So you need a digital-to-analog converter (DAC) in your circuit? This should not be much of a challenge. It is simply a matter of taking a digital word, such as 011, and turning it into an analog value (voltage or current). This task can be easily done with a few resistors and switches, right? That is right. You could string a slew of resistors and successfully change your digital word to an analog value. But you need to ask, “At what price?” Will you pay in the obvious dollars or more subtlety, will you pay in board space, accuracy, or speed?
Let's start with your application problem. In this series on DACs, we will center our attention on the use of precision DACs. Figure 1 shows a few typical precision DAC applications. Some common places where you will see these types of DACs today are as DC-biasing devices, control loops, gain control, and waveform generation.
Some of these precision DACs can be very helpful, if you need to do any type of DC-biasing to an amplifier or offset control in a system. Examples of DC-biasing applications can be found in most base stations. Appropriate DACs for these types of applications have a high level of integration and channel count of analog-to-digital converters (ADCs) and DACs in a single package.
Control loops, in motor driving applications, use DACs to control speed variations. In these applications, you measure the speed of the motor and then adjust the DAC accordingly to control speed variations. Appropriate DACs for these types of applications have excellent linearity specifications.
Some DACs excel in the AC specification arena. A waveform generator is a simple application for DACs in this application area.
For all these applications, one size DAC does not fit all. Figure 2 shows the current state of the art for DACs in the industry. This figure maps out the delta sigma ( Δ Σ), resistor string, R-2R, and current steering architectures with respect to resolution and settling time. In this series of articles, our focus will be heavily on precision DACs, which use the resistor string and R-2R topologies. Having various topologies allows us to trade-off in small size, low settling time, excellent accuracy, and repeatability, while keeping the price low.
In the upper left corner of Figure 2 , the target applications of the delta-sigma DACs are audio, instrumentation and measurement, and seismic applications. The settling time of these DACs is on the slow side; however, their resolution ranges from 16-bits to over 20-bits.
The resistor string and R-2R topologies span across a large converter resolution with setting times ranging from 0.5 us to 100s of microseconds. The resistor string DACs have smaller footprints leading to lower cost. These DACs are voltage output devices. For simplicity, think of the design as a single string of 2N matched resistors that divide down the reference voltage. In simplistic terms, as you change codes, an output amplifier taps into different points on the string to produce the appropriate voltage.
The R-2R DACs are more accurate than the resistor string devices, while also producing an analog voltage output signal. The multiplying DACs (a style of R-2R DACS) are more accurate than the resistor string DACs, but these DACs produce an analog current output signal.
The current steering DAC topology targets high-speed video and communications applications. Their accuracy does not match the resistor string or R-2R DAC. However, the converter resolution of these devices can be very high, considering the settling-time performance. Their update rates are in the mega-samples-per-second (MSPS) range, but is heavily dependent on the characteristics of the output driver.
Choosing a DAC for your applications may seem simple, but it is important to think about the appropriate DAC topology to match your application requirements.
Next month, we will center our attention on the various DAC topologies and their particular advantages and disadvantages as they cater to specific application requirements.
- Baker, Bonnie, A DAC for all precision occasions, Analog Applications Journal (slyt3001), Texas Instruments, 3Q 2008
- McCulley, Bill, Bridging the Divide: A DAC Applications Tutorial (Precision Signal Path), Signal Path Designer (snaa129), Texas Instruments, 2011
- Download datasheets: DAC7311, DAC8562, DAC8718, AMC7812, AMC7823, DAC7678, DAC5578, DAC8568, DAC7571