Digitizers convert the samples of an analog signal into digital values using analog to digital converters (ADCs). The more bits, the smaller the change in signal and ADC can detect. While 8 bits is most often used in oscilloscopes, sometimes you need finer resolution. Many digitizers and data-acquisition systems provide more bits of resolution, with 12 and 16 being common. Although more bits provides finer amplitude resolution and dynamic range, distortion and noise can add errors that go undetected with low-resolution digitizers. Additionally, increasing resolution comes with decreasing maximum sample rate/bandwidth.
The resolution of the ADC is the number of bits it uses to digitize the input samples. For an n bit ADC the number of discrete digital levels that can be produced is 2n. Thus, a 12 bit digitizer can resolve 212, or 4096 levels. The LSB (least significant bit) represents the smallest interval that can be detected. A 12-bit digitizer, for example is 1/4096 or 2.4 x 10-4. To convert the LSB into a voltage, we take the input range of the digitizer and divide by two raised to the digitizer’s resolution. Table 1 shows the LSB (least-significant bit) for a 1 V (±500 mV) input range for digitizers with resolutions of 8 bits to 16 bits.
One reason to use a high-resolution digitizer is to measure small signals. Based on the way we computed the minimum voltage level, you could use a lower resolution instrument and a smaller full scale range to measure smaller voltages. Many signals, however, contain both small signal and large signal components. For signals with both large and small voltage components, you need a high-resolution instrument, which has a large dynamic range and the ability of the digitizer to measure small signals and large ones simultaneously.
Let’s examine how a waveform would look if it is passed through digitizers with different resolutions. Figure 1 compares 12, 14 and 16-bit ideal digitizer responses to a segment of a ±200 mV damped sine waveform.
The segment selected is near the end of the waveform and has small amplitude. The 14 and 16 bit digitizers still have sufficient resolution to render the signal accurately but the 12-bit digitizer, with 100 µV resolution (based on a full scale level of ±200 mV) is unable to resolve levels smaller than 100 µV. The error in reading, for any resolution, will increase with decreasing signal amplitude. Keep in mind that this is an ideal case, later we will look at issues that limit the accuracy and precision in the real world.
source: http://www.edn.com/design/test-and-measurement/4431498/Digitizers–Finer-resolution-is-better
Comments are closed.
