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ADS1255: negative voltage measurement -150mV

Lukas Huber
Prodigy 10 points
Part Number: ADS1255
Other Parts Discussed in Thread: ADS1262, ADS1235, ADS125H02, ADS125H01

Hi there,

I am using the ADS1255 to measure an amplified signal from a load cell. 

Normally I would measure between 0-5V BUT some Loadcells output zero offset voltage to max -150mV (so -150mV to + 5V) and I need to measure this.

I am using your recommended basic connections like in your datasheet, but AIN0 is a single ended input and AIN1 is floating. 

My question now: Is it possible to measure this -150mV with this connection? I guess ADS1255 max. can measure 0V because it only is supplied with 0-5V ?!

Normally I set the channel on AIN0 as AINp Input and reference AINn is on AINCOM on GND. 

Is it possible to set the channel AINp on AINCOM on GND and AIN0 on AINn to measure this small negative voltage?

Thank you for your help.

Kind Regards,
Lukas

  • 0
    TI__Mastermind 34901 points

    Hi Lukas,

    The ADS1255 cannot measure a true negative voltage e.g. a signal below <AGND.

    Are you using a discrete amplifier in front of the ADC to amplify the load cell signal? Or does the load cell have an amplifier integrated into the device such that the output signal is already amplified? It might be possible to tune the amplifier output and level-shift the signal so this offset is actually within the range of the ADC.

    If you want to provide a system schematic or a block diagram that might also be helpful.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 10 points

    Hi Bryan,

    Thanks for your fast response. I'm always glad to hear from you.

    I am using a AD8226 amplifier right before the ADS1255 and yes you are right I could tune the amplifier output via the reference pin to get a positive voltage.

    Another solution would make me happier, because I don't have such a low voltage on my pcb which i could connect to the ref pin but it is what it is.

    Thanks for your input. I really dont know why I didn't think about the reference pin of the AD8226.

    Kind Regards,

    Lukas

  • 0 in reply to Lukas Huber
    TI__Mastermind 34901 points

    Hi Lukas,

    Yes, your conclusions are similar to what I was thinking.

    Typically the load cell output voltage is small and centered at or near the excitation voltage common-mode signal. For example, if you use an excitation voltage of 5V, then the load cell output is ~2.5V with no load applied. When you add a weight to the load cell, the output voltage increases, and this is what is measured by the ADC. In your case, the offset would be applied to the common-mode signal so the -150mV would be relative to the no-load voltage of ~2.5V and not AGND. If this is the case for your system then this would be okay for the ADS1255, since this would just be seen as 2.5V - 150mV = 2.35 V. I imagine this is what must be fed into the INA at least, as there would be no practical reason to have an INA if the INA input signal was already amplified. Therefore, I am sure you can use the VREF pin set to correct any out-of-range voltages that might otherwise be applied to the ADS1255.

    You might also consider some of our load cell / weigh scale ADCs in the future, such as the ADS1262 or ADS1235. These devices have integrated INAs that can take the place of the discrete INA in your system. Moreover, these INAs are performance-tuned for that specific ADC, so they tend to offer better performance compared to a discrete device. Again, something to consider for a future project if it is not possible to consider a new ADC for this current design.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 10 points

    Hi Bryan,

    Yes you are right, this is the case for my system. I'm using 12V excitation voltage, so 6V common mode signal and the INA amplifies 36mV at Full Scale. There is a nice link of AD8226 where the voltage, gain and so on can be added and the plot outputs the voltage (https://tools.analog.com/en/diamond/#difL=-0.01&difR=0.01&difSl=0.01&gain=1000&l=2.5&pr=AD8226&r=2.5&sl=2.5&tab=2&ty=1&vn=0&vp=5&vr=0). So in case there is a low voltage (and not GND) on VREF the ADS1255 could measure the -150mV because its then amplified as positive output and not negative.

    The ADS1235 could be a possibility in a future solution. At the moment I am using the combination of AD8226 and ADS1255 in the prototype PCB. 

    Thanks for your help.

    Kind Regards,
    Lukas

  • 0 in reply to Lukas Huber
    TI__Mastermind 34901 points

    Hi Lukas,

    Thanks for the additional information. Using a 12V excitation definitely requires some sort of scaling circuit with the ADS1255, so what you are doing makes sense. Fortunately it seems like tuning the VREF voltage will enable you to provide the correct input signals to the ADC.

    One other point: I would also recommend looking at the ADS125H01 or ADS125H02, which are high-voltage, low noise delta-sigma ADCs. These devices do require a high voltage supply (>5V), but they can then measure the 12V excited bridge directly without the need for a level-shifting circuit. You would still need to scale down the VREF voltage if you want to use a ratiometric reference configuration, as the VREF pins are only 5V-tolerant, but this solution would be a bit simpler for what you are trying to accomplish. Please keep this in mind for future projects.

    If you have any additional questions about this specific thread, please let me know. Or, if you run into new challenges with the ADS1255 design, please start a new thread. Thanks!

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 10 points

    Hi Bryan,

    I just changed on the AD8226 VS- from GND to -5V so that it can output the small negative voltage from the Load Cell to the ADC and by my surprise the ADS1255 can measure and display the -80mV (voltage verified with multimeter). I guess it will display only to GND-0,1V because of specs in the datasheet. So for now for prototype I don't need to make too complicate changes and dont have to change VREF. 

    Ok thanks, I will take a look into the ADS125H01/02.

    Kind Regards,

    Lukas

  • 0 in reply to Lukas Huber
    TI__Mastermind 34901 points

    Hi Lukas,

    Yes, the ADS1255 is capable of measuring small negative voltage when the buffer is disabled. It is not really meant for this purpose, just to give a little more headroom on the ground connection, but either way it seems like your system is working as you need it to.

    Let me know if anything else comes up.

    -Bryan