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ADS1243: Signal getting attenuated. What is the correct data rate

Part Number: ADS1243
Other Parts Discussed in Thread: ADS124S08, ADS1235, ADS124S06

Hello,

I am using a function generator to give sine wave input of 0-100Vp and trying to see how the signal reconstruction happens at the ADC output when I am reading a particular channel at 9Hz frequency. For signal frequency of 0.5Hz and 1Hz, the signal construction looks fairly ok, but when I see the waveforms for 2Hz,3Hz and 4Hz,I see some attenuation in peaks of some of the cycles. I would like to know what exactly is the reason behind this. Is my data rate( 9Hz) too slow for 2-4Hz input signal or is it a consequence of sinc filter within the ADC. If that is the case , what shall be the data rate required to re-construct 2-4Hz signal. This experiment I am doing to simulate my actual system scenario where my sensor output can vary at 3Hz or 4Hz.Attached are the waveforms for 0.5Hz, 1Hz, 2Hz ,3Hz and 4Hz.

  • Hi Siddharth,

    The phenomenon that you are seeing is a result of nyquist sampling theorem, where the frequency of the sampling must be at least 2x the frequency of the signal being measured in order to accurately measure the frequency of the input signal. While 2x is the absolute minimum, sampling at greater multiples (5x, 10x, etc) may be necessary in order to reproduce the input signal accurately. 

    If you have not seen it, our TI precision lab series on ADCs may be helpful:

  • Hi Alex,

    Thanks for your suggestion. I was also expecting the same limitation. Can you propose me some TI parts which can support my requirements?

    Basically, my requirements are as follows:

    1.I have to read 3 load cell sensors  (differential output; 6 channels) and I expect the max frequency of the input signal to be around 4-5Hz.

    2.My full scale output of the load cell is 1.25mV/V.I am currently giving excitation voltage of 5V which means my FSO is 6.25mV. So with 1.25V as Vref like what we have in ADS1243, the range of measurement shall be +1.25V to -1.25V which is optimum for me.

    3. Digital SPI interface with 3V3 compatibility is preferable.

    I am also exploring the other ADCs from TI, but your suggestion will be very helpful.

    Thanks.

     

  • Hi Siddharth,

    If you haven't seen it yet, I recommend taking a look at the ADS124S08 family of devices. I've sent an email to a colleague as well who may be able to recommend additional devices. 

  • Hi Siddharth,

    One other option for this type of application could be the ADS1235, which is a much newer ADC compared to ADS1243. The ADS1235 is designed specifically for high precision resistive bridge measurements.

    The ADS1235 can measure up to 3x load cells using a ratiometric reference configuration as shown in the image below (the 4x signal lines from the other 2x bridges would be connected to AIN0 - AIN3, while all Sense lines would connect to REFP0 and REFN0). This reference configuration offers the best performance since any variation in the excitation voltage shows up equally in the input signal as well as the reference, and therefore cancels out. This eliminates the need for a discrete reference voltage as well.

    The ADS1235 has gain options of 1, 64, and 128, so you can use the 128 V/V option to gain up your 6.25mV input signal to the maximum amount possible. At G =128 and very low data rates (<20 SPS), you can expect to achieve <20nV(RMS) noise performance per Table 1 in the ADS1235 datasheet.

    Also, the ADS1235 can support 2.7V to 5V DVDD, though the image below shows a 5V DVDD connection. So this meets your 3.3V requirement as well.

    Let me know if you have any further questions about this device.

    -Bryan

  • Thanks Brian. I will go through the datasheet in detail. Also, i will see few other devices from TI to compare. 

    I will let you know my selection preference and would like to get an approval from your side.

    Thanks,

    Siddharth

  • Hi Alex,

    I have decided to go with ADS124S08 as it suits my requirement perfectly. I looked at the datasheet and eval kit as well and have the following queries:

    1. The load cell excitation voltage is 5V in my design and the sensitivity is 1.25mV/V. So my max voltage swing with gain of 128x shall be +800mV to -800mV.

    Hence I am planning to use an external reference of 1.25V. Also, AVDD as 3.3V. Let me know if this is ok.

    2. In the eval kit the series resistor of 47E for the SCLK and DIN lines are placed near the ADC chip. Do you think it needs to placed near the MCU as the source of the signal is MCU and not ADC. I believe the intent of these series resistors is to limit any current and ringing. Let me know your thoughts.

    3. In the datsheet , it is suggested not to keep the GPIO pins floating if they are configured as input or output. If I configure them as Analog inputs, is it ok to keep them floating as I am not using them.

    4. Can I keep the AINCOM pin floating since I am not using it or I can connect it to ground. Please suggest.

    Thanks,

    Siddharth

  • Hi Siddharth,

    Let me help answer these questions:

    1. Usually in a load cell application the excitation voltage is used as the reference voltage. This reference configuration is called a ratiometric reference and is shown in Figure 84 from my last post. Note how the Sen+ and Sen- lines connect to REFP0 and REFN0. This would be the recommended configuration for this system, as any variation in the excitation voltage will show up in the signal and the reference, and be cancelled out.
    2. You are correct, these resistors are to limit ringing / overshoot on the communication lines. They should be placed close to the ADC as implemented on the EVM.
    3. The ADS124S08 datasheet says the following for unused GPIO pins:"For unused GPIO pins on the ADS124S06, leave the GPIOCON register set to the default register values and connect these GPIO pins in the same manner as for an unused analog input". Setting the GPIOCON register as such configured the pins as analog inputs, which you can leave floating
    4. AINCOM is like any other analog input, it could have been called AIN12. So please follow the datasheet guidelines for unused analog inputs

    -Bryan

  • Hello Bryan,

    Thanks for your valuable inputs. 

    For poin#1: I understand from where you are coming from. But if I use AVDD as Vref which is 5V, I am only using 1/6 of the full scale. Instead, If I use 1.25V stable external Voltage reference, the utilization of the scale is much better.

    For point#2: I can place the resistors near ADC as in the eval kit. But I feel that specially for SCLK and DIN signals, since the source is MCU and not the ADC, the resistors shall be placed near MCU. Can you give re-thought.

    Thanks,

    Siddharth

  • Hi Siddharth,

    Please see my responses to your questions in the split thread here: https://e2e.ti.com/support/data-converters/f/73/t/953524

    I asked a few questions to you in my reply, so please let me know.

    -Bryan