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ADS1246 Application sutiblity

Robert Pangrazio
Prodigy 60 points
Other Parts Discussed in Thread: ADS1246, ADS1230, ADS1232, ADS1247, ADS1248

I am wondering if anyone has any experience with the ADS1246. Specifically with the PGA feature of the device. I am using a loadcell as a sensor. At rest it has an output 0 volts from output + to output -. Each output should sit at 1/2 of the excitation or supply voltage, with for me means a nominal out put of 5v. With a full load on the load cell the will be a difference of 30mV between output- and output+.


I was thinking of using the differential input of this device, coupled with the PGA to try and eliminate the opamps I use. They are currently the most expensive part of the device.

Anyone have any thoughts on this?

 

Thanks

Bob

  • 0
    TI__Guru* 94285 points

    Bob,

    I think the ADS1246's PGA should work well for you.

    This PGA is a real amplifier, similar to an instrumentation amplifier. In some of our older components, PGAs were implemented through scaling of the sampling capacitors.

    This should work fine in your application. With a full-scale on the load cell of 30mV and a DC common-mode input of 2.5V, the input is well within the common-mode input range of the part. Since the PGA is similar to an instrumentation amplifier, the input current is extremely low (the specification table says a typical of 100pA differential current  - although this will change with the PGA gain and sampling rate).

    The ADS1246 also has two current DACs that can be used to drive the load cell (or many other types of sensors).

    While I don't know much about the specifics of your application, It should be work fine.

    I would note however, that if you are trying to build a weigh scale, that we do make the ADS1230 and ADS1232, which use similar PGAs. These two parts were specifically designed for the weigh scale market and there are reference designs in the form of evaluation modules and user guides available. If you are designing something similar, it may certainly help to download the user manuals for a quick read.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 60 points

    Your response was very helpful. 

     

    The only other consideration I have is that I am also reading from an accelerometer which has a voltage input of .8 to 2.5 volts. I had thought about adjusting the PGA depending on which input I was looking at. Do you see a problem with that idea?

  • 0 in reply to Robert Pangrazio
    TI__Guru* 94285 points

    I don't see too much of an issue with that. However with a range of .8 to 2.5V, you'll only be able to use a PGA=1 and you're at the end of your full scale with a 2.5V reference.

    I would also note that if you did end up using an ADS1247 (which does have a 2 channel multiplexer) the on-board reference is only 2.048V and a 2.5V input would be out of range.

    One last important thing. I noticed that I made a mistake on the previous post. The ADS1246 does not have the current DACs. This function is only available on the ADS1247 and ADS1248.

    Joseph Wu