Analog Circuit Design in 2018 and Beyond

Analog circuits are crucial for the performance of any real-world system. The physical world around us is analog as most of the signals, radio waves, audio, speech etc are continuous in time and value. Most of the processing takes place in the digital domain by high-speed processors, the conversion of real-world signals like audio into digital bits still need to happen by analog circuits such as analog to digital converters or ADCs. Also, all electronic projects need power. Power management ICs are usually analog circuits which manage the power and current requirements of a system. While pursuing my MS at Columbia University, I got to work on interesting analog design circuits and saw how a combination of digital circuits and software can make analog circuits even better. In this blog, we will go through some of the interesting things in the world of analog design in 2018 and beyond. If you would like to see something here, drop a comment below and we will add it in.

Analog Design Trends in 2018 and Beyond

1. Use of digital logic to improve analog circuit performance:

Analog circuits are typically designed with a pre-decided choice of bias currents, resistance and capacitance values etc. These make the analog circuits stable and ready to accept real-world signals. Usually, a designer iterates over corner cases of temperatures, humidity, transistor thresholds etc. to try and find the most optimal solution. Increasingly, analog designers are relying on digitally controlled values of bias currents, resistances and capacitances so that they can be adjusted externally by the microprocessor based on the environment such as power levels, temperature, process variation etc. This prevents the need for a one-size-fits-all solution. This also allows external calibration for high levels of accuracy.

To implement these digitally controlled values, a DAC or a digital to analog converter is typically used and the bits of the DAC are controlled externally. Current DAC is used to control currents. Digitally controlled currents, when multiplied by a fixed resistor, gives digitally controlled voltage too.

*3-bit current DAC to create a digitally-controlled current which feeds into an analog circuit *

2. Use of frequency-based circuits in analog design:

Amplifiers are building blocks of analog circuits. They are linear circuits which can amplify small signals. The accuracy of an amplifier depends on its “open loop gain”. Higher the open loop gain, the more accurate an amplifier is in real world. This comes at a cost of higher power.

What if we could use frequency domain for amplification instead? Use of frequencies would mean entering the world of Integral Control - which can lead to infinite gains at DC and zero error in the gain of the amplifier at very low powers. At university, we used voltage-controlled oscillators (VCOs) and Phase Locked Loop (PLL) based architecture to design a high-accuracy LDO regulator. We replaced the traditional op amp with a VCO - but the results were quite promising, we got low power (3.7uW), low dropout (0.1V) and high efficiency (~90%) at very lower supply voltage (0.8V) with this approach. You can refer the paper here.

Seems like a lot of analog design blocks will be re-designed with high-frequency design blocks. The challenge is to keep the system stable as the system needs a very accurate zero to stabilize 3 poles in the system. (2 poles at DC due to VCO and Cc, and one pole due to parasitic caps)

In the diagram below, VCO converts feedback voltage and reference voltage into frequencies which are then compared by a phase frequency detector (PFD) and converted back to voltage using Charge Pump (CP).

*Frequency-based voltage regulator- it replaces op amp with oscillator *

3. Use of circuit design techniques to avoid components on PCB:

PCB real estate is expensive! There is a trend of regulator and amplifier designs to move the on-PCB components into the ICs. This saves the real estate on PCB and also the component costs during large-scale productions. For voltage regulators, typical designs recommend 1-10 uF capacitors at output. However, there is a trend to avoid this capacitor and bringing this large capacitor inside the IC (to create dominant pole inside). This is done using either MOS capacitors (which are not very accurate but take less space) or MIM capacitors (which are more accurate but take more space). Such techniques do create additional challenges during design process, such as added complexity in stability analysis, but there is a good pay off in terms of saved real estate.

Creative techniques which share the same on-chip passives between circuits or using switched-capacitors circuits to build resistors are already happening and will continue to evolve in the future.

>*Capacitor built using a MOSFET inside an IC to avoid an on-PCB component *

4. Use of intelligent algorithms and machine learning to control circuit parameters like Bandwidth, Slew Rate etc:

In point 1 above we have discussed the use of digitally controlled values of bias currents, resistances and capacitances so that they can be adjusted externally by the microprocessor based on the environment such as power levels, temperature, process variation etc. This can be taken to the next step by training a microprocessor to decide its own values based on environmental parameters like temperature, humidity etc. We can train a model with training data of what values of currents, resistances or capacitances we would like our analog circuit to have at different environmental conditions. This model can then be run on the microprocessor to control the PWM of a switched capacitor or the frequency of an oscillator which can in turn control analog parameters like Bandwidth, Slew Rate, and power consumption.

Machine learning is growing rapidly fast, and I feel that its use in analog circuits is very promising.

>*Frequency of this switched cap circuit and therefore its resistance can be controlled by a machine learning model based on requirements *

5. Scaling of analog circuits:

It has been reported that transistors will stop shrinking beyond the year 2021. But the advent of 3D transistors such as FinFETs and Gate-All-Around (GAA) FETs might keep Moore's Law going (Moore’s Law states that the number of transistors in a dense integrated circuit doubles approximately every two years). Usually scaling directly affects digital circuits and their speed, as smaller transistors mean more density for the same area. Same can’t be said about the analog circuits as scaling an analog circuit is not a straightforward process. The thresholds, power supplies and hence the headroom requirements change considerably from technology to technology. But sub-100nm technologies have posed a challenge to analog circuits due to the magnitude of second order effects. In terms of design, the square relationship of current-voltage (I vs Vgs) is no longer “square” at smaller technology nodes. In these nodes, techniques like gm/Id approach are increasingly being used to size transistors in analog circuits which relies more on the empirical performance characterized by fabs rather than using square law.

This concludes this blog on the trends in analog design in 2018 and beyond. If this blog was interesting, you might also like our blogs on how to select a capacitor, inductor, voltage regulator, IC packages and more. Enjoy designing!