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XTR110 problem with 0V input

Joerg Neuhaeusler
Prodigy 40 points
Other Parts Discussed in Thread: XTR110, LM358, ISO122, TINA-TI, ISO124, LM7705, NE555, MAX232, LMC7660

Hello Collin,

Forum post referenced:  http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/135705/913927.aspx

I experienced the same problems with input ranges beginning with 0V

and output ranges beginning with 4mA.

But i need the complete 0-5V and 4-20mA range.

My source starts with 6mV but when i connect it to pin 5 it will be pulled up to 0,2V.

Over ~ 1 V all work fine and it is pretty linear. I use the circuit in FIGURE 1 and switch PIN 4 with PIN 5.

I read now in your post that i need a low-impedance driver for my input.

Can you please provide me with a tip how i can make from my (not so low impedance) circuit a

working  low-impedance driver that is able to "short" PIN 5 to the 6mV? Maybe there is a IC that did the job?.

Best regards

Joerg

  • 0
    TI__Guru 62955 points

    Hi Jeorg,

    I started a new thread so people can find your particular inquiry easier.

    It sounds like you need a buffer amplifier connected between your input source and the input to the XTR110 circuit.  Simply connect an amplifier as a buffer (short OUT to IN-)  and insert it between your source and the XTR.  An example of this is shown in Figure 8 in the upper left side of the schematic where A4 is used to buffer the input to the XTR.

    You can use the basic simulation model I've attached below to experiment with the operation of the XTR110.1665.XTR110.TSC

  • 0 in reply to Collin Wells
    Prodigy 40 points

    Hello Collin,

    thanks for fast reply.

    Yes, I already tried this with OP177 opamp but its exactly the same.

    My source wont´t be pulled up in this case but the output of the buffer instead.

    My sources output comes from an opamp too (LM358) with a parallel resistor of 10K ohms.

    But thanks for the tip looking at FIGURE 8. I discovered in FIGURE 9 the ISO122.

    I hope to find a seller to get one and give it a try (without the Isolated Power Supply).

    Which tool is required to open the .tsc file?

    Many thanks

    Regards

    Joerg

    -oops, I´ve pressed the wrong button, sry!

  • 0 in reply to Joerg Neuhaeusler
    TI__Guru 62955 points

    Hello Joerg,

    You will need to download TINA-TI from the link below:

    http://www.ti.com/tool/tina-ti

    There might be something going on here besides a simple interaction of impedances like I originally thought if you've already tried a buffer.  Perhaps you could share a full schematic so we could properly analyze your circuit and try to better understand what may be happening.

    The OPA will still have an output swing limitation which could be what you're seeing, but let's wait to see the schematic before we spend too much time speculating.  

    Just to restate the issue, when you connect your input signal to pin 5 of the XTR110 the input zero-level changes from 0.06V to 0.2V.  This occurs with or without a buffer between the input and the XTR.  Is that correct?

    I'm not sure what you mean regarding the ISO without the isolated power supply but be sure to read through the ISO datasheet and application schematics to understand how to properly operate it.

    Regards,
    Collin Wells 

  • 0 in reply to Collin Wells
    Prodigy 40 points

    Hello Collin,

    nice program, thanks.



    My lowest output is 0.006 V.

    Without source I measure a voltage on PIN 5 of ca. 1.34V.

    If I shorten PIN 5 to GROUND, I have a perfect 4mA output,

    so the Buffer/Driver should be able to do this too. For that reason, I think

    TI won´t use a ISO122 in their example if that did not work properly.

    From FIGURE 9 the only thing i use in the next try is the ISO122 or the ISO124

    because it seems it has a tricky output.

    I don´t need to isolate because all (source, XTR110, receiver) is powered by the

    same power supply.

    So, if you have any ideas (cheaper ideas), let me know please.

    Regards

    Joerg

  • 0 in reply to Joerg Neuhaeusler
    TI__Guru 62955 points

    Hello,

    You've basically come to the root of the issue with your measuring.  You have to sink the 1.34V through the 10k internal resistor of the XTR to GND through the output stage of your amplifier to get the circuit to work.  That equates to 134uA which will cause the output of an op-amp to increase away from GND if it is powered from a single-ended power supply.  Please connect the V- terminal of the OPA to a negative voltage and see if the circuit functions.  If you need to use a single-ended supply then you'll need to use an rail-to-rail output amplifier with very good swing to the negative rail and you'll have to accept the small error that will occur.  Also, if you don't have a negative rail available, consider using the LM7705 which creates a small negative voltage (~ -0.23V) that can bias the amplifier in a state where the output can reach "true zero".

  • 0 in reply to Collin Wells
    Prodigy 40 points

    Hello Collin,

    thanks. That are all good thoughts. I need a while to think about these solutions.
    but you open options. Thanks therefor.
    So, now is weekend. I whish you a nice one.

    -------------------------------------------------------------------

    Today is monday and I'm back.

    Yes with a negative supply voltage my buffer is able to reach 0V with PIN 5 connected.

    Next i try to generate a negative voltage with MAX232, NE555, LMC7660 

    or (hard to get it in germany) LM7705.

    Best regards

    Joerg