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XTR

Other Parts Discussed in Thread: XTR108, XTR101, PGA309, XTR117

Hello,

I have a need to connect either a bridge sensor (mV-Out) or a voltage out sensor ( 0.5 to 4.5 V ) to a 4-20 transmitter and have the ability to adjust the offset and span for custom sensor range use.  Both sensors operate from 5V.  Obviously, one sensor will have a common mode voltage and the other is only single-ended.  Both of these sensors are already thermally compensated.  I'm wondering which XTR would be best knowing of the offset and span needs.  Offset adjust could be quite large ( 50-75% of full scale ).  I see the XTR108 offers a lot of features that look appropriate; however, there is likely some over-kill.  Based on this limited information, are there any suggestions on whether I should use an XTR like the XTR101 or similar and then add opamp circuits separately for offsets and scaling or just go with a more complex XTR.

  • Hi Steve,

    I'm still not entirely clear on what you require. If you need to be able to switch between these sensors and adjust span and offset on the fly, then you will definitely want to use the XTR108 as this is the only programmable, 2-wire XTR we offer. It also includes an input MUX for this purpose. XTRs have either an instrumentation input (for measuring bridges) or a single ended input. A bridge sensor will not be able to drive a single ended input XTR without a bridge amplifier in the middle, and a single ended sensor will not work with an instrumentation input without additional circuitry to satisfy the common-mode requirements. As such I believe the XTR108 is your best choice based on the information you have provided.

    Regards,
    Zak Kaye
  • Hi Zak,

    I have not fully read the XTR108 datasheet but it looks like the PGA minimum gain is 6.25. If I connect a single-ended voltage that has an output of 0.5 to 4.5 volts, how can I adjust the XTR108 to get 4mA for 0.5-volts in and 20mA for 4.5 volts in? I see how it would work for a bridge mV output sensor, but can't see how you would apply it for a Vdc output sensor. This is why I'm thinking that I might need a different XTR part and use an external amplifier in between. Please let me know your thoughts.

    Thanks,
    -Steve
  • Hi Steve,
    You are correct that the XTR108's PGA has a minimum gain of 6.25. In order to interface an external 0.5-4.5V source to the XTR108 you would need an attenuator on the input. If input loading is an issue for your application, you may need an additional op amp, as the uncommitted amplifier in the XTR108 has a maximum common mode voltage of 3.5V.

    You'd need to select that attenuation to give you the required 16mA span when combined with the PGA gain and Rvi value. This would be a one-time calibration at build time, unfortunately, as the XTR108 does not have fine gain control aside from the excitation current DACs, which would not be useful in this case.
  • Thanks Alex. I'll spend some time thinking about this. As mentioned earlier, I'm trying to come up with a common circuit that can be used with a bridge sensor or a compensated Vdc sensor. If I attenuate the Vdc output and use an either or summing network into the XTR108 then this might work. I'm comparing this approach to doing something similar to the SLAU526 application note using the PGA309 and XTR117. I need to pre-condition either the Vdc sensor output or the mV bridge output to get them similar for use with the XTR chip. The idea is to use the common board with either sensor to minimize having two separate circuit boards for these two types of sensor outputs.

    Do you have any comments on these ideas?

    Thanks,
    -Steve
  • Steve,

    I think as long as you can pre-condition your Vdc voltage input - effectively taking care of your span calibration for that input - there shouldn't be any issues with reusing the XTR108 as a 4-20mA transmitter. The only caveat to all of this is that without using the excitation current sources in the XTR108, there's much more limited scope for gain calibration - that would have to be done externally. (I went into a little more detail in my response to your other question here.)