XTR111: Want to confirm the current output range 0-20mA

HI Nikhil,

1.  The XTR111 can produce close to ~0mA output, if 0V input is provided to the device; but there will be a small error due to the offsets of the device.   

The XTR111 accuracy is specified on the Electrical characteristics table of the datasheet: the internal op-amp of the XTR111 has a  ±1.5mV max input voltage offset error (Vos) , and an offset current (Ios) of 0.02% max at 25mA current span or about ~±5uA offset current). 

Please keep in mind that the external FET transistor will have some small amount leakage current, and the XTR111 accuracy will be directly dependent on the accuracy/drift and resolution of the signal source input voltage or the accuracy/resolution of the DAC.

2.  It is possible to obtain about ~0mA output when the DAC can only provide ta minimum voltage of 0.0466V using the level shift/trim circuit shown on the device datasheet on page 17. 

If the DAC or signal source cannot deliver 0V in a single-supply circuit, an additional resistor from the SET pin to a positive reference voltage or the regulator output (Figure 44) can shift the zero level for the input (VIN) to a positive voltage. This is explained on the section "LEVEL SHIFT OF 0V INPUT AND TRANSCONDUCTANCE TRIM" on page 17 of the datasheet.  The accuracy of this circuit is a function of the tolerance and drift of the resistors and the accuracy of the voltage reference.

For example, if you require a 0mA to 20mA  current output, for a DAC input range of +0.0466V to +3V, you could select RSET to 1.5kΩ.

Since the minimum voltage of the DAC range is 0.0466V, this corresponds to an ISET current of 0.0466V / 1.5kΩ= ~31.6uA. Hence, applying a 3V reference voltage, and 95.3kΩ resistor on RSET as shown below will allow for a 0mA current output for a 0.0466V input, and 20mA current output for a 3V input.  In essence, the 3V reference and 95.3k resistor is injecting a current of ~31uA into the circuit. 

The below is a quick TINA simulation using a simplified or idealized model to check the circuit transfer function after applying the level shift trim..

Thank you and Regards,

Luis

HI Nikhil,

Regarding your question:

Nikhil F said:

1. As per reference document mentioned by you "">www.ti.com/.../tidub12" for XTR111 accuracy calculations. We have query regarding accuracy calculations of DAC output. Why "VREF initial accuracy" parameter is not considered in equation 3? 

In our case 1.948 % FSR drift will occur in VREF signal, is this VREF parameter need to consider for accuracy calculations in equation 3 in our case? or it can be removed by calibration?  because DAC Output is depends upon below equation. where VREF = AVCC1

The reference design document TIDUB12 provides equation (2) for Total Uncalibrated Error, TUE, accounting for the reference initial accuracy:

Then explains that after applying a two-point best fit calibration to the design to remove the effects of gain an offset errors, and leave only the linearity errors of the DAC8551, as shown on equation (3).

Essentially, an external high precision meter is used two perform a two point DAC output measurement, one measurement slightly above zero scale and a second point slightly below full-scale, keeping the DAC well inside its linear range.  The offset and gain error of the measured result is compared against the "ideal" result, and in essence the gain error and offset errors of the circuit are measured. In essence, the transfer characteristic is a linear function in the form y=mx +b.  The offset and gain calibration is based on the idea that we can solve the straight line equation for the slope and intercept, where, the slope error is the gain error and the intercept is the offset.

The reference initial accuracy causes a DAC full-scale error, analogous to gain error or essentially slope in the linear function.  After calibrating, the measured slope and measured offset, called calibration coefficients, are stored in the microcontroller’s memory, and these coefficients are then applied during normal device operation, applying the coefficients to the DAC codes to compensate for the gain and offset error.

It is important to highlight that the calibration is only effective assuming that the reference, and passive components are very stable over time and have  low drift at the required temperature range, else different calibration coefficients will be required for different temperatures.  It is also very important that the external measurement system (measuring voltage or current during the calibration routine) is considerably more accurate than the targeted accuracy.  If the reference drifts over time or temperature the calibration will be invalid.  

The TI reference circuit on the application note uses a high precision DAC and a low-drift precision reference that offer low-drift over temperature.

An excerpt of a presentation is attached discussing the gain and offset calibration for an ADC acquisition system.  The circuit is different than this DAC application, but the same general concepts related to an offset and gain error calibration apply.\

Calibration_example.pdf

2) The TI reference design above uses a dedicated/discrete DAC8551 which offers 16-bit precision, and offers a datasheet that specifies much better linearity and lower drift.  In general, the accuracy of the system will be limited by the DAC accuracy/drift; and the voltage reference voltage stability over time and temperature drift.

As you have mentioned, in your application, the circuit uses a 12-bit DAC peripheral integrated on the  R5F564MFDDFB  microcontroller which will not offer the same level of performance as a discrete, precision16-bit DAC. It may be possible to attempt a level of calibration to reduce gain and offset errors, but keep in mind the calibration is effective to the level of the DAC voltage stability/accuracy/drift over time and temperature, as well as the drift/stability of the DAC reference.  Since this is not a TI device, please consult the microcontroller manufacturer for questions about the DAC performance.

Thank you and Regards,

Luis