How voltage references affect mixed-signal parts
The analog output of an ADC or the digital results of the DAC can be only as good as the voltage reference in your circuit.
Bonnie Baker, Texas Instruments -- EDN, August 26, 2010
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You might blame your ADC’s or DAC’s lack of output stability
on the converter itself. After all, these types of devices can be
complex. Try not to pass judgment too fast, though, because
the circuitry around your converter might be the culprit. This
circuitry, which includes a voltage reference, can change the
converter’s performance more than you may imagine.In your initial evaluation of your converter, you may not even see the ill effects of your voltage reference. In the past, when evaluating an ADC or a DAC, I would first make sure that the converter’s digital interface was in order and check to see whether the converter’s output generally represented the input signal. I then looked at the zero-input converter noise. When you measure the noise of an ADC, you short the inputs and connect close to ground. With a DAC, you program the digital input to an analog zero output.
Where might you look for an ADC
or DAC voltage-reference error? The
key to answering this question is in
the transfer function of these devices.
In Figure 1, the numerator of the
right-hand side of the ADC function has
the input signal times 2N, where N is
the number of converter bits, and the
denominator has the magnitude of the
voltage reference in volts. The 2N and
VREF values are constant. The impact
of the voltage-reference value—and
its errors—increases with an increasing
input signal.
The best way to analyze and evaluate your data converter’s voltage reference is with a full-scale output 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 will become 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 voltage reference in your circuit. When you choose your voltage-reference source, consider the following tips.
Using the system power-supply voltage at your converter’s voltage-reference pin is a good technique only when dealing with 8-bit ADCs at best. Consider the origin of the power-supply voltage. For instance, dc/dc or switching converters produce acceptable dc outputs for circuits. However, they usually have an internal switching network that produces noise on the dc signal. Even when you implement lowpass filtering, remnants of the switching action in the dc/dc converter may transmit to the output of your ADC or DAC device. You may also try to follow a dc/ dc or switching converter with a linear regulator. Linear-regulator power-supply- rejection and output noise levels are improving, but you may find that 10-bit devices and those operating at more than 10 bits still have problems.
An even riskier source for your converter’s voltage-reference pin is your computer’s USB port. The power-supply voltage from your USB port has the computer’s digital noise riding on it—a poor environment for these types of devices. For higher-resolution ADCs and DACs, the best strategy is to start your design with a low-noise, stable, stand-alone reference.
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Talkback
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Gross typo in Figure 1. Under the DAC it says the transfer function is Vout = D(in) * 2^N/Vref. It should be Vout = D(in) * Vref / 2^N.
Marc Stewart - 2010-14-9 15:04:22 PDT -
Also long term drift and tempco drift are important specs. for references. You can trim for accuracy but the drift is not so easy to cancel out.
Al Welch - 2010-14-9 13:03:14 PDT -
Presumably it should have been:
The analog output of a _DAC_ or the digital results of the _ADC_ ...
Luis Palafox-Gamir - 2010-27-8 00:03:47 PDT
