Sample Stable Signals

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.)

Go to http://rbi.ims.ca/4917-626 for descriptions of timing characteristics shown in Figure 2, as well as other information.

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