Cover story t SponSored FeATUre This means that for any given input code, once


the LSB is known, it should be possible to accurately predict the voltage output of the DAC.


LSB In practice, the accuracy of the DAC output is


subject to gain and offset errors from the DAC (intrinsic errors) and other components in the signal chain (system-level errors). For example, some DACs have an integrated output amplifier, while others require one that then becomes a possible source of additional errors. In data sheets, the most relevant


specifications are defined in the terminology section. For DACs, this section lists parameters like offset error and gain error. Zero-scale error is a measurement of the


output error when zero code (0x0000) is loaded to the DAC register. Figure 1 shows how offset and gain errors


affect a DAC transfer function for a unipolar voltage DAC. Gain error is a measurement of the span error


of the DAC, shown in purple in Figure 1. The gain error is the deviation in slope of the DAC transfer characteristic from the ideal. The ideal DAC transfer is shown in black. Offset error is a measurement of the difference


between the actual and ideal output, in the linear region of the transfer function, shown in blue in Figure 1. Note that the transfer function in blue is


interpolated to meet the y-axis for negative VOUT and identify the offset error.


Volts %FSR PPM (V × 2N)/VREF (%FSR)/100 × 2N %FSR/100 × VREF PPM/106 × 2N PPM/106 × VREF


usually specified with respect to temperature, there are other components of the system that can have an impact on the overall offset and gain of the output. So even when the INL and DNL of the DAC


are very competitive, there are other errors to consider - particularly with respect to temperature. Most recent DACs specify the total unadjusted error (TUE) as a measure of the output error taking all the various errors into account - namely, INL error, offset error, gain error, and output drift over supplies and temperature. TUE is expressed in %FSR. When the data sheet does not specify TUE for a


DAC, it can be calculated using a technique called RSS, or root sum squared - a technique used to sum uncorrelated error sources for error analysis.


There are other, smaller error sources that are


usually omitted because of their less relevant contribution like output drift, etc. Each of the specifications of each component in


the system must be converted into the same units. This can be done using Table 2. TUE is a great asset to succinctly explain how


accurate the DC DAC output is as a result of the sum of the intrinsic errors; however, it does not factor in system-level errors that differ depending on the signal chain the DAC is implemented in and its environment. It is worth noting that some DACs have a built-


in buffer/amplifier in the output stage and the data sheet specifications in this case reflect the effects of both as part of the intrinsic errors.


Figure 1. Representation of offset error and gain error for a unipolar DAC.


The effect of both gain error and offset error


can be seen in the blue plot of Figure 4. The same parameters are also defined in


relation to how they change (drift) with changes in temperature. Zero code error drift is a measure of


the change in zero code error with changes in temperature. The gain error temperature coefficient is


a measurement of the change in gain error with changes in temperature. Offset error drift is a measurement of


the change in offset error with a change in temperature. Temperature variation plays a major role in


the accuracy of electronic systems. While the intrinsic gain and offset error of a DAC are


Instrumentation Monthly January 2022


sysTeM-level errors When trying to analyse a DAC signal chain error budget for a given application, the system designer should consider and verify the different components’ contributions, paying attention to the temperature at which the system is expected to operate. Depending on the final applications, there can be many different building blocks to the signal chain, including power ICs, buffers or amplifiers, and different types of active loads that can contribute to the system-level errors.


reference source Every DAC requires a voltage reference to operate from. The reference source is one of the major contributors to the accuracy of the DAC and of the overall signal chain. The key reference performance specifications are also defined in standalone reference data


PPM/104


sheets like the ADR45xx family or as part of the DAC data sheet if the device has an internal reference source available to the user. Dropout voltage, sometimes referred to as


supply voltage headroom, is defined as the minimum voltage differential between the input and output such that the output voltage is maintained to within 0.1 per cent accuracy. The temperature coefficient (TC or TCVOUT)


relates the change in the output voltage to the change in the ambient temperature of the device, as normalised by the output voltage at 25°C.


The TCVOUT for the ADR4520/ADR4525/ ADR4530/ADR4533/ADR4540/ADR4550 A grade and B grade is fully tested over three temperatures: −40°C, +25°C, and +125°C.


The TCVOUT for the C grade is fully tested over three temperatures: 0°C, +25°C, and +70°C. This parameter is specified using two methods. The box method is the most common method and accounts for the temperature coefficient over the full temperature range, whereas the bowtie method calculates the worst-case slope from +25°C and is therefore more useful for systems that are calibrated at +25°C. For certain DACs, external reference sources


exhibit better performance compared to integrated references. The reference voltage directly impacts the transfer function, so any change in this voltage proportionally changes the slope of the transfer function - that is, the gain. It is worth noting that some DACs have a built-


in buffered internal reference and the data sheet specifications in this case reflect the effects of these internal blocks as part of the intrinsic errors.


line regulation Line regulation is defined for every standalone IC acting as a supply as the change in output in response to a given change in input. This applies to the power supply, buffers, and reference ICs that are supposed to maintain their output voltage stable regardless of the input. Line regulation is usually specified in data sheets at ambient temperature.


load regulation Load regulation is defined as the incremental change in output voltage for a change in load current. Voltage outputs are usually buffered to reduce the impact of this variation. Some DACs may not buffer the reference input. Therefore, as the code changes, the reference input impedance will also change, causing a change in reference voltage. The effect on the output is generally small


Continued on page 10... 9


LSB


Table 2. UniTs Conversion MaTrix Volts


%FSR LSB/2N × VREF LSB/2N × 100 V/VREF × 100


PPM LSB/2N × 106 V/VREF × 106 %FSR × 104


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