• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 48 subscribers
  • Views 2568 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.

Load cell conditioning circuit + ADC

Mitch English
Prodigy 20 points

Other Parts Discussed in Thread: INA128, ADS8320, ADS1259, ADS1120, ADS1262, ADS1148, ADS1158, ADS1252

I am looking for a solution for the design of a load cell conditioning circuit + ADC for a test machine. Ideally this circuit would have a bandwidth of 200+ Hz and decent resolution (14 bits). 

After some research, my understanding is that INA128, powered by a dual supply, might a good fit, due to higher CMMR

In order to keep the system ratiometric, I would like to the ADC to reference the same supply as the load cell bridge. I’m thinking a discrete 16bit SAR ADC, perhaps something similar to an ADS8320, would fit. Since its analog reference voltage can be no grater than 5V, the load cell bridge would then be powered by a +5V single supply.

Now for my problem – what is the preferred means of conditioning signal output of the INA128 to the ADC?

I am having difficultly since the output is not rail-to-rail, but would like to maximize the signal to the ADC.

I could power the INA128 from a +/- 5V supply, which would result in a minimum output range of +/- 3.6V (per shec sheet min). I would then need to offset and amplify this to achieve 0-5V for the ADC. But what if the output range is higher, say +/- 4.1 (per the spec sheet typ)? Would I also need to clamp the voltage?

Thanks in advance!

Mitch.

  • 0
    TI__Mastermind 44760 points

    Hi Mitch,

    Welcome to the TI E2E Forums!

    Typically, the ADC input will be differential so when using an INA it common to connect:

    1. The INA's output to the ADC's positive input
    2. and the INA's "REF" input to the ADC's negative input.
    3. Apply an appropriate common-mode voltage to both the INA "REF" in and ADC negative input.

    Also, it is common to use a higher-resolution delta sigma ADC for measuring load cell inputs (Look at ADS1120, ADS1148, ADS1259, or ADS1262).
    Using a higher-resolution ADC solves the problem with limited input range and makes the signal conditioning easier because you may not need to offset and amplify the signal to match the 0-5V ADC input range. For example, if you were to use a 16-bit ADC but only use half of the input range, you would still achieve about 15-bits of resolution (using 1/4th the input range provides 14-bits of resolution, etc... - so higher resolution can make matching the ADC's input range unnecessary) .

    Many delta sigma ADCs have an integrated PGA which would remove the need for the INA, but devices like the ADS1252, ADS1158, or ADS1259 would still require an input buffer or amplifier stage.

    It's possible to use a SAR converter for this application as well, but it would be more important to match the input signal to the SAR's input range... NOTE: Another way of achieving the matched signal to ADC input range with a SAR converter is to simply change the SAR's reference voltage, because the reference sets the full-scale range.

    Hope that helps,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 20 points
    Hi Chris,

    Thanks for the feedback, much appreciated!

    I never thought to connect the INA “REF” input to the ADC’s negative input... that makes sense. Since I only require 14 bits resolution, even if I only use half the range of a 16bit ADC I will still be fine.

    The only part I’m still not clear on is how to select an appropriate common-mode voltage, and how to ensure the output of the INA does not drop below 0V.

    i.e. to continue my example scenario, let’s say the INA is powered by +/- 5V, so the minimum output range would be +/- 3.6V.

    I believe an appropriate common-mode voltage would be 1.8V, in order to maximize the output range of the INA (0 to 3.6V). I’ll also feed that 1.8V to the ADC negative input. But, it’s still possible that the INA’s output voltage could swing below 0V, even with the 1.8V offset. Would that not damage the ADC?

    Thanks,
    Mitch.
  • 0 in reply to Mitch English
    TI__Mastermind 25865 points

    Hello Mitch,

    We recently released a Vcm vs. Vout tool for instrumentation amplifiers and it includes the INA128. Here is the link:

    www.ti.com/.../sboc452

    I think you're going to be the first one to use it, so please let us know what you think! FYI, it is based on a series of articles that you may find interesting:


    www.edn.com/.../Instrumentation-amplifier-VCM-vs-VOUT-plots--Part-1

    www.edn.com/.../Instrumentation-amplifier-VCM-vs--VOUT-plots--part-2

    www.edn.com/.../Instrumentation-amplifier-VCM-vs--VOUT-plots--Part-3