D/A Converter Specifications
The major performance specifications of a D/A converter include resolution, accuracy, conversion speed, dynamic range, nonlinearity (NL) and differential nonlinearity (DNL) and monotonicity.
The resolution of a D/A converter is the number of states (2n into which the full-scale range is divided or resolved. Here, n is the number of bits in the input digital word. The higher the number of bits, the better is the resolution. An eight-bit D/A converter has 255 resolvable levels. It is said tohave a percentage resolution of (1/255) × 100 = 0.39 % or simply an eight-bit resolution. A 12-bit D/A converter would have a percentage resolution of (1/4095) × 100 = 0.0244 %. In general, for an n-bit D/A converter, the percentage resolution is given by (1/2n − 1) × 100. The resolution in millivolts for the two cases for a full-scale output of 5 V is approximately 20 mV (for an eight-bit converter) and 1.2 mV (for a 12-bit converter).
The accuracy of a D/A converter is the difference between the actual analogue output and the ideal expected output when a given digital input is applied. Sources of error include the gain error (or full-scale error), the offset error (or zero-scale error), nonlinearity errors and a drift of all these factors.
The gain error is the difference between the actual and ideal output voltage, expressed as a percentage of full-scale output. It is also expressed in terms of LSB. As an example, an accuracy of ±0.1 % implies that the analogue output voltage may be off by as much as ±5 mV for a full-scale output of 5 V throughout the analogue output voltage range. The offset error is the error at analogue zero.
Conversion Speed or Settling Time
The conversion speed of a D/A converter is expressed in terms of its settling time. The settling time is the time period that has elapsed for the analogue output to reach its final value within a specified error band after a digital input code change has been effected. General-purpose D/A converters have a settling time of several microseconds, while some of the high-speed D/A converters have a settling time of a few nanoseconds. The settling time specification for D/A converter type number AD 9768 from Analog Devices USA, for instance, is 5 ns.
This is the ratio of the largest output to the smallest output, excluding zero, expressed in dB. For linear D/A converters it is 20 × log2n, which is approximately equal to 6n For companding-type D/A converters, it is typically 66 or 72 dB.
Nonlinearity and Differential Nonlinearity
Nonlinearity (NL) is the maximum deviation of analogue output voltage from a straight line drawn between the end points, expressed as a percentage of the full-scale range or in terms of LSBs. Differential nonlinearity (DNL) is the worst-case deviation of any adjacent analogue outputs from the ideal one-LSB step size.
In an ideal D/A converter, the analogue output should increase by an identical step size for every one-LSB increment in the digital input word. When the input of such a converter is fed from the output of a counter, the converter output will be a perfect staircase waveform. In such cases, the converter is said to be exhibiting perfect monotonicity. A D/A converter is considered as monotonic if its analogue output either increases or remains the same but does not decrease as the digital input code advances in one-LSB steps. If the DNL error of the converter is less than or equal to twice its worst-case nonlinearity error, it guarantees monotonicity.