• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 46 subscribers
  • Views 1386 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1248: Measurement of 4 wheatsone bridge precision load cells

Mackenzie Fairbanks
Prodigy 20 points
Part Number: ADS1248
Other Parts Discussed in Thread: ADS124S08, ADS1274, ADS1220

Hello, I am a university student working on a Capstone project involving the use of 4 load cells in a full wheatstone bridge configuration. I do not have much experience with analog design so please forgive me if my question is very simple or misguided. I plan on using the the 4 differential inputs of the ADC to simultaneously sample the 4 load cells. Without any prior signal conditioning or amplification, should this be signfigant enough to measure signals in the 10 mV range? Another question I have is about the 60-Hz attenuation, is the 20 SPS stated in the datasheet for the multiplexed signal? i.e., to get good 60-Hz rejection can each channel only be sampled at a maximum of 5 SPS? (20 / 4 channels)

Again I apologize if this question is very simple, or is misguided, this is my first real experience with any sort of analog design. Any help would be greatly appreciated!

  • 0
    TI__Guru* 97775 points
    Mackenzie,


    Yes, without any front end amplification, you should be able to measure signals in the 10mV range. The ADS1248 has programmable gain amplifier (PGA) integrated into the device. If you use a reference of 2.048V and a PGA gain of 128, then the input range would be ±16mV.

    However, I would consider using the ADS124S08 instead. If you're making a bridge measurement, then you probably want to tie the bridge excitation to the supply and then to the reference. The ADS1248 reference input only goes as high as AVDD-1V (4V if the supply is 5V). This might be a bit inconvenient if you were planning to set up the bridge measurement from the supply. The ADS124S08 has a similar PGA and the reference will not be limited.

    If you do decide to use either device, I'd get the user manual for the evaluation module and look at the schematic. It may give you ideas about what you want for the final circuit. Also, what sort of resolution do you want for your measurement? Read through the datasheet and look over the noise performance for whatever configuration you think you want. Then compare that to what you think your largest input signal will be.

    As for the second question, you are correct the 20 SPS is for any given ADC data read. If you need to sample four channels at this data rate, then the fastest that you'd be able to collect data on any given channel is 5 SPS. Note that neither device will simultaneously sample 4 channels. There is only one ADC, with a multiplexer in front. If you absolutely need to simultaneously sample 4 channels, then you'll either need to go to multiple devices or with something like the ADS1274, which has multiple ADCs.

    When you finally decide on something to use, post back and we can discuss the schematic.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 20 points

    I apologize, i misspoke when I said we had to simultaneously sample 4 channels. We are using 4 separate load cells to measure a signal in the frequency range of .1 to 20 Hz and wanted to use a sampling rate of at least 200Hz for each channel, so we do not need to sample them simultaneously, however we do need to sample them at least 200 S/s each, so it seems as neither of these devices can perform this task while effectively filtering out 60 Hz noise. Could you possibly recommend another device or configuration of devices capable of performing such a task? 

    Also as far as resolution goes, we wanted to achieve at minimum, 14 samples of noise free resolution, I see this could achieved at a sufficient sampling rate, but it seems there will be very little 60 Hz attenuation at that sampling rate.

  • 0 in reply to Mackenzie Fairbanks
    TI__Guru* 97775 points

    Mackenzie,


    As far as I know, we don't have any digital filters that are able to filter 60Hz noise but run faster than 60SPS. That's a little unusual for a filter response. As an alternative, you'd need to run the data rate much faster and then use some sort of notch filter on the input.

    The ADS1248, ADS1220, and ADS124S0x are all capable of running a data rate fast enough to capture 200SPS for 4 channels. Of those, I think the ADS124S0x has the best noise performance and should be able to meet 14 bits of noise free performance at a high enough data rate. However, none have 60Hz rejection for a faster data rate.


    Joseph Wu