Why you should integrate your signal chain

Guest columnist Tamara Schmitz, senior principal applications engineer at Intersil, looks at the signal chain from sensor to processor and considers the pros and cons of integration.

Sensors bridge the gap between the physical and electrical worlds. They can be RF, proximity, ambient light or temperature sensors. The next step in the mixed-signal chain is the input amplifier, which must accept the signal from the sensor without loading or distorting it.
  
If filtering is needed – and in most cases it is – it may be wrapped around this amplifier or added in ­series to the system. Filtering is an art by itself. There is a handful of programs available online to help design the circuit a system ­requires.

The analogue-to-digital converter (ADC) is the crucial selection in the signal chain and is often the first block chosen. The choice will det­ermine the number of bits in the system, its speed and one of the main power-consuming blocks.

Choices of topology create different trade-offs. 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 one to understand seems to be the delta-sigma ADC. 

This is an oversampling ADC that 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. 

Successive approximation ADCs, called “SARs” because they use a successive approximation register, offer a trade-off of resolution and speed. They operate in a range from 1kHz to a couple of megahertz and offer mid-range precision. 

An integrating converter is slow, taking time to average the input signal. That makes the integrating converter great for DC measurements since it can filter power-supply noise (50Hz or 60Hz).

The opposite side of the trade-off is the flash converter. This can operate over 1GHz, but is limited to about 10 bits of accuracy. To achieve this speed, a flash converter burns lots of power because it calculates the conversion in one step.

If you choose to back off on the power and use a two-stage solution, it is called a multi-stage converter. If you back off even more to three or more stages, you would typically call it a pipelined converter – these run from around 500kHz to 500MHz and can provide resolutions up to 16 bits.

The digital signal processing is commonly provided by a microcontroller or an FPGA and many of these parts are available with built-in ADCs and DACs. Built-in converters are sufficient for simple solutions, but may not provide the performance available in discrete ADC and DAC packages. If you do decide to use a discrete package, there is an array of speed, resolution, power and performance issues, just like the ADCs. There is a delta-sigma topology for DACs that oversamples similar to its high-quality ADC counterpart. 

Two simpler DACs are the R-2R and the resistor string. While the R-2R configuration relies on matching, the resistor string can guarantee monotonicity (each increase in input voltage gives a correlated increase in output voltage). The output amplifier buffers the DAC from whatever load needs to be driven. In some cases, this amplifier must also perform current-to-voltage conversion, depending on the output signal of the DAC. Filtering may be required in this stage, similar to the requirements of the input half.


If cost is your primary concern, then using a microcontroller with built-in ADC and DAC may be your best option. The next option is for the designers who prefer to choose ADCs and DACs in operating pairs – similar characteristics and usually from the same vendor.

Many designers are pushing for the integration of both sides of the signal chain. Technology has advanced enough to allow the op amp and some filtering to be co-­packaged, if not included in the same die as the converter. But why might you choose to ­de-integrate?

The product or application might be new and not fully defined. Second, there is no flexibility. What if you wanted to upgrade the filter to a higher order to compensate for a new, powerful interferer? What if you wanted to try a new configuration of converter? Integration makes our lives easier with system designs – as long as it doesn’t limit our capability at the same time.