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Digital to Analog Converters, DACs

Buy digital-to-analog converters DAC in Ukraine

A digital-to-analog converter, also known as a digital-to-analog converter or DAC for short, is a device that converts digital values to analog values. The D / A converter is mainly composed of 4 parts, namely a weight resistor circuit, an operational amplifier, a voltage reference and an analog switch. A digital to analog converter is commonly used in an analog to digital converter.

DAC (digital to analogue converter) concept

 A converter that converts discrete signals in the form of binary digital values to analog values based on standard values (or reference values), called a DAC.

The most common digital-to-analog converter converts parallel binary digital values to DC voltage or DC current. It is often used as the output channel of a computerized process control system and is connected to an actuator to realize automatic control of a manufacturing process. D/A converter circuits are also used in the design of analog-to-digital converters using feedback technology.

Composition and characteristics of the DAC

The DAC is mainly composed of digital registers, analog electronic switches, positional power supply, summing operational amplifier, and a voltage reference (or constant current source). Use the digits of the digital value stored in the digital register to control the analog electronic switches of the corresponding digits so that the digit with the digit 1 generates a current value proportional to its position weight in the position weights network, and then an operational amplifier monitors each Sum current value and converts it to a value voltage.

Depending on the difference in the bit network, various types of DACs can be formed, such as a resistance-weighted DAC, an R-2R inverted T-resistance DAC, and a single-digit current DAC. The DAC conversion accuracy of the impedance weighted circuit depends on the reference voltage VREF as well as the accuracy of the analog electronic switch, op amp and the value of each impedance weighted. Its disadvantage is that the resistance value of each weighting resistor is different. When the number of discharges is large, the resistance value varies greatly, which makes it very difficult to ensure accuracy, especially in the manufacture of integrated circuits. Therefore, an integrated DAC. This circuit is rarely used alone.

It consists of several identical R and 2R network sections, and each section corresponds to an input bit. The nodes are connected in series to form an inverted T-shaped network. Inverted T-Resistor R-2R DAC is a kind of faster speed and more applications. Compared to a weighted resistance network, since it only has two resistance values R and 2R, it overcomes the disadvantages of a large resistance value of the resistance weight and a large difference in resistance.

The current-type DAC switches the constant current source to the resistor network. The DC power supply has a large internal resistance, which is equivalent to an open circuit. Therefore, together with the electronic switch, the effect on the conversion accuracy is relatively small. The saturation ECL switch circuitry allows this DAC to achieve high conversion speed and high conversion accuracy [1].

DAC presentation

The main characteristics of a digital-to-analog converter include the following aspects:

  • Resolution

Refers to the ratio of the minimum output voltage (the corresponding digital input has only the least significant bit as "1") and the maximum output voltage (the corresponding digital input has all valid digits as "1"). For example, an N-bit D / A converter has a resolution of 1 / (2 ^ N-1). In practice, the method of expressing the size of the resolution is also expressed in the number of digits of the input digital quantity.

  • Linearity

The magnitude of the non-linear error represents the linearity of the digital-to-analog conversion. And the percentage of the ratio of the deviation of the ideal input and output characteristics to the full-size output is defined as non-linear error.

  • Conversion accuracy

The conversion accuracy of the D / A converter depends on the structure of the D / A converter IC and the configuration of the interface circuit. Unless factor in other D/A conversion errors, D / A precision is resolution. Therefore, in order to obtain a highly accurate D / A conversion result, you must first ensure that a D / A converter with sufficient resolution is selected. At the same time, the accuracy of the D / A conversion is also related to the configuration of the external circuit. When the error of the external circuit device or power supply is large, it will cause a large D / A conversion error. When these errors are exceeded at a certain level, digital-to-analog conversion will occur.

In the D/A conversion process, the main factors affecting conversion accuracy are offset error, gain error, nonlinear error, and differential nonlinear error.

  • Conversion speed

The conversion speed is usually determined by the tuning time. From the moment the input signal changes from all zeros to all ones until the output voltage stabilizes within the FSR ± ½LSB range (or the range specified in FSR ± x% FSR), this period of time is called the tuning time, i.e. maximum DAC response time, so use this to measure conversion speed.

  • DAC temperature coefficient

At full scale conditions, for each 1° C rise in temperature, the percentage change in power output is defined as a temperature coefficient.

  • Power supply rejection ratio

For high quality D/A converters, changing the voltage of the power supply used by the switching circuit and the op amp minimally affects the output voltage. Typically, the ratio of the percent full scale voltage change to the percent power supply voltage change is called the power supply rejection ratio.

  • Operating temperature range

Typically, the main environmental and operating conditions that affect D/A conversion accuracy are changes in temperature and power supply voltage. Since the operating temperature will affect the weighted resistance circuit of the op-amp, nominal accuracy can only be guaranteed over a specific operating range.

The operating temperature range of the best DAC is -40 ° C to 85 ° C, and the operating temperature range of the poor DAC is 0 ° C to 70 ° C. Most devices have static and dynamic indicators.

Measured at an operating temperature of 25 ° C, the effect of operating temperature on various accuracies is described by temperature coefficients such as temperature coefficient of displacement, temperature coefficient of gain, temperature coefficient of linear error, etc.

  • Offset error

Offset error (or zero point error) is defined as the deviation between the analog output value and the ideal output value when the digital input is all 0. For unipolar D / A conversion, the ideal analog output value is the zero volt point. For bipolar D / A conversion, negative full scale is ideal. The amount of deviation is usually expressed as the number of parts of the least significant digit or the percentage of deviation relative to the full scale.

  • Gain error

The slope of the input and output transfer characteristic of a D / A converter is called the gain or scale factor of the D / A conversion, and the deviation between the actual converted gain and the ideal gain is called the gain error (or scale error). The gain error uses the complete code after the offset error is eliminated.

The deviation between the output value and the ideal output value (full scale) when expressing the input data is usually expressed as the number of least significant digits or as a percentage of the deviation value relative to full scale.

  • Nonlinear error

The non-linear error of a D / A converter is defined as the maximum deviation between the actual conversion characteristic curve and the ideal characteristic curve and is measured as a percentage of full scale deviation. Converter circuit design typically requires that the nonlinear error does not exceed ± 1 / 2LSB.