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Digital filter on ADS1248

Other Parts Discussed in Thread: ADS1248

Hi everybody

I have a basic questions about digital filter on ADS1248:

I want to make a board that measure type K and J of thermocouples "TC" on 4 channel, sample rate is 5 sample/second. the questions are:

1-can the internal digital filter remove for example 50Hz noise? i want to know more about this digital filter.

2-Do we need an external filter? if we need what type of filter do you recommend?

"it is important for me that f-3db is less than 50Hz"

best regard

Mahdi

  • Mahdi,


    At a data rate of 5SPS, the digital filter will reject 50Hz (and 60Hz) noise. You can see the filter frequency response at 5SPS in Figure 54, on page 27 of the datasheet.

    In general, you really don't need much input filtering. There is a brief discussion about input filtering in one of the applications reports on RTD ratiometric measurements with the ADS1248 at the following:

    http://www.ti.com/lit/an/sbaa201/sbaa201.pdf

    Also, I'd look at the ADS1248 evaluation board schematic. It has some basic filtering on the input pins and it's capable of making accurate measurements. The EVM user guide can be found here:

    http://www.ti.com/lit/ug/sbau142b/sbau142b.pdf



    Joseph Wu

  • Hi Joseph

    thank you a lot for your answer.

    I have 2 more questions on this topic:

    1- Is there any relationship between this 2 filters (internal digital and external analog RC filters)?

    2- Does it have any benefit if we use external RC filters? I want to make an industrial board

    best regard

    Mahdi

  • Can you display the thermocouple interface with ADS1248 ?

    if you want better performance, the default CMMR is PGA1 - 90 dB, PGA32 - 125 dB, PSRR is 135 dB, 

    You can choose additional high performance amplifier.

    Additional RC filter is OK.

  • Mahdi,

    The external filter doesn't really affect the performance of the digital filter. The two are cascaded so you would get the effect of both.

    I would use some modest filtering on your board. As I mentioned earlier, I'd look at the filtering on the EVM. The benefit would be two things. First, it acts to remove higher frequency noise (as a filter would). Second, the capacitor acts as a small reservoir of charge so that the input sampling of the ADC has charge to easily draw from.

    Joseph Wu

  • Gaven

    One presentation to look at would be the one in the following link. It describes different temperature measurement applications including temperature measurement.

    http://www.ti.com/lit/an/sbaa180/sbaa180.pdf

    Joseph Wu

  • Hi Joseph

    thank you a lot,

    best regard

    Mahdi