Hello,
I am following a TI reference design for a prototype build and have run into an issue where the output deviates from the expectation at low input voltages ~ <1.5V. I did not see this particular problem mentioned in the e2e forum, but if it has already been answered please point me in the right direction.
The circuit is to transmit 4-20mA to an external receiver from a 0-5V input. (valid input signal range is 0.5-4.5V), which matches the TI reference design exactly so that is where i began my design. The schematic is attached.
All resistors are 1% tollerance, the resistors setting Iset and the 4-20 scalling will be changed out to 0.1% parts to reduce static error. The static error, as built, is not a particular problem, it is that the error % changes over input voltage (between 0.5 and 4.5V). I have attached my measurements showing the deviation.
The tollerance of the spescification is ~+/- 0.185mA (25uA Ideally) between the ideal Iout and the actual Iout. As the chart showes Iout deviates from this at VinTx <1.199V.
| Desired Voltage | Voltage out of Sensor (V) | Voltage into Tx (V) | Voltage@ Rsns (V) | Current @ Rsns (mA) | Expected voltage into Tx (Ideal V) | Expected Current (Ideal mA) | Difference (mA) | Difference (V) | Deviation (mA |%|) | Deviation (V |%|) |
| 4.500 | 4.496 | 4.595 | 7.04 | 18.483 | 4.5966 | 18.386 | -0.096 | 0.000 | 0.523 | 0.035 |
| N/A | 4.229 | 4.383 | 6.71 | 17.616 | 4.3828 | 17.531 | -0.085 | 0.000 | 0.485 | 0.005 |
| N/A | 2.104 | 2.701 | 4.118 | 10.811 | 2.6811 | 10.724 | -0.087 | -0.007 | 0.810 | 0.743 |
| N/A | 1.803 | 2.463 | 3.75 | 9.845 | 2.4401 | 9.760 | -0.085 | -0.009 | 0.870 | 0.940 |
| 1.700 | 1.699 | 2.381 | 3.622 | 9.509 | 2.3568 | 9.427 | -0.082 | -0.010 | 0.870 | 1.028 |
| 1.600 | 1.600 | 2.302 | 3.502 | 9.194 | 2.2775 | 9.110 | -0.084 | -0.011 | 0.923 | 1.076 |
| 1.500 | 1.498 | 2.222 | 3.378 | 8.868 | 2.1958 | 8.783 | -0.085 | -0.012 | 0.970 | 1.193 |
| 1.400 | 1.402 | 2.146 | 3.268 | 8.580 | 2.1189 | 8.476 | -0.104 | -0.013 | 1.226 | 1.277 |
| 1.300 | 1.300 | 2.065 | 3.157 | 8.288 | 2.0373 | 8.149 | -0.139 | -0.014 | 1.709 | 1.362 |
| 1.200 | 1.199 | 1.985 | 3.049 | 8.005 | 1.9564 | 7.825 | -0.179 | -0.015 | 2.291 | 1.463 |
| 1.100 | 1.100 | 1.906 | 2.940 | 7.719 | 1.8771 | 7.508 | -0.210 | -0.015 | 2.799 | 1.540 |
| 1.000 | 1.000 | 1.827 | 2.836 | 7.446 | 1.7970 | 7.188 | -0.257 | -0.017 | 3.582 | 1.669 |
| N/A | 0.945 | 1.784 | 2.777 | 7.291 | 1.7530 | 7.012 | -0.279 | -0.018 | 3.975 | 1.770 |
| 0.900 | 0.901 | 1.749 | 2.729 | 7.165 | 1.7177 | 6.871 | -0.294 | -0.018 | 4.274 | 1.820 |
| 0.800 | 0.822 | 1.686 | 2.645 | 6.944 | 1.6545 | 6.618 | -0.326 | -0.019 | 4.929 | 1.906 |
| 0.700 | 0.700 | 1.590 | 2.516 | 6.605 | 1.5568 | 6.227 | -0.378 | -0.021 | 6.075 | 2.134 |
| 0.600 | 0.600 | 1.510 | 2.409 | 6.324 | 1.4767 | 5.907 | -0.418 | -0.023 | 7.072 | 2.255 |
| 0.500 | 0.509 | 1.439 | 2.312 | 6.070 | 1.4038 | 5.615 | -0.455 | -0.025 | 8.095 | 2.506 |
What would be causing the deviation to slip at low input voltages? Where should I focus my design efforts?
Any assistance in understanding the XTR111 is appreciated
Thank you
