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How Voltage References Affect Mixed Signal Parts

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How voltage references affect mixed-signal parts
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  How voltage references affectmixed-signalparts Bonnie Baker   - August 26, 2010   Click here to download a PDF You might blame your ADC’s or DAC’s lack of output stability on the converteritself. After all, these types of devices can be complex. Try not to pass judgmenttoo fast, though, because the circuitry around your converter might be the culprit.This circuitry, which includes a voltage reference, can change the converter’sperformance more than you may imagine.In your initial evaluation of your converter, you may not even see the ill effects of your voltagereference. In the past, when evaluating an ADC or a DAC, I would first make sure that theconverter’s digital interface was in order and check to see whether the converter’s output generallyrepresented the input signal. I then looked at the zero-input converter noise. When you measure thenoise of an ADC, you short the inputs and connect close to ground. With a DAC, you program thedigital input to an analog zero output.Where might you look for an ADC or DAC voltage-reference error? The key to answering thisquestion is in the transfer function of these devices. In Figure 1 , the numerator of the right-handside of the ADC function has the input signal times 2 N , where N is the number of converter bits, andthe denominator has the magnitude of the voltage reference in volts. The 2 N  and V  REF  values areconstant. The impact of the voltage-reference value—and its errors—increases with an increasinginput signal.  Figure 1  D OUT  is the decimal representation of ADC’s output code, V  IN  is the ADC’s input voltage, N is the ADC’s and DAC’s number of bits, V  REF  is the reference voltage in volts, and DIN is the decimalrepresentation of the DAC’s input digital code.  The best way to analyze and evaluate your data converter’s voltage reference is with a full-scaleoutput signal. A voltage reference with an offset error creates an ADC or a DAC gain error. If your voltage reference is noisy or marginally stable, you will also see this noise or instability, which willbecome worse when the converter’s output is close to full-scale.The analog output of a DAC or the digital results of the ADC can be only as good as the voltagereference in your circuit. When you choose your voltage-reference source, consider the followingtips.Using the system power-supply voltage at your converter’s voltage-reference pin is a good techniqueonly when dealing with 8-bit ADCs at best. Consider the srcin of the power-supply voltage. Forinstance, dc/dc or switching converters produce acceptable dc outputs for circuits. However, theyusually have an internal switching network that produces noise on the dc signal. Even when youimplement lowpass filtering, remnants of the switching action in the dc/dc converter may transmit tothe output of your ADC or DAC device. You may also try to follow a dc/ dc or switching converterwith a linear regulator. Linear-regulator power-supply- rejection and output noise levels areimproving, but you may find that 10-bit devices and those operating at more than 10 bits still haveproblems. An even riskier source for your converter’s voltage-reference pin is your computer’s USB port. Thepower-supply voltage from your USB port has the computer’s digital noise riding on it—a poorenvironment for these types of devices. For higher-resolution ADCs and DACs, the best strategy is tostart your design with a low-noise, stable, stand-alone reference. References 1. Baker, Bonnie, and Miro Oljaca, “How the voltage reference affects ADC performance, Part 1,”Baker, Bonnie, and Miro Oljaca, Analog Applications Journal, Texas Instruments, March 2009.2. Crampton, Ray; Dennis Hudgins, and Dave Heisley, “Noise Management in Portable RFSystems,” Texas Instruments.
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