The Trade-off and Analysis of the Selection Requirements of Low-speed Digital-to-analog Converters
For a low-speed DAC selection, it is important to decide whether the design is closed-loop , open- loop , or "set and then do not need to interrogate again." Each design requires a DAC with some key performance specifications.
Closed loop system
The closed-loop system includes a feedback path to detect and calibrate any errors. Sensors monitor output based on physical parameters such as servo motors, flow valves, or temperature detection units. The sensor then feeds data back to the controller, and the controller uses this information to decide if correction is needed.
DACs and analog-to-digital converters (ADCs) are key components at the heart of closed-loop systems. The DAC is used in the feed-forward path to regulate the system and the ADC is used in the feedback path to monitor the effect of these adjustments. Together they apply and detect analog control signals to truly adjust the parameters they control.
Motor control is an example of such a closed-loop system, as detailed in Figure 1. First, a desired output (set point) is added to the controller, and the controller compares this output with the feedback signal. If correction is required, the controller adjusts the DAC's input code and the DAC then generates an analog voltage at its output. The DAC's output voltage is amplified by a power amplifier to provide the motor with the required drive current.
In the next stage of this closed-loop system, a tachometer is used to measure the rotation speed of the motor. The rotation signal is the actual output or variable process of the closed-loop system. The ADC digitizes the output of the tachometer and sends the data to the controller where the algorithm decides whether any corrections need to be made on the DAC output and on the final motor. In this way, the error is reduced to an acceptable level. Ideally, feedback allows the closed-loop system to eliminate all errors, effectively limiting any error sources such as noise, temperature, external forces, or other unwanted signals.
The performance of closed-loop systems depends on the exact feedback path, including sensors and ADCs. In essence, the feedback path compensates for the error in the feedforward path. Because the DAC is in the feedforward path, its integral nonlinearity (INL) error is automatically compensated. The INL error is the deviation between the actual transfer function and the ideal transfer function at the DAC output. However, the DAC must have good differential nonlinearity (DNL) and must be monotonic with respect to the number of bits specified in the datasheet. The DNL error is the difference between the actual voltage change at the analog output of the DAC and the ideal voltage step (equivalent to 1 least significant bit (LSB) step in the DAC input code). A monotonic DAC means that the analog output always increases or stays the same as the digital encoding increases (and vice versa). DNL specifications that are always greater than -1LSB mean monotonicity. Figure 2 shows the transfer function of the DAC analog output voltage relative to the DAC input code.
If the DAC is not monotonic, there will be a region where negative feedback becomes positive feedback. This can cause oscillations that can eventually destroy the motor.
Figure 1: An example of a closed-loop system
Figure 2: DNL transfer function
Open loop system
The open loop system has no feedback path. This means that the system itself must be accurate. Open-loop control is useful for well-defined systems in which the relationship between the input code and its action on the load is known. If the load is not very predictable, closed-loop control is best used.
An example of an open-loop system is shown in FIG. In this example, the DAC drives the SET voltage pin of the Linear Technology regulator LT3080. The SET pin is the regulation set point for the input and output voltages of the error amplifier. The LT3080 has an output voltage range from 0V to absolute maximum rated output voltage.
The resolution of the DAC determines the step size adjusted by the SET pin. For example, an 8-bit DAC with a 5V reference has an LSB of 5V/28 = 19.5mV. A 12-bit DAC with the same 5V reference has a 1.2mV LSB, and a 16-bit DAC has a 76μV LSB. This means that for an ideal DAC, the analog output should increase by 76μV for every increment of the digital code.
Other important parameters in an open-loop system include offset, gain error, reference voltage error, and stability of these parameters over time and temperature changes. INL is particularly important because the INL of the DAC has a direct effect on the overall linearity of the system compared to a closed-loop system.
Figure 3: Open Loop System Example
"Set up without further questions"
A third application where the linearity of the DAC plays an important role is a system that "sets up without further intervention." In such systems, adjustment or calibration is performed only once, perhaps at the time of manufacture or installation. Therefore, such systems are initially a closed-loop system and then become open-loop again. Therefore, any parameters related to the initial accuracy (offset, gain error, INL) are not critical because these parameters are compensated during adjustment. But once feedback is removed, stability becomes critical. The data sheet performance specifications that indicate stability include gain error drift, offset, and reference drift.
Figure 4 shows an example of an application that requires no further questions after setting. In this figure, a lower resolution DAC drives a programmable gain amplifier that sets the voltage on the precision DAC offset adjustment pin. This lower resolution DAC is used to effectively calibrate the precision DAC gain offset during initial system calibration. This adjustment code can be stored in non-volatile memory and loaded each time the system is powered up.
Fig. 4: Example of a system that does not require further questions after setting
Learn more about DAC DC Performance Specifications
Once you have decided on the closed-loop, open-loop, or "set-and-not-need-to-use" system types, it is time to choose the best DAC. As mentioned before, some applications require coarse adjustments, which means that the system requires only a limited number of variable settings. In this case, an 8- or 10-bit resolution DAC is usually sufficient. For systems that require more fine-grained control, 12-bit DACs provide sufficient resolution. In today's market, 16-bit and 18-bit DACs provide the finest resolution per LSB.
The LTC2600 is a 16-bit, 8-channel DAC designed for closed-loop systems. Looking at its DC performance specification will find this to be obvious. The typical INL is ±12LSB with a maximum of ±64LSB. The typical INL vs. input code curve shows these performance specifications in the lower part of Figure 5. 16-bit monotonicity and ±1LSB DNL error allow precise control in the feedforward path. As mentioned earlier, the feed forward error is not important to the closed loop system as long as the DAC is monotonic.
In contrast, the new LTC2656 is an 8-channel DAC. All eight DACs provide 16-bit monotonicity and excellent ±4LSB INL error, making the device suitable for both open- and closed-loop systems. The typical INL code-dependent curve of all 8 DACs in the LTC2656 package is shown in Figure 5. In the 16-bit 8-channel DAC category, the LTC2656 provides the best INL.
The high linearity achieved by the 8 DACs in a single package is not an easy design task. The package pressure and voltage drift with temperature must be considered in the design. It is much easier for a single DAC to achieve a tighter INL performance specification. For example, Linear Technology's LTC2641 is a single 16-bit DAC that provides the highest DC performance specifications for ±1LSB INL and DNL.
In addition to INL and DNL, ​​other important DC performance specifications to consider are offset error (or zero scale error) and gain error (full scale error). The offset error indicates how well the actual transfer function matches the ideal transfer function at (or near) zero-scale input coding. The offset error is very important for precise control applications up to the ground. The LTC2656 offers a very low ±2mV maximum offset error.
The gain error indicates how well the slope of the actual transfer function matches the slope of the ideal transfer function. Gain and full-scale errors are sometimes used interchangeably, but full-scale errors include both gain and offset errors. The LTC2656 provides a maximum gain error of ±64LSB, which is equal to 0.098% of the full-scale (64/65536) and is a very small maximum gain error.
DACs with very good offset and gain errors may allow the system not to run the calibration cycle of the software in the controller or FPGA. A DAC that drifts very little over time and temperature also makes the design simpler because the system engineer does not need frequent calibration.
Figure 5: Comparison of LTC2656 and LTC2600
Figure 6: LTC2656 block diagram
±10V Output DACs
The previously mentioned DACs are for single-supply or unipolar 0V to 5V systems. However, some closed-loop, open-loop, or "set-after-time" systems require ±10V DACs. For these high-voltage systems, designers can perform gain and level shifting with unipolar 0V to 5V DACs with programmable gain amplifiers, or they can provide ±10V signals directly from the DAC.
Linear Technology offers single-, dual-, and quad-channel DACs for customers to choose from, these DACs provide up to ± 10V output voltage. The LTC1592 is an example of a single 16-bit DAC that provides two unipolar and four bipolar software programmable output voltage ranges, including 0V to 5V, 0V to 10V, ±2.5V, ±5V ±10V and -2.5V to 7.5V. Therefore, the same DAC can be used for both unipolar and bipolar systems without having to completely reset the controller. For example, changing the DAC output range from 0V to 5V to ±10V requires only changing to two bits in the DAC serial bitstream.
in conclusion
The DAC is a key component of an open-loop, closed-loop, or "set-and-don't care" system. Each of these systems requires a DAC to provide different levels of accuracy and resolution. At certain resolutions, there are always some factors that need to be weighed, such as price, package size, reference accuracy, and output impedance. For a system with the highest accuracy, it is important to select the DAC not only to consider the number of bits provided on the first page of the data sheet, but also to take into account DC performance specifications such as INL, DNL, ​​offset error, and gain error. How high accuracy.
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