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Original question:

TIPD158: TIPD158

TIPD158: 4-20mA generation issue

Shubham Yadav
Prodigy 10 points

Part Number: TIPD158
Other Parts Discussed in Thread: OPA317, LM324

Hello sir,

Based on our requirement to generate 4-20mA signal for PLC, i am following your TIPD158 reference design as well user guide. As i am on design phase, before that i have prototye the board on zero PCB and try to read the loop current and it was coming around 58mA. I have attached my schematics below for your reference, please review it and let us know if there are any changes required. 
Below are my test set up for your reference:
1) Instead of OPA317IDBV i have used LMV324AIPWRG opamp as we have ordered OPA317 not recieved yet.
2)  Instead of FCX690BTA-TP i have used BC547 transistor as we have orderedFCX690BTA-TP not recieved yet.
3) Instead of 60.4 ohm I have used 20 ohm resistor connected at emitter.
Does above testing changes will create any issue in 4-20mA loop o/p.
 Please review it and let us know if any discrepancies found.
Waiting for your reply,
4-20mA loop design.pdf
Thanks & Regards,
Shubham 

  • 0
    TI__Guru 56933 points

    Hi Shubham,

    These changes seem reasonable to me.  

    Did you fabricate this PCB or is this on a breadboard? Note that the LM324 DBV package does not match the symbol on your schematic.  

    Could you measure a few key voltages? What is the voltage on the inverting, noninverting, and output node of the op amp look like?  What have you set your DAC voltage to be?

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 10 points

    Hello sir,

    We haven't fabricated PCB till now we are first testing the design on zero PCB, once its authentication is done then we will go for PCB fabrication. Yes, we have ordered OPA317 and FCX690BTA transistor as well 60.4 ohms resistor. Once, it will recieved i will test it again by mounting it  and forward you the readings.
    I think the issue may be with OPAMP and transistor which i am using i.e LM324 and BC547. Opamp not able to drive the transistor perfectly, as well as transistor gain may be hampering our result. Please, let me know whether what i am thinking is correct? Or what else can be done.

    Looking for your feedback...
     Regards,
    Shubham

  • 0 in reply to Shubham Yadav
    TI__Guru 56933 points

    Hi Shubham,

    Could you measure a few key voltages? What is the voltage on the inverting, noninverting, and output node of the op amp look like?  What have you set your DAC voltage to be?  This will be illuminating.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 10 points

    Hello sir,

    Now the ckt is working fine after replacing the op-amp with OPA317 and transistor with FCX690BTA  i.e as per the original design recommended by the TI. But there are some offset errors in the o/p
    i.e When DAC o/p 0v, o/p current is 4.27mA...( error 0.27mA)
    When DAC o/p 3.3v, o/p current is 22.67mA ( error: 2.67mA)

    Please, let me know how to eliminate this error,

    Waiting for your response !!!

    Thanks & Regards,
    Shubham

  • 0 in reply to Shubham Yadav
    TI__Guru 56933 points

    Hi Shubham, how accurate is the VCC3V3 supply? Any error in that supply will directly contribute to system error as well.  If you remove R25 or R26, you can isolate the offset error contribution of the VCC3V3 supply.  In addition, have you confirm that the DAC is truly 0V or 3.3V? Many DACs integrated into MCUs have large offset and gain errors.

  • 0 in reply to Paul_Frost
    Prodigy 10 points

    Hello Sir,

    Actually, i am currently testing the prototype before doing the fabrication and assembly of the ckt. I have mounted the design on zero PCB and given supply from bench top multimeter, so the supply is accurate, it is not fluctuating and i had verified it from my side as well. Moreover, one things i want to share with you as per TI design series resistor value should be 30.542k for DAC supply coming from mcu.But the resistor combination which i am using coming around 30.5k so there is offset of 42 ohm ( This can create issue). Similar on the other side of supply as well there is some offset in the error.

    I think this are the reasons for the offset, correct me if i am wrong?

    Please, let me know from your side what could else be the reason of the offset in the o/p.

    Waiting for your response,

    Thanks & Regards,
    Shubham

  • 0 in reply to Shubham Yadav
    TI__Guru** 100785 points

    Shubham,

    There are many potential sources of error, but they can easily be analyzed. If you go to the writeup of the TI Design here:

    https://www.ti.com/lit/pdf/TIDU299

    You'll need to reference the schematic in Figure 2. We're having some problems posting pictures on the E2E forum right now (sorry).

    You can find equation 4, which gives the IOUT calculation based on the DAC code. You can set this equation up in two ways. First IOUT when the DAC is at 0V:

    IOUT = (VREG/R5) * (1 + R3/R4)

    and IOUT when the DAC is at full scale (in your case 3.3V):

    IOUT = (VREG/R2 + VREG/R5) * (1 + R3/R4)

    Based on these equations, you can see that the IOUT is directly dependent on the VREG voltage. It's the largest contributor to the gain error, and you could even put 3.4V into the equation and see that the value changes 3%. You could also see that the resistors add a certain amount of error. All of these 4 resistors (R2, R3, R4, R5) should be precision resistors and potentially contribute to the error as well. Note that R3 and R4 become a current multiplication in the circuit and precision is required particularly there.

    Lastly, the offset of the DAC and the offset of the op amp also contribute to the error. Ideally, this offset is 0V, but in practice a several hundred microvolts can also affect the loop current.

    Regardless, you can review the design guide and check these values against some of your measurement numbers. Your multimeter should be high resolution to get the best results in comparing the values against calculation. 

    Joseph Wu