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ADS131M08: DAQ measurement

Pradeep Raaj K
Prodigy 10 points
Part Number: ADS131M08
Other Parts Discussed in Thread: TIDA-010036

Hi there,

We are designing a particular DAQ system with 48 differential channel inputs. The boards are separated into 3 different boards with a microcontroller on each board. The timing needed to be synchronized. The 16 channel in each board needed to be sampled at 60Hz per channel. The impedance of the sensor is around 25 ohms. The maximum voltage needed to be measured is ±0.5V. The minimum voltage needed to be measured is 2uV.

1.Whether any amplification is needed at the front?

2.Is it possible to achieve this with 2 ADS131M08 per board?

 

  • 0
    TI__Mastermind 18900 points

    Hi Pradeep,

    Given that it sounds like you're measuring power supply mains, or a frequencies that is notoriously around 50/60Hz power supply frequency, I can confidently say that the ADS131M0x series is made to do that. I'm going to answer the questions a bit out of order.

    Pradeep Raaj K said:
    2.Is it possible to achieve this with 2 ADS131M08 per board?

    I would definitely check out the datasheet section 9.1.5 Multiple Device Configuration. More details about how to do this below and in the datasheet section.

    Pradeep Raaj K said:
    The 16 channel in each board needed to be sampled at 60Hz per channel.

    I will note that given the digital filter limits your bandwidth, the device data rate will be sampling a bit faster f_data = 60Hz, but if you use the highest OSR to get f_data = 250SPS (or Hz) the -3dB bandwidth should end up around 62.5Hz. The Filter specifications are in the datasheet electrical characteristics sections and in the body of the datasheet within section 8.3.7.2 Digital Filter Characteristic (figure 8-5) for more info.

    This might put some stress on the MCU as the device is really designed to be read as soon as data as ready. If you have a pause in data collection, there's a section in the datasheet the explains how the FIFO might fill up and data might become corrupted. More info can be found in section: 8.5.1.9.1 Collecting Data for the First Time or After a Pause in Data Collection (you'll definitely want to pick the option to quickly read twice so you don't have to resynchronize all the devices).

    Pradeep Raaj K said:
    The timing needed to be synchronized.

    Realistically, you can get synchronized within 1 t_CLKIN period because of how the ADS131M0x synchronization is defined. If you quickly toggle SYNC once for all ADCs, some might be reset and need to resettle where some will continue converting as the SYNC really isn't a conversion start pin. Its just checking clock edges to see if they match up. So, if you force a reset on all ADCs, they might happen within 1 t_CLKIN period of one and other. Section 8.5.2 Synchronization has you covered in the datasheet.

    There is also some phase offset registers if you want to get the channels as close as possible so that should help if the goal of "synchronization" in the long term. I will also reiterate this is a simultaneously sampling ADC so we have 8 ADCs (1 for each channel) that is capturing data simultaneously.

    Let me answer your other questions more directly.

    Pradeep Raaj K said:
    1.Whether any amplification is needed at the front?
    Pradeep Raaj K said:
    The minimum voltage needed to be measured is 2uV.

    Setting FSR to +/-600mV (G=2), which is setting the PGA that is integrated into the ADS131M08, sounds like a good fit to me. G = 2 gives you 1.69uVRMS noise which is below the smallest voltage you want to sense. Calibration for gain and offset is mandatory but you'll definitely be able to measure those levels.

    The design guide of this TI Reference Design shows some data that confirms this: https://www.ti.com/tool/TIDA-010036  

    But yeah, seems like a good application to for the ADS131M0x to me.

    Best,

    -Cole

  • 0 in reply to Cole Macias
    Prodigy 10 points

    Thanks for your valuable comments. The application is for measurement of heat flux measurement.

    Regarding this,

    I will note that given the digital filter limits your bandwidth, the device data rate will be sampling a bit faster f_data = 60Hz, but if you use the highest OSR to get f_data = 250SPS (or Hz) the -3dB bandwidth should end up around 62.5Hz. The Filter specifications are in the datasheet electrical characteristics sections and in the body of the datasheet within section 8.3.7.2 Digital Filter Characteristic (figure 8-5) for more info.

    I would like to clarify this: 

    Considering only one ADC, 8 channels are used at the same time, the sampling frequency requirement is 60Hz. I'm hoping that the DATA RATE Table:7.1, represents the output data rate.In that case we need to look at the second row (i.e) f_data=500Hz , which is near to the 60x8=480Hz . Or does it represent per channel sampling rate in this simultaneous sampling ADC? Kindly correct me if I'm wrong.

    Regarding the selection of microcontroller can you suggest a microcontroller from TI which suits our purpose? We though of a microcontroller which has the following specification:

    32-Bit Cortex-M0+, with CPU frequency 64MHz

    64Kb of flash and 18Kb of SRAM

    Two SPIs (32 Mbit/s) with 4- to 16-bit programmable bitframe, one multiplexed with I2S interface.

    To power the Components from single power source(5V), to separate the analog and digital power source of the IC , Is it okay to proceed with two LDO's with Pi filters and Ferrite bead will work or do I need to have a isolated DC-DC for the digital side? In TIDA-010036 a Ferrite bead of 10 ohm(R8) is used in the schematic, something like that will work?

  • +1 in reply to Pradeep Raaj K
    TI__Mastermind 18900 points

    Hi Pradeep,

    Pradeep Raaj K said:
    Or does it represent per channel sampling rate in this simultaneous sampling ADC? Kindly correct me if I'm wrong.

    Yeah this can get a little confusing for Delta-sigma ADCs but the whole “Nyquist/2 and sampling frequency” is related to f_mod or the frequency that the modulator samples. This is on a similar magnitude to CLKIN or ~MHz. So data rate is the result of changing the filter associated with the modulator but stream.

    More plainly, 250SPS is the data rate, ~MHz is the sample rate.

    More info can be found here:

    https://training.ti.com/ti-precision-labs-sar-delta-sigma-basic-operation?context=1139747-1140267-1128375-1147914-1147913 

    Pradeep Raaj K said:
    Regarding the selection of microcontroller can you suggest a microcontroller from TI which suits our purpose?

    Unfortunately, I am no expert in MCUs. I suggest finding something from TI here: https://www.ti.com/microcontrollers-mcus-processors/microcontrollers/overview.html

    And then with a part number you can make another post on E2E and see if you can get some clarification. In general though, those specs look fine from the ADC perspective.

    Pradeep Raaj K said:
    s it okay to proceed with two LDO's with Pi filters and Ferrite bead will work or do I need to have a isolated DC-DC for the digital side?

    Yeah that sounds great to me! That’s what the reference designs are for, to show a more wholistic view of how to design with the IC. 

    Best,

    -Cole