In the previous column, I described the operation of three types of analog-to-digital converters (ADCs). Each converter takes a specific period to "convert" a voltage into a digital value. If you plan to measure a voltage from a sensor that puts out a signal that varies only slightly over time, some converters can keep up with the changes. If your signal changes rapidly, though, you may need to capture a sample of the unknown voltage and maintain it for the entire conversion period. Special devices, called sample-and-hold amplifiers or track-and-hold amplifiers, perform this function. (I'll use the abbreviation SHA for sample-and-hold amplifier.)
If during the conversion period your unknown signal can change by more than 1/2 the voltage "weight" of your ADC's least significant bit (LSB), you will need an SHA between a sensor and an ADC. Think of an SHA as freezing your unknown signal temporarily. An SHA quickly samples an unknown signal and produces a proportional charge on a low-leakage capacitor. The capacitor maintains the charge long enough to provide a stable voltage at the ADC's input during the conversion time.
Watch Your Hertz
The reality of how conversion rates and small signal changes affect the frequency response of a measurement system may cause a rude shock. Consider a 12-bit successive-approximation converter with a 10-V input range and a 25 [m]sec conversion rate. Half of the voltage attributed to the LSB amounts to 1.22 mV. So, for accurate conversions, an unknown signal must not change at a rate above 1.22 mV/25 [m]sec, or 48.4V/sec. For a 10-V full-scale input signal, the maximum frequency comes to a few hertz!
So, like most successive-approximation converters, this 12-bit ADC requires an SHA. Several circuit topologies let designers build an SHA suited to their needs, or designers can buy commercial SHA ICs. The SHA in Figure 1 includes a sampling capacitor in a closed-loop circuit.
Unfortunately, SHAs don't offer perfect performance. Like all circuits, they alter signals that pass through them. If your design requires a SHA, you should know how manufacturers describe and quantify the characteristics that will affect your signal. The diagram in Figure 2 identifies some of these characteristics and shows the timing relationships of the input and output signals for a typical device. Data sheets from various manufacturers may provide specifications in different ways. (Always consult data sheets for test conditions and measurement criteria.)
Last year at Hannover Fair, lots of people were talking about Industry 4.0. This is a concept that seems to have a different name in every region. I’ve been referring to it as the Industrial Internet of Things (IIoT), not to be confused with the plain old Internet of Things (IoT). Others refer to it as the Connected Industry, the smart factory concept, M2M, data extraction, and so on.
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