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XTR111: mV to mA conversation

Part Number: XTR111
Other Parts Discussed in Thread: INA333, INA818, INA333-HT, INA188, OPA2197, OPA2192-Q1, TLV2197-Q1, OPA197

I would like to design a circuit, whose signal voltage is in the range -1mV to 11mV. This signal voltage should be converted in to 4 to 20mA. Means when signal voltage is at -1mV , it should be converted as 4mA. when signal voltage is at 11mV , it should be converted as 20mA. Please suggest me.

  • You can use a summing amplifier to increase the minimum voltage from −1 mV to 3 mV.

    The XTR111's offset voltage is up to 1.5 mV, so it might be a good idea to amply the input signal.

  • Hello,

    Agree with Clemens. 

    We can get a circuit that will give this functionality on Monday of next week.  In the meantime, can you tell me what is the signal created by?  Is it low impedance, meaning that it can drive current without changing the voltage?  The easiest solution will require the input signal source to be able to drive ~10's of microamps without a problem.

    Regards,
    Mike

  • Hi Naga,

    As Mike implies, you need to provide us some design and accuracy requirements. I do not know what performance specification you are looking for. 

    As an example, here is a circuit. It is not perfect, but it may be close enough in terms of accuracy. The Va of 400mV source may be able to obtain from the 3V via voltage divider. 

    OPA387 -1mVto11mV to 4-20mA 03152024.TSC

    If you have other questions, please let us know. 

    Best,

    Raymond

  • Hi Naga,

    I have not heard from you. I assumed that the previous V-to-I converter will meet your requirements. 

    If you have other questions, please let us know. 

    Best,

    Raymond

  • .The circuit should convert the mV output of the strain gage bridge into a corresponding mA output. The minimum accuracy of the output current of the 4-20 mA module shall be +/- 0.1%. 

    Relationship between output current (mA) of the circuit vs. input voltage (mV) from the strain gage bridge. 

    Please suggest the ti IC or circuit , which will meet above requirements.

     

  • Hi Naga,

    Relationship between output current (mA) of the circuit vs. input voltage (mV) from the strain gage bridge. 

    Please provide us the design requirements of the strain gauge sensing circuit. I included the INA333  instrumentation application note below. You need to tell us the strain gauge's sensing range, accuracy, supply rail voltage and current in your application. The instrumentation amplifier is able to amplifier the differential uV to mV strain gauge signals, then the amplified voltage signal can be converted to current information.   

    https://www.ti.com/amplifier-circuit/instrumentation/products.html?keyMatch=INSTRUMENTATION%20AMPLIFIER

    TI has very large instrumentation amplifier selection. If you tell me your strain gauge requirements, we can recommend one for you, including the cost and accuracy performances.

    https://www.ti.com/lit/ug/tidub00/tidub00.pdf?ts=1711091038970&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FTIPD170

    In the I vs. V diagram above, I believe that the voltage signal on X-axis is from strain gauge bridge which is from -1.0mV to 11.0mV.  

    The minimum accuracy of the output current of the 4-20 mA module shall be +/- 0.1%. 

    In the I vs. V diagram above, I believe that the voltage signal on X-axis is from strain gauge bridge which is from -1.0mV to 11.0mV.  +/- 0.1% requirements are V to I conversion requirements, which is different from strain gauge sensing requirements. Is this weight measurement application?

    If this is battery driven and low power sensing application, please specify. 

    Best,

    Raymond

  • Hi Raymond,

    This requirement is not for a Battery driven application.

    Strain gauge is supplied with Regulated 10V DC supply. Voltage signal on X-axis is from strain gauge bridge which is in the range -1mV to 11mV. This mV voltage is the input to V to I conversion circuit. The minimum accuracy of the output current of the 4-20 mA shall be +/- 0.1%. 

    Kindly see below for more details about strain gauge:

    The load cell range is 0.0 mV to 10.0 mV with a force range of 0.0 lbf. to 12,500 lbf. Therefore a 1.0 mV variation corresponds to a variation in force of 1,250 lbf.  Being able to measure one additional mV on the positive or negative side is needed. That is the reason for -1mv to 11mV

  • Hi Naga,

    From your specification, the input strain gauge is operated between 349.5Ω and 352Ω range. I selected INA818 instrumentation amplifier (IA) for the analog front end. The IA amplifies mV signals to V signals for the XTR111's V-to-I conversion. The IA's output range should be configured between 1V to 5V for the differential input signal of -1mV to 11mV. We can vary the IA's Gain and Verf using the transfer function below to meet the IA's output requirements.  

    You will need to calibrate the strain gauge input vs. 4-20mA current for your application, since the input differential signal range is very small. 

    If I put two circuit together, the simulation looks like the following. 

    The strain gauge's input and output relationship is shown below. 

    Here is the simulation.

    INA188 Strain Gauge + XTR111 V2I 03252024.TSC

    Please note that XTR111's Vref is unable to drive the stain gauge bridge due to its impedance. 10V/350Ω  requires >28mA current to drive it. Also, you probably will need lower drift 10Vref to drive the strain gauge, though you did not specify these requirements. 

    You probably will need REF5010 bandgap + OPA182 zero drift amplifier as buffer to construct a 10Vref precision reference voltage to drive the strain gauge. 

    https://www.ti.com/amplifier-circuit/op-amps/precision/products.html#2192=Zero%20Drift&sort=451typ;desc&

    If you have other questions, please let me know. 

    Best,

    Raymond

  • Is XTR111 automotive qualified?  is there any other IC from TI , which will meet my requirement , along with automotive qualified or operating junction temperature from -55 to 150deg?

  • Hi Naga,

    The XTR111 is not automotive certified. Although XT111's operating temperature is specified -40C to +125C, its datasheet performance is specified and guaranteed from -40C to +85C. The part will operate outside of the temperature range as specified, but its performance will be degraded somewhat, which may be outside of datasheet specification. 

    is there any other IC from TI , which will meet my requirement , along with automotive qualified or operating junction temperature from -55 to 150deg?

    The standard automotive industrial temperature operating range is from -40C to +125C. -55C to 125C is considered as defense or space grade IC specification. -55C to 150C is considered as high temperature operating range, which it typically has different sets of performance temperature range.  

    For instance, INA333-HT IA can work for your transducer application from -55C to 150C or higher. I am not aware any V-to-I convertor converter that is capable to withstand in -55C to 150C range (without degradation), unless you design the V-to-I with discrete high temperature IC components.   

    https://www.ti.com/lit/ds/symlink/ina333-ht.pdf?ts=1712567019525

    https://www.ti.com/amplifier-circuit/instrumentation/products.html#sort=1192;desc&

    Anyway, whenever we are talking about the high temperature operating environments, the cost of the product will increase. These are considering special operating application which it has to operate reliably in high-temperature environments beyond the capabilities of standard temperature ranges.

    If you need further assistant, please let us know.

    Best,

    Raymond  

  • as per the INA 188 datasheet, 

     Input Voltage: (V–) + 0.1 V to (V+) – 1.5 V 

    For a single supply of 10V ,  input voltage range is 0.1V to 9.5V . whether this will work for differential input signal of -1mV to 11mV?

    also as per the datasheet offset voltage is 55uV which will also add/subtract to the differential input signal of -1mV to 11mV. As per your simulation gain of INA188 is 354.55 V/V, this gain will amplify offset also. How can i achieve 0.1% accuracy for 4 to 20mA current signal?

  • Hi Naga-san,

    For a single supply of 10V ,  input voltage range is 0.1V to 9.5V . whether this will work for differential input signal of -1mV to 11mV?

    This is INA188's configuration and input & output ranges, which is in agreement with the simulation. 

    I do not understand "input range is 0.1 to 9.5V" question. Please explain. Below is the simulation from the previous reply. 

    You may download the analog calculate in the link below. 

    https://www.ti.com/tool/ANALOG-ENGINEER-CALC

    How can i achieve 0.1% accuracy for 4 to 20mA current signal?

    Both INA188 and XTR111 are precision parts. The XTR111's V-to-I should be able to meet 0.1% accuracy without issues. The worst case is when Iout is at 4mA, the errors are 4mA ± 4uA.

    At 4mA, Vin is approx. 1.00021V from the simulation, and INA188's Vos overall errors at the input seem to be not significant, see the image below. In addition, we may be able to remove the Vos error, if this is critical application or calibrate it out. 

    If you have additional questions, please let me know. 

    Best,

    Raymond

  • Hi Raymond,

    in my application, in place of XTR111, for voltage to current conversion, can i use howland current pump? if yes, could you please help me with the simulation along with suggested automotive grade op amp for this requirement.

  • Hi Naga,

    could you please help me with the simulation along with suggested automotive grade op amp for this requirement.

    Here is the Howland current pump for the v-to-I conversion. 

    OPA197-Q1 Howland Current Pump 04292024.TSC

    Here is the related application note. 

    https://www.ti.com/lit/an/sboa441/sboa441.pdf?ts=1714400703961&ref_url=https%253A%252F%252Fwww.google.com%252F

    I picked OPA2197 for the application with a reasonable cost. There are some op amps (e.g. (TLV2197-Q1, OPA2192-Q1), but I do not know if these are able to meet your other requirements, see the link below. 

    https://www.ti.com/amplifier-circuit/op-amps/precision/products.html#1498=Automotive&sort=1261max;desc&

    If you have other questions, please let me know. 

    Best,

    Raymond

  • Hi raymond,

    while simulating the file in TINA , i am getting error " missing syntax element  Line#256.

  • Hi Naga,

    I do not see the issues, if I combined the two circuit together, see the simulation. 

    INA188 Strain Gauge + Howland Ipump 04302024.TSC

    Best,

    Raymond

  • Thank you for the simulation file. I am adjusting the gain of INA188  ,  reference voltage as per my application. when the load cell voltage is -1mV,  INA188 should amplify this to 1.5V.  when the load cell voltage is 11mV, INA188 should amplify this to 7.5V. For this the needed gain of INA188 is 500, ref voltage is 2V. as per the INA188 datasheet for 500 gain, required Rg is 100.2 ohm. when i am simulating for -1mV load cell voltage condition , i am seeing INA188 output as 1.53V , similarly for 11mV condition INA188 output as 7.53V. so 30mV error is coming in both conditions , why this error is coming, how to calibrate this? Also plz provide the spice file for INA188.

  • Hi Naga,

    The issues are likely caused by the howland current pump, where your Rsense and RL voltage delta exceeds the supply voltage of 10Vdc and it is not linear on the circuit. 

    Vdelta = 20mA*375 + 20mA*165 = 7.5+3.3 =10.8Vdc > 10Vdc. Anyway, I increase the Vs to approx. 15Vdc. 

    INA188 Strain Gauge + Howland Ipump 500Gains 05022024.TSC

    The above DC output characteristics have to be linear within the transducer's operating range. 

    when i am simulating for -1mV load cell voltage condition , i am seeing INA188 output as 1.53V , similarly for 11mV condition INA188 output as 7.53V. so 30mV error is coming in both conditions , why this error is coming, how to calibrate this? Also plz provide the spice file for INA188.

    Due to the gains of 500V/V, the INA188's offset voltage will play a role, thus Vout vs. Vsensor ranges need to be calibrated it out. Two ways you may calibrate out the Vos errors. 

    1. adjusting Vref to compensate the Vos error. 

    2.  calibrate Vout vs. the input from -1mV to 11mV and determine the slope of the Vout and compensate the errors using CMU. 

    The Howland current pump also has errors, so will be the best to calibrate out 4-20mA vs. -1mV to 11mV Vsensor linear range. 

    Regarding to the PSpice file, it is already in the simulation. Please right click on INA188 --> Enter Macro and you should see the .txt file. 

    or you can download it from the link below. 

    https://www.ti.com/product/INA188#design-tools-simulation

    Please let me know if you have other questions, 

    Best,

    Raymond

  • I have understood about adjusting the vref to compensate error but i did not understood your 2 point (calibrate Vout vs. the input from -1mV to 11mV and determine the slope of the Vout and compensate the errors using CMU. )

  • Hi Naga, 

    The 2nd point is called system calibration. Since we are dealing with linear curves, we can calibrate out all intermediate errors within the circuits between the output and the input. 

    1. Your input: Force (say it is force or weight or pressure) vs. -1mV to 11mV linearly.

    2. INA188 amplifies -1mV to 11mV to A --> B Volts linearly.

    3. OPA197 converts A-->B Volts to 4-20mA linearly. 

    4. ADC converts 4-20mA to voltage again, which as Vout (linearly)

    Assume the temperature, environmental behaviors, influence variables and effects are well understood within the system's acceptable error bands, you can calibrate the Vout vs. the input, where Vout is 0-3.3V in your final ADC conversion (CMU) and input is -1mV --> 11mV or Force in step 1. 

    since these are linear functions, you can calibrate two discrete points along the overall linear transfer function and establish an accurate linear relationship mathematically that describes the function between the Vout and the input. You can always to use additional input parameters to validate the linear curve and make sure that the linear function relationship is valid. 

    If you have other questions, please let me know. 

    Best,

    Raymond