Digital to Analog

You can use a simple µC to continuously program an audio DAC so that it operates in a 20-bit resolution mode ( Figure 1 ). After power-on, the PCM1710 delta-sigma DAC (Burr-Brown Corp, www.burr-brown.com) operates in its default 16-bit resolution mode.

A typical way to add two binary words and provide an analog output is to use several digital ICs that drive a DAC. The circuit in Figure 1 eliminates the use of several digital-IC packages and, hence, the need for the digital power supply. The circuit simultaneously carries out addition and subtraction on two 8-bit binary words and presents the output in bipolar analog form.

Current-output D/A converters usually deliver their best accuracy if the current-output (summing-junction) pin remains at a constant dc voltage, because of the finite output impedance of the current-out DAC output (Figure 1). Two sources contribute to the output impedance. One is the impedance of the R-2R ladder used to binary-scale the bit currents; the second is the nonlinear output impedance of the transistor current switches.

JDAC and CENT_DAC are both simple Digital to Analogue converter to use with your IBM PC/AT or compatible. Those circuit are connected to Centronics parallel port and does not affect the normal port usage in any way. The circuits are transparent to computer, so it is not possible to program to see if those devices are connected or not.

High-speed DACs, such as Analog Devices' AD9776/78/79 TxDAC family, offer differential outputs, but, for low-end ac applications or high-precision level-setting applications, a single-ended current-output DAC with a differential-conversion circuit provides a novel approach to generating differential-waveform-control functions.

The circuit in Fig 1 loads 4 address bits and 8 data bits into IC8's 8-bit octal, two-quadrant multiplying DAC from a PC parallel port. Initially, the circuit loads the desired data on the D0 through D7 pins of the parallel port into shift register IC1 when the parallel port's STR pin goes low. Next, the circuit loads the desired address from the port's D0 through D3 pins into register IC2 when the port's ALF pin goes low.

The discrete comparator/DAC approach is already common in certain fields. Automatic test equipment, nuclear pulse-height discriminators, and automated time-domain reflectometers often use the technique whereby one comparator input is driven by the DAC, and the other is driven by the signal to be monitored.

The DS1602 and DS1603 from Dallas Semiconductor offer a simplified hardware solution for keeping time as well as tracking powered up time of a system. The DS1602 and DS1603 can be read and written directly by a microprocessor or microcontroller using simple software; however, a more creative software algorithm can be used to track years, months, days, day of week, time of day, etc.

Many embedded applications require the generation of analog signals. Although separate D/A converter IC?s exist on the market today, their extra price makes them prohibitive in cost sensitive embedded control designs. The following application note describes two DAC designs for generation of complex analog waveforms.: (PWM and R-2R Ladder).

To effect A/D and D/A conversion using ASICs, you usually need mixed-signal analog and digital processing. Mixed-signal silicon is more expensive than purely digital circuitry to process and test. In the case of FPGAs, you need external conversion circuits. However, it is possible to perform A/D and D/A conversions using strictly digital circuits. Figure 1 shows a dual 10-bit D/A converter that uses stochastic logic to provide the two converted analog voltages and their product.

This design idea describes a simple circuit to generate a programmable negative control voltage. It takes the output of a single supply digital-to-analog converter (DAC) and produces a variable negative voltage. The DAC output from 0V to +2.5V is converted to 0V to -5V at the output. These early DACs (such as the MX7837/MX7847 and MAX523) require both a positive and a negative supply rail to accommodate their negative output.

The transmitter output drivers present a low impedance to inbound surges and must be able to drive sufficient current in the primary of the transmit transformer in order to produce the required output pulse at the network interface. The receiver inputs present a high impedance to inbound surges and require very little input current to operate. For these reasons, the transmitter and receiver pins require different protection techniques.

A typical DSP system first converts an analog signal to digital, then performs windowing, and finally does a fast-Fourier transform. However, you can accelerate this process by swapping the converting and windowing steps and implementing the windowing in hardware. The multiplying DAC in Fig 1 performs the windowing of an input signal and produces the following output signal, where the digital code N ranges from 0 to 4095:

Early DACs contained standard R-2R ladder networks, and produced a negative output voltage. These early DACs, such as the MAX7837/7847 and the MAX523, require both positive and negative supply rails to accommodate their negative output. With the transition to single-supply ICs, however, many modern DACs operate with a single supply rail and an inverted R-2R ladder network.

Precision DACs are essential in many consumer, industrial, and military applications, but high-resolution DACs can be costly. Frequency-to-voltage converters have good nonlinearity specifications—typically, 0.002% for the AD650—and are inherently monotonic. This Design Idea shows how you can use a frequency-to-voltage converter and a DDS (direct-digital-synthesizer) chip for precise digital-to-analog conversion.

In an R-2R DAC design with supply voltages exceeding ±5V, large voltage glitches (up to 1.5V) can occur during the DAC's major-carry transitions. These glitches can propagate through the output buffer amplifier and appear at output. The slewing of the level shifters that control the top (VREF+) and bottom (VREF-) single-pole double-throw switches (S0 to SN) causes the glitches (Figure 1).

Despite the popularity of digital devices, real world signals are typically represented by analog signals. Digital control systems process real world analog signals by using an analog-to-digital converter (ADC) to convert analog signals to digital. Conversions back to analog signals are accomplished using a digital-to-analog converter (DAC). Maxim offers a complete line of precision DACs from 8 to 16 bits. It is important to find a DAC that meets to requirements of the application.

You can effectively double the sample rate of a DAC by interleaving two DACs into a single unit. Updating each DAC on an alternating basis and switching to the appropriate output double the effective throughput of the overall system. It is essential to overall performance that you use a high-quality, high-speed switch in the multiplexing of the DACs' outputs.

The availability of a seemingly limitless variety of monolithic DAC chips makes it easy to implement most digital-to-analog-conversion applications with a single off-the-shelf device. Sometimes, an unusual set of requirements necessitates a multichip approach, however. One example of such a requirement is the need for nonvolatility of the DAC's setting in power-up and -down cycles.

An interface circuit you can use for high-speed D/A conversion, using the serial port in the 50-MHz TMS320C25-50 DSP chip. An AD7568 12-bit DAC provides eight output channels and a serial input. You can interface the serial input to the TMS320C25 with a minimal amount of additional logic circuitry. The AD7568 provides great flexibility because it can work from one 5V supply, and it provides four-quadrant multiplication.