Why Power Choices Matter in Precision Signal Chains

As we design our analog circuitry, we seem to reflexively dive into the specification details for each component in the circuit. That gets you an “A” for enthusiasm, but the final design may suffer from a power perspective. Here’s why and how to avoid that mistake.

If you are working on an ultrahigh precision circuit optimized for fast settling with low latency, you seem to have your circuit design marching orders. You select components that exhibit low noise and higher speeds for this high-precision circuit to keep the latency under control. An appropriate block diagram for this type of circuit is shown in Figure 1. It contains many components that deliver the analog signal across the isolation barrier.

Figure 1: An adaptable voltage/current measurement signal chain with low-latency measurement. (Image source: Analog Devices)

Given this scenario, the precision component details immediately come into view. The Analog Devices’ ADG5421F switch is a ±60 volt fault protection and detection, 11 ohm (Ω) R_ON, dual single pole, single throw (SPST) switch. This device identifies when the difference between two inputs exceeds the power supply and generates an error message.

The gain stage is an Analog Devices LTC6373, a 36 volt fully differential, programmable-gain instrumentation amplifier. The LTC6373 gains the input sensor signal to the subsequent appropriate analog-to-digital converter (ADC) levels. The filter is a single-pole discrete resistor-capacitor (R-C) circuit. This filter performs an anti-aliasing function to attenuate higher frequency signals.

The analog-to-digital conversion circuitry starts with the Analog Devices ADA4896-2, a dual, low-power, rail-to-rail output amplifier. This serves as the ADC’s driver amplifier. Next is Analog Devices’ AD4630-24 24-bit, 2 megasample per second (MSPS), dual-channel successive approximation register (SAR) ADC, which performs high-precision conversion with zero latency.

The voltage reference portion to the ADC consists of Analog Devices’ LTC6655LN, a series voltage reference with a fixed 5 volt output, and Analog Devices’ ADA4523-1BCPZ, a zero-drift amplifier that forms a buffer between the voltage reference and the ADA4523 reference input pin.

Finishing off the design, Analog Devices’ ADuM14xE digital isolator isolates and further protects the circuitry.

Go beyond the precision specifications

STOP. Now, imagine that you are now the analog designer for your group. Congratulations: you have joined an elite class of engineers, though it will remain an ongoing learning experience. With that in mind, I trust you will allow me to make some recommendations that you might not have heard before. To start, throw your analog tutorials aside and consider a different way to tackle some fundamental problems, beginning with the power supply system, the quiescent current, noise, and precision.

In my many years of analog design, this technique often only occurred to me at the end of my design cycle, and yes, I went through painful redesign and refining efforts. Now you don’t have to.

Also, it is clear that the industry trend is towards lower power dissipation and smaller geometries. Fortunately for us, we are not so far into this specific design that we cannot take a step back and look at the circuit fundamentals: supply voltage, quiescent current, and the power supply rejection ratio (PSRR).

The change that I am suggesting is to tailor-fit the power solution as a critical value proposition of the signal chain design, equal to—if not above—the precision goals. The design steps and considerations are now:

1. Create your signal chain block diagram.
2. Select possible components in the diagram.
3. Define the:
   1. Supply voltage
   2. Supply current
   3. PSRR

If you don’t consider the power specifications early on and in line with your design proposal, the design complexity increases by leaps and bounds. This is due to the addition of complex background power supply designs, with an ensuing increase in power consumption due to the quiescent current of the components. The result? An overall unsatisfactory design that may send you back to the drawing board.

So, let’s look at the power supply voltage, current, and PSRR budgets.

Designing with power in mind

The switch (or protection) and gain (or programmable instrumentation amplifier) stages require more extensive power supply voltage ranges, such as ±15 volts to provide ample signal range for the input sensor. The ADC driver, reference, reference driver, and isolation blocks require average power supply ranges, such as ±5 volts and +5.5 volts to accommodate signal requirements. And finally, the ADC’s digital interface to the controller requires lower voltages, such as +1.8 volts, keeping the overall power consumption low (Table 1).

Table 1: Respective device power requirements. (Table source: Analog Devices, modified by Bonnie Baker (for Power))

Per Table 1, there are three positive analog supplies (15 volts, 5.5 volts, and 5 volts), three positive digital supplies (5 volts, 5 volts, and 1.8 volts), and two negative analog supplies (-15 volts and -5 volts). The numbers in Table 1 point out the analog and digital power supply requirements. The ADA4896-2 dominates the PSRR system picture, and the generated quiescent power is 487 milliwatts (mW): minimal power at optimal performance.

Conclusion

As shown, the right combination of switches, amplifiers, and 24-bit ADC, such as Analog Devices AD4630-24, can help you create an ultra-high-precision system with the optimization of fast settling and low latency characteristics. However, to deliver the best overall performance, power must be given due consideration. The ADG5421F, LTC6373, ADA4896-2, AD4630-24, LTC6655LN, ADA4523, and ADuM14xE combine to create an adjustable voltage/current measurement signal chain that meets the application requirements for low latency measurements—while minimizing power consumption and forming an ideal instrumentation solution.