Mixed-signal solutions are an inevitable trend. We want our electrical devices to interact with us. Our car doors should unlock as we approach. Our computer should turn itself on as we sit down in front of it—or at least enhance its screensaver to show how excited it is that we are returning. The screens on our phones should dim to save power when we are in low-light conditions. And, thermostats should keep our homes in a comfortable range of temperatures.
Humans are analog, meaning they are infinitely variable, while electronics communicate with their own language of voltages and currents. In the electronic realm, the initial readings from sensors (see the figure) are typically conditioned by input circuits, converted to digital signals, processed, converted back to analog signals, and properly conditioned to interact with us again.
The basic block diagram has remained relatively constant, absorbing changes in operating frequency, channel bandwidth, processing power, and technology. Before starting a discussion of system-level issues, let’s look at each of the blocks (from left to right) in a little more detail.
Steps In The Mixed-Signal Chain
Sensors bridge the gap between the physical world and the electrical one. All of the technology examples given above need sensors. They can be RF, proximity, ambient light, or temperature sensors. Temperature and pressure sensors are two of the most popular.
In the industrial market, there is growing interest in using accurate flow sensors. Proximity sensors bounce ultraviolet light to signal when something comes close. Ambient light sensors are a vital sentry in portable devices with any kind of screen, saving power and prolonging battery life.
The next step in the mixed-signal chain is the input amplifier, which must properly accept the signal from the sensor without loading or distorting the signal. The wide variety of sensors suggests that a broad range of amplifiers may be needed to pair correctly with the sensor. Buzzwords include the instrumentation amplifier, chopper-stabilized amplifier, low-noise amplifier, and input bias cancellation amplifier. Each system solution has different requirements and should be matched with the amplifier optimized for its needs.
If filtering is needed (in most cases, it is needed), it may be wrapped around this amplifier or added in series to the system. Filtering is an art all by itself. A handful of programs is available online to help you design the circuit your system requires. One new tool that combines practical circuit techniques with the expertise of years in filter design is Intersil’s iSim Active Filter Designer (http://web.transim.com/iSim/). It’s powerful and it’s free.
The analog-to-digital converter (ADC) is the most crucial selection in the signal chain and is often one of the first blocks chosen. Your choice will determine the number of bits in the system, the speed of the system, and one of the main power-consuming blocks. Choices of topology create different tradeoffs, depending on what’s required.
The mere mention of a few topology names is enough to send an ordinary person running in another direction: delta-sigma, pipelined, successive approximation, flash, and integrating. The most daunting topology to understand seems to be the delta-sigma ADC. This oversampling device typically operates at low frequencies, although a few break the megahertz barrier. Delta-sigmas offer the highest resolution of the pack, like 24 bits, in applications for weighing, temperature control, and instrumentation.
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