DAC Digital-To-Analog Converter
Digital-to-analog converter, also known as D / A converter, referred to as DAC, is a device that converts digital quantities into analog. The D / A converter is basically composed of 4 parts, namely the weight resistance network, operational amplifier, reference power supply and analog switch. Digital-to-analog converters are generally used in analog-to-digital converters. Analog-to-digital converters are A / D converters, or ADCs for short, which are devices that convert continuous analog signals into discrete digital signals.
DAC, as the name implies, is to convert digital signals into analog signals.
The N-bit digital signal corresponds to 2 ^ N-1 analog levels of different sizes. The main performance indicators of DAC include resolution, maximum operating frequency, settling time, power consumption, etc.
The DAC is mainly composed of digital registers, analog electronic switches, bit weight networks, summing operational amplifiers and reference voltage sources (or constant current sources). Use the digits of the digital quantity stored in the digital register to control the analog electronic switches of the corresponding bits, so that the bit with the number 1 generates a current value proportional to its bit weight on the bit weight network, and then the current value is calculated by the operational amplifier. Sum and convert to a voltage value.
According to the different bit weight networks, different types of DACs can be formed, such as weight resistance network DAC, R–2R inverted T-shaped resistance network DAC, and single-value current network DAC. The conversion accuracy of the weighted resistance network DAC depends on the reference voltage VREF, as well as the accuracy of the analog electronic switch, operational amplifier, and each weighted resistance value. Its disadvantage is that the resistance of each weight resistor is different. When there are many digits, the resistance is very different. This brings great difficulty to ensure accuracy, especially for the production of integrated circuits. Therefore, the integrated DAC This circuit is rarely used alone.
It consists of several identical R, 2R network sections, each section corresponding to an input bit. The nodes are connected in series to form an inverted T-shaped network. The R–2R inverted T-shaped resistor network DAC is a kind of faster operating speed and more applications. Compared with the weight resistance network, because it only has two resistance values of R and 2R, it overcomes the shortcomings that the resistance value of the weight resistance is large and the resistance value is large.
The current-type DAC switches the constant current source to the resistance network. The internal resistance of the constant current source is extremely large, which is equivalent to an open circuit. Therefore, together with the electronic switch, the influence on its conversion accuracy is relatively small. Saturated ECL switch circuit makes this kind of DAC can realize high-speed conversion with high conversion accuracy.