Cover story t SponSored FeATUre


this is not possible, the data sheets of the devices in the signal chain can be used to derive the information needed, once the two main data points have been collected at ambient. Each device in the signal chain contributes to


the errors, and each board is different from the other, so it should be calibrated individually.


TempCal: CalibraTion aT operaTing TemperaTure


The best level of calibration is achieved by measuring the errors in the application environment at the operating temperature, and systematically correcting them when writing to the DAC to update the output. To calibrate the DAC using this method,


at the temperature the system will be operating at, measure the DAC output at


code ZSLIN and FSLIN. Construct the transfer function as follows:


section by calculating the gain and offset error at ambient and applying Equation 14.


where:


GEamb = Gain error at ambient temperature VOE,amb = Offset error (V) at ambient temperature


Calibrating the DAC signal chain at ambient


temperature accounts for the system-level errors. However, the change of the external errors due to temperature change are not accounted for ; thus, this method of calibration is not as accurate as the TempCal method. Drift in intrinsic DAC errors, namely offset


and gain errors, due to change in operating temperature can then be accounted for using the data sheet specifications. This is what we call SpecCal. The typical value of offset error drift is listed in the specifications table of the AD5676R data sheet, and the offset error vs. temperature typical performance characteristics (TPC) indicates which direction the error drifts depending on an increase or decrease in temperature from ambient.


The change in gain error due to where:


VOE = Offset error (V) VFS,LIN,ACT = Actual output at FSLIN VZS,LIN,ACT = Actual output at ZSLIN VFS,LIN,IDEAL = Ideal output at FSLIN VZS,LIN,IDEAL = Ideal output at ZSLIN Note that offset error can be positive


or negative. Figure 5 shows the output error achieved


for the EVAL-AD5676 evaluation kit with the TempCal method.


where: Figure 6 shows the output error achieved


for the EVAL-AD5676 evaluation kit with the SpecCal method.


Figure 5. System output error in LSB with TempCal at different temperatures.


SpeCCal: CalibraTion uSing SpeCifiCaTionS


If it is not possible to measure the errors in the application environment at the operating temperature, it is still possible to achieve a high level of calibration using the AD5676R data sheet and the DAC transfer function calibrated at ambient temperature. To calibrate the DAC using this method,


measure the DAC output at codes ZSLIN and FSLIN at ambient temperature. Construct the transfer function as described in the TempCal


Instrumentation Monthly January 2022


temperature is indicated in the gain error vs. the temperature TPC. Determine the gain error in per cent of FSR from the graph and apply Equation 16.


Now that we have estimated the offset


error and gain error at the operating temperature, we can use Equation 17 to determine the input codes for the SpecCal output.


source can be accounted for using the reference data sheet by considering the reference drift at the temperature of interest. Changes to the reference voltage alter the actual output span and, thus, the LSB size. This should be accounted for if an external reference is used. The temperature vs. output voltage TPC can be used to determine the change in the output span due to the reference drift.


where: ConCluSion


This article outlined some of the main causes of error in a DAC signal chain, including DAC intrinsic errors that are defined in the data sheet and system-level errors that vary depending on the system and must be considered in an open-loop application. Two methods of calibration have been


discussed, one for when the DAC can be calibrated at the temperature the system will be operating at and the second for when it is not possible to calibrate at the operating temperature, but measurements can be taken at ambient temperature instead. The second method uses the TPCs and the specifications outlined in the data sheet of the DAC and other ICs in the signal chain, to account for gain and offset error drift. The TempCal method can achieve much better accuracy than the SpecCal one. For example, Figure 7 shows how, for the EVAL- AD5676 board at 50°C, the TempCal method achieved a level of accuracy very close to the ideal, while the SpecCal method still managed to deliver an improvement from the NoCal data.


Figure 7. System output error in LSB with NoCal, SpecCal, and TempCal at 50°C.


Temperature variation plays a major role in


Figure 6. System output error in LSB with SpecCal at different temperatures.


The internal reference was used in this


instance. An external reference can add to the overall error. Errors due to the reference


the accuracy of electronic systems. Calibrating at the system operating temperature counteracts most errors. If this is not possible, temperature variation can be tackled using the information available in the DAC and other ICs’ data sheets to achieve accuracy of an acceptable degree.


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