Why is voltage reference necessary in mixed-signal designs? It comes down to accuracy and stability when dealing with digital power conversion.
Last time, in my piece on using integrated A/D converters onboard a microcontroller or system-on-chip (SoC), I said: “An external voltage reference pin may allow a higher voltage source (compared to the digital power rail for the microcontroller itself) for a wider analog input range, or a more stable source for higher accuracy.”
That was a tad oversimplification; so, how voltage references factor into conversion is worth a second look.
Power supplies are not what they used to be
In the before-maker times, analog circuitry often ran on ±12 VDC or ±15 VDC power, while digital circuits usually ran on +5 VDC. Staying away from “the rails”, analog signals were usually kept in the ±10 VDC range, making unit-per-millivolt scales easy mental math. Supply voltages were often very clean with linear regulation. Noise was still in the background, but a large signal-to-noise ratio (SNR) covered it most of the time. The A/D and D/A converter bit resolutions were small, so step widths were also much larger than noise.
Then digital-first thinking took over. Shrinking digital semiconductor processes made +5 V operation mostly obsolete—with one notable exception we’ll see shortly—with stops at +3.3 V, +1.8 V, +1.2 V, and even sub-volt operation. Those clean but wildly inefficient linear-regulated power supplies gave way to more efficient but noisier switched-mode power supply technology. Mixed-signal processes brought analog circuitry on digital chips, often running at lower digital power supply voltages. Converter resolutions increased, narrowing step widths. All this worked against analog signal-to-noise ratio, raising concerns for converter accuracy.
There may be a pin for that, or not
Back to my opening statement—of course, the operative word is “may.” Lower-end parts often provide only an internal reference voltage for the A/D or D/A converter. This is especially common in a low pin count package, with no room for an external voltage reference pin. Small packages tend to go on small boards. An internal voltage reference saves external parts and takes up less space. If that’s what there is to work with, work with it.
For bigger parts, there is often a choice between an internal reference voltage, and one supplied on a VREF pin. One of the downsides of using the internal reference is that it’s probably offset with a range of zero to its value. That puts the midpoint of the converter in the middle of that range. A level shifting circuit based on an external op-amp may center things to better advantage.
Many manufacturers have thought of that with VREF lining up the converter range to ±VREF and the midpoint at zero volts. Even just doubling the range can improve signal-to-noise ratio. Also, having control over VREF may be more digital-friendly. Instead of working with something like 2.5 V or 3.3 V, a VREF might be set to a binary value like 4.096 V, an exact figure matched to a 12-bit converter with 4096 values.
Here is how the precision LM4040 voltage reference breakout looks like. Source: Adafruit
Steal some volts and add stability
One might ask: if a part is running at say, 3.3 VDC, how does one get a higher voltage for VREF? The good news is VREF does not need a lot of current behind it. There may be a higher voltage running around on a board. For instance, if a board has a USB port, there’s 5 VDC available close by. Small voltage reference parts can transform that into a lower VREF. If a switching regulator is needed, there are small parts requiring an external inductor and a few other parts to create a higher voltage to run into a voltage reference.
So, why is a voltage reference part necessary? It comes down to accuracy and stability. Remember, the digital power supply may vary, in some cases up to ±5%. Using the digital power for a converter pours all the variation and noise from it straight into the conversion results. A voltage reference is much more tightly controlled. Initial accuracy values are usually in the 0.05% to 0.5% range. Noise is often measured in the hundreds of microvolts or less. Temperature coefficients are also kept low; boards aren’t always running at room temperature and VREF can drift.
An external VREF and the parts needed to create it can help converter accuracy and ease in using the results. There are good selection guides with more on how voltage references factor into conversion from several manufacturers online. It’s an advanced topic, but if you’re spending more for more accurate A/D or D/A converters, investing in an external reference makes sense.
This article was originally published on Planet Analog.
After spending a decade in missile guidance systems at General Dynamics, Don Dingee became an evangelist for VMEbus and single-board computer technology at Motorola. He writes about sensors, ADCs/DACs, and signal processing for Planet Analog.