push the charge into an integrator.

    However, the sampling cap significantly smaller than that of a SAR. In the SAR, the input is sampled into a network of capacitors that are scaled through factors of 1/2 with the number of elements determined by the number of bits for the ADC. For linearity, the matching of these elements is critical, and therefore are a bit larger in layout. For the delta sigma, the input sampling cap is usually a single capacitor. I would note that there are times that there are small networks of input sampling capacitors and we will change the size to enable different gains, but the input capacitance of a delta sigma is always smaller than a SAR.


    Joseph Wu