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Knowledge of digital potentiometer

Date:2018-03-24 17:36:48        Hits:574 
Introduction to digital potentiometers and explained.
Knowledge of digital potentiometer
Introduction to digital potentiometers and explained.
Digital potentiometer, also known as a numerical control potentiometer, is devices (integrated circuit) that use digital signals to control its resistance. Digital potentiometer compared with the mechanical potentiometer, change with programmable resistance, resistance to vibration, low noise, long life, important advantages such as resistance to environmental pollution, therefore, has been in the automatic detection and control, intelligent instruments and meters, consumer electronics and other successful applications in many important fields. However, the digital potentiometer has a high rated resistance error, large temperature coefficient, narrow passband, and small current (generally 1 ~ 3mA), which limits its application to a great extent.
 
Digital potentiometer product features.
1. Using the principle of the sensor, it has good linearity, precision, and temperature stability.
2, the software was adopted to realize functions, can be customized according to the required changes.
3. The working mode is non-contact, avoiding the wear of traditional potentiometer, long life, and high reliability.
4.Because the brush substrate in canceled the traditional potentiometer, effective stroke up to 360 °, blind area measurement is realized.
5. Output signal types (0 to 5 v / 0 to 10 v / 4-20 ma/serial digital signal output), convenient signal collection and processing.
6, can be realized through software effective stroke and the change of output signal, meet all kinds of special requirements.
7. Wide application and flexible use.

Digital potentiometer principle Introduction.
The word potentiometer generally has a bus interface, which can be programmed through a single chip or logic circuit. It is suitable for a variety of programmable analog device, such as programmable gain amplifier, programmable filter, programmable tones/volume control circuit, linear regulated power supply, and truly achieve the "put analog devices on the bus" (that is, the single-chip microcomputer through the bus control system to simulate the function block) this new design concept.
Since the digital potentiometer can replace the mechanical potentiometer, there is a similarity in principle. The digital potentiometer is an integrated three-terminal variable resistor and its equivalent circuit. When the digital potentiometer is used as a voltage divider, its high-end, low-end and sliding end are respectively represented by VH, VL, and VW. When used as adjustable resistors, RH, RL, and RW are used respectively.
The digital control part of digital potentiometer includes four digital circuit modules, including addition and subtraction counter, decoding circuit, saving and recovery control circuit and non-volatile memory. Using series, and the plus/minus counter in the input pulse and the control signal can be realized under the control of plus/minus counter, counter the cumulative data provided to decode circuit control switch array directly, but also save data transmitted to the internal storage. When the external counter pulse signal stops or the chip selection signal is invalid, the output end of the decoding circuit is only valid, so only one MOS tube guide is selected.
 
The memory of the digital control part is a kind of power-off non-volatile memory. When the circuit is powered on again after being powered off, the original control data is still stored in the digital potentiometer, and the resistance value between the middle tap and the two ends is still It is the result of the last adjustment.Therefore, the use efficiency of a digital potentiometer and mechanical potentiometer is basically the same. But due to the switch with the method of "disconnect" after first connection, so during the effective period of input count, the digital potentiometer resistance and expectations may have the certain difference, can only reach the expected value at the end of the adjustment.




Advantages of digital potentiometers.
1. The service life of digital potentiometer is long, usually in million units. Advantages, digital potentiometer variable resistance can be configured to
2 end, potentiometer center tap can be connected with high or low, to H termination highest voltage or low voltage side.
 
The disadvantage of a digital potentiometer
1. the digital potentiometer is restricted by CMOS technology.
2. cannot meet the negative power supply directly.
3. The temperature coefficient is too large. Disadvantages
4. the rated resistance is large.
5. The passband is narrower.
 
Distinguish the performance of digital potentiometers.
Digital potentiometer, or Digipot, facilitates the digital control and adjustment of analog circuit's resistance, voltage, and current. Digital potentiometers are usually used for power supply calibration, volume control, brightness control, gain adjustment, and offset/modulation current regulation of optical modules. In addition to basic functions, digital potentiometers provide many other functions to enhance system performance and simplify the design. These features include different types of non-volatile memory, zero detection, dithering key interface, temperature compensation and write protection. These functions are designed for different applications.
 
Basic digital potentiometer design.
The potentiometer is actually a three-terminal element (see figure 1a). Low-end VL is connected internally to the device or as pin output for easy design. The structure of the three-terminal digital potentiometer is essentially an adjustable partial pressure resistor with a fixed end to end resistor.
Variable resistance is a dual - end potentiometer with a variable resistance of the tap and a resistance string (see figure 1b). Adjust the tap position of the variable resistor digital potentiometer to change the end - to - end resistance of the digital potentiometer.

FIG. 1. (a) the structure of the three-terminal digital potentiometer is essentially an adjustable partial pressure resistor with a fixed end to end resistor. (b) a variable resistor is a double-ended digital potentiometer, which is connected to one end of the potentiometer.
In short, the digital potentiometer is analog output controlled by digital input, similar to the definition of a number/analog converter (DAC). Unlike DAC, DAC provides a buffered output, and most digital potentiometers cannot drive low resistance loads without an external buffer.
For digital potentiometers, the maximum tap current range is hundreds of microampere to milliampere. When the digital potentiometer tap is connected to the low resistance load, variable resistor and real digital potentiometer, it is important to ensure that under the condition of the worst job in the range of acceptable WIPER tap current. The worst load of variable resistance occurs when VW approaches VH. At this point, there may be no resistance to the current in the circuit except for the tap resistance. However, some applications may require a lot of the tap current, this kind of circumstance, need we consider the pressure drop of the potentiometer tap, the pressure drop limits the output dynamic range of digital potentiometer.
 
Improve design according to application requirements.
Digital potentiometers have a wide range of applications, and some designs may require additional devices to meet the "fine-tuning" requirements for digital potentiometers. For example, digital potentiometer Ω end-to-end resistance in the range of 10 k and 200 k Ω, and control the LED brightness often need small resistance. To solve this problem is DS3906, the fixed resistor in parallel with the chip and 105 Ω, 70 Ω to 102 Ω equivalent resistance. This configuration can be obtained under the 0.5 Ω step adjustment, accurate to adjust the LED brightness. Another solution is the multi-channel digital potentiometer, such as MAX5477 or MAX5487, which can be combined with multiple channels to obtain different resistances to achieve the resolution requirements of the digital potentiometer.
 
In some cases may need to be the more specific function of digital potentiometer, voltage or current regulation of temperature compensation to need, such as the optical module of optical drives bias, can choose a variable resistance based on the look-up table. Some digital potentiometers integrate EEPROM (calibrated data used to store temperature changes) and internal temperature sensors (for measuring ambient temperature). The digital potentiometer retrieves the corresponding value in the lookup table according to the measured temperature and adjusts the variable resistor. A digital potentiometer based on a temperature lookup table is usually used to modify the nonlinear temperature response of a circuit element, such as a laser diode or a photodiode. The temperature response of a nonlinear resistor can also be established according to the application requirements.
 
Nonvolatile memory is introduced into the digital potentiometer common function circuit, low-cost standard block based on EEPROM nonvolatile digital potentiometer (NV) during power-on reset (POR) into a known state. EEPROM can ensure the number of repetitions of 50,000 times, which greatly improves the reliability of the system relative to the mechanical potentiometer. One-time programming (OTP) digital potentiometer, such as MAX5427/MAX5428/MAX5429, adopts fuse setting to permanently save the default tap position. Like a digital potentiometer based on EEPROM, the OTP digital potentiometer is initialized to a known state. However, the POR state of the OTP digital potentiometer cannot be rewritten once it is programmed. Therefore, OTP is suitable for factory programming or product calibration. The fuse permanently sets the POR tap position of OTP digital potentiometer without locking the tap position. The tap of some OTP digital potentiometers can be adjusted after fuse programming; Some OTP digital potentiometers are permanently set up to obtain an accurate, calibrated resistance divider. Some digital potentiometers provide locking registers, or digital control input, which makes the digital potentiometer interface high resistance and avoids improper tap adjustment. It also reduces the power consumption of EEPROM digital potentiometer.
The digital potentiometer can complete the voltage and current calibration in the power supply or other systems that require factory calibration. Digital potentiometers can improve the manufacturer's production capacity and improve the calibration precision and repeatability index, compared with the time-consuming and inaccurate manual calibration of mechanical potentiometer or separation resistance. In addition, the no potentiometer is convenient for remote debugging and recalibration. When multiple voltages and/or currents need to be calibrated, it is ideal to use the DS3904/DS3905 and other NV digital potentiometers (figure 2). In this case, a small digital potentiometer can replace three mechanical potentiometers. Replacing a mechanical potentiometer with a digital potentiometer can also improve the flexibility of the circuit layout since the digital potentiometer does not require mechanical adjustment during installation or maintenance. Calibration is a typical application of OTP or EEPROM writing protection, in which EEPROM writing protection is more conducive to design.

FIG. 2.DS3904/DS3905 three-way non-volatile digital potentiometer, ideal for systems requiring calibration of multipath voltage/current. This small IC can replace three mechanical potentiometers.
Although it is not a digital potentiometer, the sampling/maintaining voltage reference of a simple single-line digital control interface such as DS4303 can also be used for product calibration (figure 3). Compact design is very accord with calibration requirements, voltage reference output before locked by control signal depends on the input voltage, output after lock, unless the reprogramming or power, otherwise the output will not change, has nothing to do with the input voltage. The latest products store the locked output voltage in the EEPROM, and the power can be restored after the power is plugged in.

FIG3. Non-volatile sampling/maintenance voltage reference DS4303, although not a digital potentiometer, it is ideal for product calibration. When calibrated, the DS4303 output (VOUT) depends on the input voltage (VIN) before the controlled signal (ADJ) is locked.
The improved key interface is a complement to the traditional interface (add/subtract and rotate control). This interface is used by the digital potentiometer MAX5486 with buffer output. This shaking button interface is based on the pressed time of the button and controls the tapping motion with the changing speed. The key interface does not need the microcontroller, which reduces the complexity of system design. Dithering key interface is especially important for volume control.
Digital potentiometers designed for audio applications usually provide zero detection circuits, and zero detection can suppress the audible noise when the tap is jumping from one location to another. This function enables the zero detection circuit to delay the tap action to VL close to VH. Many of the zero detection circuits also provide the maximum tap change delay, convenient dc regulation, and other specific circuits.
 
Conclusion
The simple and volatile digital potentiometer is still practical in system design, while the digital potentiometer and variable resistor designed for special applications provide more functions. At present, many designers hope to replace mechanical potentiometers, improve the reliability of the system and the performance of the whole operating temperature range, save the system microprocessor, or suppress the click/chip. For these requirements, digital potentiometers fully display its advantages, and the application of digital potentiometers is becoming more and more common.