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Question regarding XTR106

Other Parts Discussed in Thread: XTR106, OPA2277, LM358

Hi all,

I have some questions regarding the bridge balancing (Equation 3).

  1. Is R_B the Resistance of the Bridge (V_Ref to I_Ret) like on Page 1 of the datasheet? 
  2. Does V_Trim define the max voltage range that is mapped to the 4-20 mV ourtput range?
  3. If I have an input range of +-2 V meaning a 4 V span in total. Does this mean V_Trim is 2V?
  4. What happens If i do not use R_1 and R_2 at all?

Thanks in Andvance and forgive my stupid questions but I am a mechanical engineer so...

Regards Hannes

  • Hannes,

    1.  R_B is the resistance of a 4-resistor bridge with 4 nearly equal value resistors. Measure from "top" to "bottom" of the bridge. This is also the value of each individual resistor, though when connected as a bridge you can't isolate just one resistor--the other three are in parallel with it so it will measure 75% of the actual resistance.

    2, The purpose of R1 and R2 (the bridge balance circuit) is to adjust the mismatch of the resistors in the bridge to produce zero volts differential bridge output with zero force or weight. V_Trim is the maximum initial pre-trim voltage output from the bridge with zero force or weight. It will depend on the manufacturing tolerance of the bridge you are using.

    3. A 2V output from a bridge is extraordinarily high. A bridge of this type would normally only have an output of perhaps 10mV, or so, at full load. The initial offset of a bridge with no force or weight might be a few mV. So you trim for zero output with no weight and at full weight you would get 10mV.

    4.  Without R1 and R2 the bridge output may be non-zero. Thus the current output of the XTR106 would not be 4mA at zero scale.

    Does this help?

    Regards, Bruce.

  • Bruce,

    thank you very much. I thing i got that. Just 2 last questions. Is it correct, that R_1 is very high for small V_Trim?  Is it correct the way I integrated R_1 and R_2?

    Thanks and Regards

    Hannes

  • Hello Hannes,

    I was looking at your equations and I verified that you calculations are correct.  The XTR106 is typically used with bridge resistances in the range of 350 Ohms to 20 kOhms.  Having a bridge resistance of 130 kOhms will likely work but as you've found if you want to trim out the offset errors the resulting values for R1 and R2 need to be very large in order to not dominate the bridge response significantly. 

    Your placement of R1 and R2 is not correct though.  The potentiometer shown in the circuit diagram will allow for both positive and negative offset correction whereas your current circuit will only allow for a single-ended correction.  Your placement or R1 is correct but you should use a potentiometer connected as shown in the diagram for R2.

    Regards,
    Collin Wells
    Precision Linear Applications

     

  • Hi Collin,

    I have a use case where my bridge output is almost real without any Vtrim. Vtrim is in the range of 2uV at 0.253mV excitation. My bridge resistance is 350 Ohm.

    In that case, should I still use the R1 & R2 resistors? I need to use 218 MOhms resistors for R1 & R2.

    Also Im using the LM358 instead OPA2277, input to the opamp from the bridge @ 0.253mV excitation is 0.083 mV. I measured the current to Opamp pin 8 which reads 1.7 mA & the excitation for load cell reads 0.853 mA. Im unable to make the current to Opamp reduce to 1.6 mA.

    What would happen if OpAmp Current + Excitation Current > 2.5mA?

  • Hello,

    If your sensor is linear then you won't need to use the linearization circuitry included in the XTR106 and you can directly connect the bridge.

    If the XTR106 and external circuitry consume more than 4mA, then the system will not be able to output the zero-scale level of 4mA.  We wrote a blog that directly addresses this topic here:  Blog:  2-Wire 4-20mA – IRET < 4mA

  • Hi,

    The blog was really useful.

    One issue which Im facing now is that we are unable to get the Vref5 for the bridge. It is 2.6V [Pin14 - Pin7 ]& Vref2.5 [Pin13 - Pin7] is about 1.4V.

    My Vss is 24V DC and Im using the similar configuration as per the Figure 5 in datasheet w/o linearisation.

    My Pin 12 is connected to 1 & 11. Im not sure what am I missing.

    Part number for Q = TIP31C & OpAmp OPA2277P.

    I tried replacing the XTR suspecing it might have failed. But the result is the same.

  • First, all measurements on the XTR106 circuit should be performed referenced to pin 6, "IRET", not pin 7 "IOUT". It sounds like the regulator outputs are collapsing. Is it possible that your circuitry is pulling too much current from the regulator outputs? I know you're using a circuit configuration close to that shown in Figure 5 but please share your actual schematic so we can look at all the connections and component values.
  • Another issue is that the current to the bridge is branching to OPA2277. The incoming 732 uA i s split to 591uA to Iret & 191uA to Pin5 of OPA. Im not sure why.

    Is there any maximum load limitation other than the usual Max Current x Load Resistor drop.?

  • Hi Sudarshan,

    The schematic will not work as shown because the 1k load and a 20mA output current result in a 20V drop across the load resistor.  This subtracts from the 24V supply leaving only 4V to power the XTR106 which is below the minimum level of 7.5V as stated in the product datasheet.  Replace Rload with a 100 Ohm resistor and let us know your results.

  • Thanks for the input, Collin. I missed to note that the Vss for Chip is from the loop supply. The scheme works now as expected.

    The output current seems to be oscillating and is not constant for constant input differential voltage.

    Kindly suggest if any filtering needs to be implemented for reducing the drift.

  • Hi Sudarshan,

    Please share your complete schematic and we'll take a look. The device does not require a filter to output a constant dc current.
  • Hi Collin.

    1. I have some more clarifications. The voltage between Pin14-Pin6 reads 4.43V , while the Pin2-Pin5 reads 2.97V on few instances. Does that mean that the OpAmp is drawing more than the capacity?
    2. Also the Zero (4mA) setting/tuning is not possible even while I inject 0.000mV on the Pin2-Pin5, the current flowing on the load [700 Ohm] is 4.78 mA while its supposedly to be 4.00 mA.
    3. While Im reading the OPA2277 output across Pin1-Pin7 [Leads on Pins], the current  flowing on the load is 2.92 mA. While Im not reading the pins [PINS OPEN] the current  flowing on the load is 25.13mA. And I don't have an explanation for the same.

    Attaching my scheme for your reference.

  • The scheme is same as provided in earlier thread - Collin.
  • Hi Sudarshan,

    1.) Pin 14 is not attached to anything in the schematic drawing you sent earlier. Do you mean the +5V VREG output (pin 13)?

    2.) This sounds like your circuitry is drawing too much current which is why the zero-scale level is not 4mA

    3.) This sounds like a grounding issue caused by accidentally shorting the IRET pin to the Vloop GND through your measurement circuitry EARTH GND connections. Be sure you're using a floating meter and I think you'll find this doesn't happen anymore.