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Figure 2 is an example of how an ADC is connected in a basic data-acquisition system (based on the ADS8326). The reference voltage is supplied by the REF3240; both reference and signal inputs are buffered by op amps. The ADC is supplied power, a reference voltage and the input signal voltage by analog circuitry; the digital interface to a host microcontroller or DSP is often a simple serial interface. High-speed applications may require more involved signal drive circuitry; the data sheet for most ADCs usually shows the recommended circuitry needed around the ADC.
DACs
A digital-to-analog converter block diagram would be similar to Figure 1, viewed from right to left: the DAC accepts a digital input, which is then decoded into a form that the DAC circuitry can use. The quantizer then creates a voltage or current corresponding to the digital input value. The sampler is not always on the analog side because most DACs have a means to latch the digital value provided by the interface.
While ADCs generally accept only voltage inputs, the output of a DAC may be a voltage or a current. Figure 3 shows an example of a multiplying architecture, which usually has a current output, using an external op amp to convert the current to a voltage. Many DACs provide this conversion circuitry in the device itself. Like the ADC, the DAC requires a reference input and may require analog circuitry on its output. The digital interface is not shown. This DAC8811 provides the current output, which is converted to a voltage through the output op amp and an internal feedback resistor.
How to read a data converter data sheet
As with any electronic component, the data converter data sheet is an essential resource. When deciding on a converter to use for a design, much of what you need to know is usually prominently displayed on the first page of the data sheet. The most important parameters for a data converter are speed , resolution and accuracy .
Speed
For an ADC, speed refers to the time it takes to convert an analog input into a digital value – the actual specifications are acquisition time and conversion time, which when combined limit how fast the ADC can output a conversion result. This can also be expressed as throughput, which is usually shown as the maximum sampling frequency.
For a DAC, the limiting factor on speed is usually the settling time, which is the time it takes for the output to settle to a new value from a previous value within a specified error band. A 1-µs settling time implies that the converter may be suitable for updating its output at a rate up to 1 MHz.
Resolution
As described previously and with the example shown in Equation 1, resolution refers to the either the number of bits (N) that the converter has as its input or output, the number of counts or codes (which is simply 2 N ), or the value of the least-significant bit in volts or amps. Higher resolution means that the converter can discriminate between smaller changes on the input signal or provide more precise control over an output signal.
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