Offset error is the deviation of the DAC output from that of the ideal transfer function when gain error is zero. Gain error is the deviation of the slope of the converter's transfer function from that of the ideal transfer function (see Figure 1). These errors are characterized by a number of AC and DC performance specifications that determine the converter's static and dynamic performance.Ī number of factors affect static or DC performance. In addition, DAC operation is also affected by nonideal effects beyond those dictated by quantization and sampling. In an ideal DAC, the analog outputs are exactly one least significant bit ( LSB) apart, where one LSB is the full-scale analog output amplitude divided by 2 N, and N is the DAC resolution expressed in number of bits. The Nyquist theory states that the signal frequency (that is, the DAC output) must be less than or equal to one-half the sampling frequency to prevent sampling images from occurring in the frequency band of the DAC output. Sampling determines the maximum bandwidth of the DAC output signal according to Nyquist criteria. Quantization determines the maximum dynamic range of the converter and results in quantization error or noise in the output. Quantization and sampling impose fundamental, yet predictable, limits on DAC performance. In other words, the DAC output attempts to represent an analog signal with one that features finite resolution and bandwidth. However, the DAC's output is a signal constructed from discrete values (quantization) generated at uniform, but finite, time intervals (sampling). N is the number of digital input bits (resolution)Īnalog signals are continuous time-domain signals with infinite resolution and possibly infinite bandwidth. The analog output of the DAC output is the digital fraction represented as the ratio of the digital input code divided by 2 N times the analog reference value. For N bits, there are 2 N possible codes. The number and size of the fractions reflect the number of possible digital input codes, which is a function of converter resolution or the number of bits (N) in the input code. Then the digital input drives switches that combine an appropriate number of these fractions to produce the output. To generate the output, a reference quantity (either a voltage or a current) is divided into binary and/or linear fractions. The digital input may be TTL, ECL, CMOS, or LVDS, while the analog output may be either a voltage or a current. The transfer function for an example 3 bit DAC is shown in Figure 1. In these systems, the performance required of the DACs will be influenced by the capabilities and requirements of the other components in the system.Ī DAC produces a quantized (discrete step) analog output in response to a binary digital input code. DACs are often incorporated into digital systems in which real-world signals are digitized by analog-to-digital converters (ADCs), processed, and then converted back to analog form by DACs. Loudspeakers, video displays, motors, mechanical servos, radio frequency (RF) transmitters, and temperature controls are just a few diverse examples. The circuits that perform this step are digital-to-analog converters (DACs),and their outputs are used to drive a variety of devices. In many systems, this digital information must be converted back to an analog form to perform some real-world function. Real-world analog signals such as temperature, pressure, sound, or images are routinely converted to a digital representation that can be easily processed in modern digital systems.
0 Comments
Leave a Reply. |