ADC resolution

Figure 1.32 Increase in sensitivity via increase in ADC resolution, (a) A tiny signal of impurity at 8 3.90 appears clearly with a 16-bit digitizer, (b) The signal at 8 3.90 is smaller when a 12-bit digitizer is used, (c) The signal at 8 3.90 is hard to see when the spectrum is recorded with an 8-bit digitizer.

There is a poor ADC resolution in the intensity direction five bits represent a true reading of 2, so that a true value of 2 is represented by 11111 in the digital recorder. The true reading is always rounded down to the nearest integer. This means that possible levels are 0/31 (=binary 00000), 2/31 (=00001), 3/31, etc. Hence a true reading of 1.2 would be rounded down to 18/31 or 1.1613. Explain the principle of ADC resolution and show why this is so. 

Thus, resolution of ADC board limits not only the dynamic range but also the overall system accuracy. Alternatively, a higher resolution may be required to retain a certain level of accuracy within a given dynamic range. For example, a 0.5% accurate transducer with 80 dB dynamic range requires at least 12-bit ADC resolution. 

Amplifying a low-level signal by 10 or 100 times increases the effective resolution by more than 3 and 6 bits, respectively. On the other hand, increasing an ADC resolution cannot benefit the overall system accuracy if other components, such as amplifier or transducer, have a lower resolution. 

Other instrumental considerations are computer related and include factors such as memory size, word length and ADC resolution (9). Most modern FT nmr spectrometer systems are designed to satisfy the above requirements, but one must always verify proper operating parameters for the analysis being run. 

The plot shows that the signal has a high gradient at 2.5 V of approximately 50mV/°C and a span of more than 4 V. Nonhnearities are compensated in the subsequent digital processing if required. With a 10-bit ADC resolutions of 1°C and better are possible. 

The resolution of an ADC is expressed in terms of channels. It represents the maximum number of discrete voltage increments into which the maximum input pulse can be subdivided. ADC resolutions range from 4096 to 16,384 channels. Since commercial amplifiers can provide a maximum 10-V pulse, an ADC with a resolution of 4096 channels may subdivide 10 V into 4096 increments. Another quantity used is the conversion gain of the ADC. The conversion gain may be considered as a subset of the resolution. An ADC with a resolution of 16,384 channels may be used, depending on the application, with a conversion gain of 4096, or 8192, or 16,384 channels. 

Fig. 5.51 Time-of-flight curves for two different source and detector positions at the forehead, and instrument response functions. Left hnear scale, right logarithmic scale. Source-detector distance 6 cm, Laser 2.5 mW, 785 nm, 50 MHz. Acquisition time 20 s, ADC resolution 4,096 channels, time scale 400 ps/division...

Fig. 5.53 20 steps of a TOF sequence recorded at an adult human head by TCSPC memory swapping. Acquisition time 100 ms per curve, ADC resolution 1,024 channels. Diode laser 785 nm, 2.5 mW, detector H5773-20. Left Source-detector distance 8 cm, count rate 1.8T0 s. Right Source-detector distance 5 cm, count rate 4.5T0 s ... 

Fig. 5.127 File size versus acquisition time for the continuous flow and the FIFO mode. For the FIFO mode the amount of data depends on the average count rate, for the continuous flow mode on the time per step of the sequence and on the ADC resolution...

To illustrate the calculation, consider a pressure transducer with a 5 V full-scale range rated at 100 bar connected to a data logger with 8 bit ADC resolution and a 1 V full-scale range. According to the equation, the resolution equals 0.08 bar or about 0.08% ... 

ADC resolution is the total number of channels available within the ADC. It varies from model to model, but MCAs for germanium systems ought incorporate a 16k (16384), 8k (8192), or 4k (4096) channels ADC. It would be normal for an MCA to have available as many memory channels as there are ADC resolution channels. 

The theoretical effect of quantization by the ADC is discussed in Section 10.2.3. Above about 1 count, the ADC adds (in quadrature) the square root of 1/12 count (about 0.28 count). Below about 0.5 count, the ADC can yield any value between zero and 0.5 count. Typically, the actual noise added by the ADC should be on the order of less than 0.5 count (in quadrature), and the ADC resolution is typically selected so that the noise inherent in the signal chain is equal to about 2 ADC counts. 

Verify that ADC resolution is not contaminating noise measurements Source... 

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