Integrate More Analog Into Your Digital Designs

Many digital devices incorporate analog circuits, also called analog building blocks, that augment analog-to-digital interfaces for microprocessors, ASICs, and FPGAs. These analog building blocks can include internal voltage references, analog-to-digital converters (ADCs), or digital-to-analog converters (DACs).

If we ask the various manufacturers how good their analog parts are, they will reply that they’re quite good. Indeed, the circuits usually are quite amazing when we consider the environment inside the IC: heat, noise, ground bounce, crowding and constricted space, multiple layers, and ground lines running in different directions.

Digital ICs, however, have hundreds or thousands of digital gates changing state at megahertz or gigahertz rates. They tend to share power supplies and grounds. Given this congestion, it’s difficult to place decoupling capacitors inside the IC where they’re really needed. The noise floor inside the IC is quite high since the logic thresholds protect the digital circuits.¹ In effect, the digital signal is cleaned up (i.e., noise is ignored) at every stage.

Logic thresholds don’t protect analog circuits, so we can’t say that any analog voltage above a threshold is a one or below another threshold is a zero. The noise accumulates stage by stage in analog. Consequently, analog ICs are naked, and what you see is what you get. There is no proverbial discrete fig leaf or threshold to mask or protect anything.

Where does this leave us? We must always trade one parameter for another, with the application dictating the proper tradeoff of analog function—building blocks. But when the demand for economy of space and cost pushes analog circuits onto digital substrates, design challenges emerge.

Tradeoffs Are Unavoidable

The semiconductor industry has a specific, perfect example of how digital interference increases the noise floor—a chopping op amp. The classic “chopper” interrupted the signal path to recalibrate the amplifier and greatly reduce offset and drift. Later IC developments combined two parallel amplifiers built on a common silicon substrate. This design produced excellent matching between the two amplifiers. One amplifier was then chopped, and the resulting error signal was applied to both amplifiers.²

In current technology we improve op-amp performance by reducing offset, temperature drift, and 1/f or pink noise at low frequencies.^3,4 However advantageous this is for some applications, we degrade performance in another area, which is the point of the story.

The unavoidable chopper switching noise propagates onto the bias and substrate. This noise occurs at higher frequencies and is caused by switching a small chopper switch with low current. Now imagine switching hundreds and thousands of digital gates (with many switches each) on the same substrate with the analog circuits. Such is the challenge facing microprocessor designers as they add analog circuits to the part.

Look at how some IC data sheets specify analog parts. Generally, only the resolution and the reference voltage range are specified. Most of the specifications that analog designers expect to see in a stand-alone precision ADC, DAC, or voltage reference are missing. Noise, dynamic nonlinearity (DNL), integral nonlinearity (INL), offset, gain, and temperature coefficients (tempco) just aren’t there. This is a not too subtle hint that you are trading something for known precision.

Is this bad? Not necessarily. It depends on the application. In real estate, the most important factor is location, location, location. In electronic design, it’s application, application, application. If we need to measure a voltage with a 10% tolerance, the built-in ADC may be adequate. But if we need more precision, an external ADC may be a better choice. Once again, it depends on the application

Resolution And Accuracy

There’s a marked difference between resolution and accuracy in data converters. The resolution, or number of bits, is usually expressed in powers of two. It’s a good number that indicates how many steps are in the full-scale value. But it’s like asking how many steps are on a ladder. Resolution reveals nothing about accuracy or linearity—or, specifically, whether the steps are all the same size and evenly spaced. Without adequate specifications, we cannot understand what happens as the temperature changes.

The figure illustrates the lack of accuracy and linearity. The straight, sturdy ladder with the man on it represents the reliable, predictable steps that we would expect in a precision ADC or DAC. The left ladder has some obvious problems. It may be noisy and disjointed (segmented poorly), skipping steps, and not monotonic. The next ladder is uneven and compressed in some places, which means the ADC or DAC advertised as 12 bits (a poor 12 bits) is really a good 8-bit converter. The steps on the right ladder are stretched and irregular. It might not provide the expected resolution just when you need it. Does this mean that the empty three ladders are unusable? Again, it depends on the application. Perhaps the application only needs a low-accuracy measurement, so one of these ladders will work.

In envisioning data converter performance, the number of the rungs on a ladder can represent the resolution.

Accuracy And A Voltage Reference

The first consideration is the voltage reference. It sets the full-scale value, controls the initial accuracy, and contributes to the tempco.

To reduce system costs, the digital designer may include an ADC inside a microprocessor or FPGA. An easy way to evaluate the noise environment is to add extra bits of resolution to the ADC and see what happens. Later, one might describe it as 8, 10, or 12 effective bits. This is not a bad thing. Extra bits may be useful even if they are just noise. Again, it is the application that matters.

Suppose we want to measure something that doesn’t change quickly so we have time to average. If the noise is truly random, then the noise adds by the RMS and the signal adds directly every time we double the number of samples. Therefore, normalizing the signal level to unity, two samples will yield 3-dB better signal-to-noise ratio (SNR). Four samples will increase SNR by 6 dB, and so on.

But if the noise is coherent (i.e., the noise is the same from sample to sample), the SNR doesn’t improve. In most cases, the total noise is a combination of random and coherent noise. The noise in your application may improve by 1 dB or 2 dB with each doubling of the samples. This not only is good to know to troubleshoot and reduce noise sources, it also may be just the tool to make the application practical or possible.

An ADC’s internal analog voltage references also will experience difficulties inside a digital device. For example, the data sheet for a device with a 12-bit ADC with a ±1% reference says that an external reference can be used for more accuracy. What do the numbers tell us? The ADC is 1 V full scale, and 12 bits is 4095 levels of 0.000244 V each, or 0.024% if everything is perfect.

Now we overlay the reference voltage at ±1%, which is one part in 50 or greater than 5-bit (32) least significant bit (LSB) levels. Not to worry—there is a useful tool, a spreadsheet to make an error budget for the combination of the ADC and reference or a DAC and reference.⁵ In any case, remember that the error isn’t what’s important. It’s knowing if the error is acceptable for the applications.

Open Loop, Closed Loop, And Servo Loops

There are two obvious divisions between convertor applications: open loop and closed loop. Open-loop applications require the most stringent accuracy. An ADC, for example, might require a precise voltage with a particular, predictable output code. A DAC, in contrast, is an arbitrary waveform generator in which a specific digital code always produces a given output voltage. Closed-loop applications use the center of the ADC or DAC range because they are inside a servo loop.

Servo action corrections mean that the absolute accuracy is less important, as long as the converter is monotonic. Therefore, if one increments a voltage in a rising direction, the ADC output code will always increment in the upward direction. The code cannot change direction (go down or decrement). Another way to say this is that the sign must never change. The same must be true when decrementing the ADC’s voltage to be monotonic. DACs also need to be monotonic in servo or feedback applications. An example might be a motor controller that keeps the motor speed constant despite changes in the motor powerline voltage or the load applied.

If the ADC or DAC (integrated into a digital device) is inside a servo loop or in low-accuracy open-loop applications, the internal voltage reference may be adequate. If the application is more demanding, ADCs, DACs, and voltage references are available from IC companies like Maxim Integrated Products.

External Voltage References

Any circuit that handles small or sensitive signals can benefit from clean, low-noise power. A voltage reference can be such a stable low-noise power source for analog circuits. It can be inside an FPGA if a separate power pin is provided, or it can be an external analog circuit that feeds the FPGA.

Voltage references help FPGAs by providing a stable reference for ADCs and DACs. A reference is a small power supply with guaranteed initial accuracy and stability over temperature. For even more accuracy, many voltage references can be externally trimmed by a few percent using a digital potentiometer.^{6, 7} This trimming enables designers to compensate for ADC and DAC full-scale gain errors. Not limited to FPGA applications, voltage references also can provide low power to radio low-noise amplifiers (LNAs), op amps, multiplexers, and filters.⁸

PWM And Logic Translators

Typical digital power supplies have a voltage tolerance of ±5% to ±10%, which is quite reasonable for a digital supply. The digital parts ignore the voltage tolerance and reject the noise because of the thresholds inherent in the digital system.⁹

FPGA outputs commonly use DACs or pulse-width modulation (PWM) signals to control motor valves and other actuators. External DACs offer higher resolution, a cleaner voltage reference, and typically better frequency response. But the improvement with PWM is subtler.

In many cases the DAC will require less low-pass filtering compared to a PWM and, therefore, produce a faster response time or wider frequency response. But with PWM signals, an FPGA can count a high-frequency clock, making the time axis very precise. However, the voltage axis is imprecise and noisy because the power supply that the PWM uses is the same digital supply that the rest of the digital parts use.

Conversely, the PWM is an analog output where voltage level and noise are not rejected or accommodated by a threshold. The digital (dirty) PWM output needs to be translated to a clean analog signal connected to a precise voltage reference. This could be done with transistors, but it is easier to use dual-supply logic translators.

The translator can use the analog supply, which will likely be quieter. If more precision is necessary, a low-noise voltage reference can be used. This optimizes the PWM signal so it is precise in both time and amplitude. This precision will also minimize the low-pass filter complexity typically necessary to smooth the PWM signal into a slowly varying dc signal.

Conclusion

Digital design is difficult enough on anybody’s good day. That is why we have included references to application notes and analog building blocks useful in augmenting analog-to-digital interfaces for ASICs and FPGAs. They help save designs from errors, miscalculation, or last-minute disruptive changes. The saving is more than moving circuits to external devices. The savings include design time, which impacts time-to-market and manufacturing, testing, and debug time. Avoiding a spin or redo of a device is never a bad thing and may even allow a project to succeed and reach the market when others may fail.