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overview
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In the 1950s and 1960s, DC performance specifications such as integral nonlinearity, differential nonlinearity, monotonicity, no missing codes, gain error, offset error, and drift were mainly used to represent the performance characteristics of data converters. At the time, these specifications were sufficient because most early applications (except for PCM and radar) involved only DC or low frequency signals in applications such as industrial measurement and process control. In the 1970s and 1980s, with the emergence of microprocessors and digital signal processing (DSP) technology, in order to meet the needs of more complex signal processing applications, converters need to measure the signal-to-noise ratio (SNR), spurious-free dynamic range ( SFDR) and other dynamic performance specifications.
overall architecture process
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Modern data converter applications cover the entire spectrum from low frequency industrial measurements to wideband radio receivers. Although the importance of dc characteristics decreases with increasing signal frequency, it still plays an important role in many applications. For example, in IF sampling applications, large gain and/or offset errors can cause signal clipping, degrading SNR and SFDR performance. In applications requiring matched converters, such as interleaving, simultaneous sampling, and I/Q signal processing, matching the relative gain and offset between converters is critical. The purpose of this article is to describe various DC performance characteristics of a data converter so that the reader understands the significance of the corresponding section in the ADC or DAC data sheet.
Explanation of technical terms
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ADC:
Analog-to-digital converter, or A/D converter, or ADC for short, usually refers to an electronic component that converts an analog signal into a digital signal. A common analog-to-digital converter converts an input voltage signal into an output digital signal. Since the digital signal itself has no practical significance, it only represents a relative size. Therefore, any analog-to-digital converter needs a reference analog quantity as a conversion standard, and the more common reference standard is the largest convertible signal size. The output digital quantity represents the magnitude of the input signal relative to the reference signal
technical details
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It should be noted that for DAC and ADC, either the input or output is a digital signal, so its signal has a quantized nature. In other words, an N-bit word represents one of 2 N possible states, so an N-bit DAC (with a fixed reference voltage) can only have 2 N possible analog outputs, and an N-bit ADC can only have 2 N possible digital output. As mentioned earlier, analog signals are typically voltages or currents. The resolution of a data converter is expressed in several different ways: least significant bit (LSB), parts per million of full scale (ppm FS), millivolts (mV), etc. Different devices (even from the same manufacturer) are characterized in different ways, so converter users must understand how to convert between the different specifications in order to make meaningful comparisons between different devices. The least significant bit sizes for various resolutions are shown in the figure
The ideal transfer characteristics of a 3-bit unipolar DAC and a 3-bit unipolar ADC are shown. In a DAC, both the input and output are quantized and the graph consists of 8 points - although the lines formed by these points can be discussed, it must be remembered that the actual transfer characteristic is not a straight line but a number of discrete points
The input to the ADC is analog in nature and not quantized, but its output is quantized. Therefore, the transfer characteristic consists of 8 horizontal steps. When considering the offset, gain, and linearity of an ADC, we focus on the line connecting the midpoints of these steps—often called the code center. For DACs and ADCs, digital full scale (all "1s") is equivalent to 1 LSB below analog full scale (FS). (Ideal) ADC conversions occur ½ LSB above zero, and every 1 LSB thereafter until 1½ LSB below analog full scale. Since the ADC's analog input can accept any value, while the digital output is quantized, there can be up to ½ LSB of error between the actual analog input and the exact value of the digital output. This is called quantization error or quantization uncertainty, as shown. In AC (sampling) applications, this quantization error results in quantization noise, which is described in detail elsewhere.
The example of uses a unipolar converter whose analog port only has a single polarity. This is the simplest type, but bipolar converters are generally more efficient in real-world applications. Bipolar converters fall into two categories: the simpler category is simply a unipolar converter with a negative offset of exactly 1MSB (many converters are designed so that the offset can be turned on or off as needed so that it can be used as a unipolar or bipolar converters); another type, called sign-to-magnitude converters, is more complex, with N bits of magnitude information and an additional bit equivalent to the sign bit of the analog signal. Sign-magnitude DACs are very rare, and sign-magnitude ADCs are primarily used in digital voltmeters (DVMs). Unipolar, offset binary and sign magnitude are shown.
The four DC errors of a data converter are offset error, gain error, and two types of linearity errors (differential and integral). Offset error and gain error are similar to offset error and gain error in an amplifier, as shown in the figure for the bipolar input range. (Although offset error and zero error are equal in amplifiers and unipolar data converters, they are not equal in bipolar converters and must be carefully distinguished.
summary
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The transfer characteristics of DAC and ADC can be expressed as a straight line obtained by D=K+GA, where D is a digital code, A is an analog signal, and K and G are constants. In a unipolar converter, the ideal value of K is 0; in an offset bipolar converter, it is –1MSB. Offset error is the difference between the actual value of K and the ideal value. Gain error is the difference between G and its ideal value, usually expressed as a percentage difference between the two, but can also be defined as the contribution of gain error to the total error at full scale in mV or LSB. Typically, the user of the data converter can adjust for these errors. Note, however, that the amplifier offset is trimmed at zero input and the gain is trimmed near full scale. The tuning algorithm for bipolar data converters is complex.