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XTR115 impedance

Paul Peterson
Prodigy 160 points

Other Parts Discussed in Thread: XTR115

I performed a quick test yesterday with the transmitter that has the XTR115 in it.  The Ra is the load resistor in series with the XTR, and was 5000ohms.  The current loop runs through both.  An AC signal is applied as shown below.   Vb is the voltage drop across the transmitters current loop circuit.(in the case of the 2 wire transmitter it is also the power to the transmitter).  The transmitter is powered from the 5V regulated power out of the XTR115.

Since the transmitter is a 2 wire device, it runs off the current from the current loop. So, the transmitter current consumption must be less than 4mA, since that is the lowest signal level the transmitter can signal.  So, when the transmitter is signaling 4mA, the NPN transistor is on, but not much.  My guess is somewhere in the 1mA range. 

 So I performed a series of tests at different DC current signaling levels to get the NPN turned on more.  Here are the results. In the 11mA results, the NPN is supplying at least 7mA of current, so it’s on pretty good.  This was as high as I could go, as the resultant voltage drop across the 5000 ohm resistor lowers the voltage at the transmitter to the point where the transmitter quits working.

 Zm is an approximation of the impedance magitute. Zm = R/Va x Vb. 

As you can see ZM is big numbers.  This is where our design is 131 ohms which is way below the minimum. 

I was surprised that the impedance didn’t come down as the NPN transistor was turned on more.  It appears to me that Zm is independent of the DC current through the NPN.  So, where is the impedance coming from?  My guess is it is the XTR115….but how??? 

  • 0
    TI__Guru 64875 points

    Hi Paul,

    Your results didn't seem to get attached, would you try attaching them again? 

    That said, I'm not sure I need to see them to figure out what's going wrong.  To accomdate a 5000 Ohm load in a 4-20mA system you would need a 100 V loop supply (0.02 A * 5000 Ohms) which is way above the maximum voltage potential that the XTR115 can operate on. 

    Basically you can't violate Ohm's Law and once the output current * output resistance (V = I * R) exceeds the compliance voltage the design simply can not function and the current will level off.  A simple estimate is to simply use the loop-supply as your compliance voltage.  In practice the compliance voltage is below the supply voltage because we must take into account the driving limitations of the internal amplifier, base-emitter drop, and the saturation voltage of the transistor.

    Try reducing your load to 500 Ohms and see if your problems go away.

  • 0 in reply to Collin Wells
    Prodigy 160 points

    Collin,

    Thanks for the reply and that is a similar line of thought that we had as well, but apparently there is a certification test for HART compliance that is failing.  HART places about a 100mV signal on the 4-20 loop.  The impledance we need appears to come from a lowpass filter going into the 4-20mA side and the HART modem needs a hi pass filter to get the AC signal.   I know I am missing something fundamental here.

    Thanks, Paul  

  • 0 in reply to Paul Peterson
    TI__Guru 64875 points

    Hi Paul,

    I believe the HART compliance tests are run with the high-pass filter + analog test filter in parallel with a 250 Ohm load.  Please place a 250 Ohm load to the loop-supply GND on the IOUT output.  Then connect the HART compliance filtering circuit at the node where the 250 Ohm load and IOUT meet. 

  • 0 in reply to Collin Wells
    Prodigy 160 points

    Here are some figures.  The first figure would be the test configuration for a 2-wire transmitter. 

     

    The 2nd figure shows how to connect a 3-wire device.  Most of the HART Physical Layer tests use the 500 Ω resistor.  I think there are (2) tests that deviate from this.

     

    Figure 3 shows how I had to connect the 3 wire device.

     

    The Kepco power supply is a bipolar power supply, which I’ve never heard of until now.  Basically it’s a big OpAmp.  You connect the signal generator directly to it and dial in the DC offset needed.  This is needed because of the 5000Ω resistor has a big voltage drop across it.  The DC offset needs to be increased enough to get the required DC at the 2-wire transmitter. 

     

    Since I didn’t have the Kepco P/S I had to connect the signal generator output to the (-) input of the Loop power supply.  The (+) of the Loop power supply is connected to the resistor which in turn is connected to the return of the signal generator (which is the shell of the BNC connector and is earth ground).  I was able to generate enough DC voltage offset from the power supply to get the loop to function properly.  This was new to me as I’ve never connected a signal generator to a power supply like this before.  )I tested with a sister companies 2-wire transmitter to verify that it worked as intended.)  The signal generator has a DC offset, but it is not large enough to get the Loop DC level high enough to operate the loop. The transmitter power was provided by a separate power supply.

     

    The HART Communications Foundation (HCF) test frequencies are 200, 500, 950, 1.6K, 2.5K and 10K Hz.  The test is to adjust the AC signal to where Va or Vb is 1Vpp and then measure the other voltage…  basically a voltage divider.  Since a transmitter is a high Z device, to get better resolution a 5000 Ω resistor is used.  This requires the higher DC voltage.  HCF provides a graph to convert this raw Z into its components. It’s not exact, but close enough.

  • 0 in reply to Paul Peterson
    TI__Intellectual 1245 points

    I am attaching the pictures:

  • 0 in reply to Paul Peterson
    TI__Guru 64875 points

    Hi Paul,

    Unfortunately your images did not make it.  Sometimes there are issues uploading them as images so please try uploading them as a file and I'll download them and take a look with out local HART compliance expert to see what guidance we can provide.

  • 0 in reply to Collin Wells
    TI__Guru 64875 points

    Hello,

    Okay, I got the images from Jeovid's post.  Our local HART expert is out of the office until Monday but I will check with him when he gets back regarding this test.  We have expertise in testing our XTR transmitters for HART compliance in the following tests:

    -Carrier Transients
    -Output Noise During Silence
    -Analog Rate of Change

    We are not familiar with the "Impedance Test" you are referring to but will get with our local HART engineer and go through the HART spec to find the test you're referring to when he gets back.  We have had several customers successfully use the XTR115 with both the DS8500 (Maxim) and A5191HRT (On Semi) HART MODEMs so I know the XTR115 can pass the HART standards.