Baker’s Best; Will the right voltage reference please stand up?
Figure 1 (above) Block diagram of an ADC/Voltage reference configuration. The voltage reference output (VOUT) determines the full-scale range of the ADC.
For some converters the full-scale range is equal to the voltage reference’s output voltage, or VOUT. For other converters the full-scale range is equal to twice VOUT. You can find this relationship between the ADC’s full-scale range (FSR) of the voltage reference chip’s (VOUT) output value in the ADC’s product data sheet.
In a perfect world, that is all you have to do as you find the IC chip with the correct output voltage for your ADC. However, in our non-perfect world the voltage reference chip has initial output errors, and an inherent inability to drive the ADC’s reference pin directly. If you want to learn about voltage reference driving circuitry, refer to references 1 and 2. But first let’s come to terms with the reference’s initial output errors.
When it comes to the voltage reference output errors, what you should be concerned about is initial accuracy, temperature drift, and noise. Part of this evaluation is to convert all of these errors to units of volts.
Usually the specification unit for the voltage reference’s initial accuracy is a percentage. This unit can be converted to millivolts with one of these two equations:
Usually the specification units for the reference’s temperature drift is ppm/°C, or parts per million per degree Celsius. You convert this unit to millivolts with the following equations:
Where ppm/°C is the reference maximum temperature drift value, max app temp is the maximum temperature to which you will expose your application circuit board.
The noise specification unit for the reference is µVPP, or microvolts peak-to-peak. This is an easy conversion. Simply divide the specification by 1000 to change it from microvolts to milivolts. Also you need to use a multiplier of 2 if the ADC’s FSR is equal to twice the voltage at VREF.
Now take these three values and add them together using the root-sum-square equation:
Figure 2 The maximum application temperature is 85°C.
Figure 2 shows these results using the ADS8887: 18-bit ADC, REF5041 – VOUT = 4.096V, and LM4032-4.1 B grade – VOUT = 4.096V. Table 1 has all of the specifications for these devices.
Table 1 Maximum chip specifications for offset, noise, and drift. In this table maximum offset is analogous to the offset error for the ADC and initial accuracy for the voltage reference chips.
Table 2 Calculated offset, noise, and max drift values that appear in Figure 2. The max drift value is obtained with an 85°C temperature.
By the way, you convert the ADC’s SNR to converter noise with this formula:
How do you minimise these errors? The dominant error in this evaluation is the offset error for the ADC and the initial accuracy error of the reference chips. You can digitally calibrate these errors at the ADC output. One can reduce reference noise with analogue filtering prior to the ADC’s VREF pin. The drift error is by far the hardest, if not impossible, to calibrate.
References
1. Baker, Bonnie, “Taking the mixed signal voltage reference to a higher level,” EDN Baker’s Best, Sept 23, 2010.
2.Baker, Bonnie, “Go for the gold with voltage-reference circuits,” EDN Baker’s Best, November 18, 2010.
3.Baker, Oljaca, “How the voltage reference affects ADC performance, Part 1,” Analog Applications Journal, 2Q 2009 (SLYT331), Texas Instruments.
4.Oljaca, Baker, “How the voltage reference affects ADC performance, Part 2,” Analog Applications Journal, 3Q 2009 (SLYT339), Texas Instruments.
5.Baker, Oljaca, “How the voltage reference affects ADC performance, Part 3,” Analog Applications Journal, 4Q 2009 (SLYT355), Texas Instruments.
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