Digital potentiometer, also known as CNC programmable resistor, is a new CMOS digital and analog mixed signal processing integrated circuit that replaces traditional mechanical potentiometer (analog potentiometer). The digital potentiometer is controlled by a digital input and produces an analog output. Depending on the digital potentiometer, the maximum tap current can range from a few hundred microamps to a few milliamperes.

Ⅰ Product Features

◆Produced according to sensor principle, with good linearity, accuracy, and temperature stability.

◆Using software to realize the functions, it can be customized according to the changes in the use requirements.

◆The working method is non-contact, avoiding the abrasion of the traditional potentiometer, long life, and high reliability.

◆Since the brush substrate in the traditional potentiometer is eliminated, the effective stroke reaches 360°, realizing no blind spot measurement.

◆Multiple output signal types (0-5V/0-10V/4-20mA/serial digital signal output), convenient for signal acquisition and processing.

◆The effective stroke and the output signal can be changed through software to meet various special requirements.

◆Wide application range and flexible use.

Ⅱ Principle 

The potentiometer is an instrument constructed on the principle of compensation. A high-precision measuring instrument based on the principle of mutual compensation between the measured voltage and the known voltage. There are two types: AC and DC. It is used to measure voltage, current, and resistance, the AC potentiometer can also measure magnetism. The digital potentiometer adopts the numerical control method to adjust the resistance value, which has obvious advantages such as flexible use, high adjustment accuracy, no contact, low noise, not easy to be stained, anti-vibration, anti-interference, small size, long life, etc. It can replace mechanical potentiometers in many fields.

Digital potentiometers generally have a bus interface and can be programmed through a microcontroller or logic circuit. It is suitable to form a variety of programmable analog devices, such as programmable gain amplifiers, programmable filters, programmable linear stabilized power supplies, and tone/volume control circuits. It truly realizes "put analog devices on the bus" (that is, the microcontroller controls analog function block through of bus control system)

Since the digital potentiometer can replace the mechanical potentiometer, the two have similarities in principle. The digital potentiometer belongs to the equivalent circuit of an integrated three-terminal variable resistance device. When the digital potentiometer is used as a voltage divider, its high end, low end, and sliding end are represented by VH, VL, and VW respectively; and when used as an adjustable resistor, they are represented by RH, RL, and RW, respectively.

The digital control part of the digital potentiometer includes 4 digital circuit modules, including an up/down counter, a decoding circuit, a save and restore control circuit, and a non-volatile memory. Use serial input and parallel output up/down counters to realize up/down counting under the control of input pulses and control signals. The counter provides the accumulated data directly to the decoding circuit to control the switch array and also transmits the data to the internal memory. When the external counting pulse signal is stopped or the chip selection signal is invalid, only one output terminal of the decoding circuit is valid, so only one MOS tube is selected to be turned on.

The memory of the digital control part is a non-volatile memory after power failure. When the circuit is powered on again after a power failure, the digital potentiometer still saves the original control data, and the resistance value between the middle tap and the two ends is still The result of the last adjustment. Therefore, the use effect of the digital potentiometer and the mechanical potentiometer is basically the same. However, because the switch works in the "connect first and then disconnect" method, the resistance value of the digital potentiometer may be different from the expected value during the valid period of the input count, and the expected value can only be reached after the adjustment is completed.

Ⅲ Construct programmable oscillator with digital potentiometer

The digital potentiometer (digiPOT) is versatile and has a wide range of applications, for example, to filter or generate AC signals. However, sometimes the frequency must be able to be changed and adjusted according to application requirements. In this type of design, a programmable solution that supports frequency adjustment through an appropriate interface is extremely useful, and in some cases, is very helpful for development. This article introduces a simple and easy way to construct a programmable oscillator, in which the oscillation frequency and amplitude can be adjusted independently by using a digital potentiometer.

Figure 1. Programmable Wien bridge oscillator with stable amplitude, where the resistor is replaced by digiPOT.

Figure 1 shows a typical diode-stabilized Wien bridge oscillator, which can be used to generate an accurate sine wave signal of approximately 10 kHz to 200 kHz at the output (VOUTPUT). The Wien bridge oscillator has two bridge circuits, one is formed by a band-pass filter and the other is formed by a voltage divider. In addition to the ADA4610-1 rail-to-rail precision amplifier, this example also uses the AD5142digiPOT, which contains two independently controllable potentiometers, each with a resolution of 256 steps. The resistance value is programmed through SPI, as shown in Figure 2. Alternatively, the AD5142A controlled by I²C can be used. Both can be used as 10 kΩ or 100 kΩ potentiometers.

In the classic oscillator circuit shown in Figure 1, the paths of R1A, R1B, C1, and C2 form positive feedback, while R2A, R2B, and two parallel diodes D1 and D2 or their resistance RDIODE form negative feedback. In this case, you can use formula 1:

In order to achieve continuous and stable oscillation, the phase shift in the loop gain needs to be eliminated. Expressed by the formula, the oscillation frequency:

Among them, R represents the programmable resistance value on AD5142:

D represents the decimal equivalent of the programmable digital code in AD5142, and RAB represents the total resistance of the potentiometer.

In order to maintain oscillation, the Wien bridge oscillator should be relatively balanced, that is, positive feedback gain and negative feedback gain must be coordinated. If the positive feedback (gain) is too large, the oscillation amplitude or VOUTPUT will increase until the amplifier saturates. If negative feedback is dominant, the oscillation amplitude will be attenuated accordingly.

Figure 2. AD5142 circuit

In the circuit shown here, the gain R2/R1 should be set to about 2 or greater. This will ensure that the signal starts to oscillate.

However, turning on the diodes in the negative feedback loop alternately can also cause the gain to be temporarily less than 2, which stabilizes the oscillation.

Once the required oscillation frequency is determined, R2 can be used to tune the oscillation amplitude regardless of frequency. It can be calculated by the following formula:

Therefore, the variables ID and VD represent the diode forward current and diode forward voltage through D1 and D2, respectively. If R2B is short-circuited, an oscillation amplitude of approximately ±0.6 V will occur. When the magnitude of R2B is correct, balance can be achieved, so that VOUTPUT converges. In the circuit shown in Figure 1, R2B uses a single 100 kΩ digital potentiometer.

By using the described circuit and 10 kΩ double digital potentiometer, the 8.8 kHz, 17.6 kHz, and 102 kHz oscillation frequencies can be tuned with resistance values of 8 kΩ, 4 kΩ, and 670 Ω, respectively, with a frequency error as low as ±3%. Increasing the output frequency may affect the frequency error. For example, at 200 kHz, the frequency error will increase to 6%.

When using this type of circuit in frequency-related applications, do not exceed the bandwidth limit of the digital potentiometer, as this value is a function of the programmable resistance. In addition, the frequency tuning shown in Figure 1 requires that the resistance values of R1A and R1B are the same. However, the two channels can only be set in sequence and will result in a transient critical intermediate state. For some applications, this situation is unacceptable. In these cases, a digiPOT (such as AD5204) that supports daisy-chain mode can be used so that the resistance value can be changed at the same time.

Ⅳ The difference between digital potentiometer and digital-to-analog converter

There are two options for using digital input to control and fine-tune analog output: potentiometer">digital potentiometer and digital/analog converter (DAC), both of which use digital input to control analog output. The analog voltage can be adjusted through the digital potentiometer; through DAC Both the current and the voltage can be adjusted. The potentiometer has three analog connections: positive pin, middle pin (or analog output), and ground pin (see Figure 3a). DAC has three corresponding pinpoints: the positive pin corresponds to the positive reference voltage, The middle pin corresponds to the DAC output, and the ground pin may correspond to the ground or negative reference voltage end (see Figure 3b).

Figure 3. Digital potentiometer and digital-to-analog converter

There are some obvious differences between a DAC and a digital potentiometer. The most obvious difference is that a DAC usually includes an output amplifier/buffer, while a digital potentiometer does not. Most digital potentiometers require external buffers to drive low-impedance loads. In some applications, users can easily choose between DAC and digital potentiometer; and in some applications, both can meet the demand. This article compares DAC and digital potentiometer so that users can make the most appropriate choice.

D/A converter

DAC usually adopts a resistor string structure or R-2R ladder architecture. When a resistor string is used, the DAC input controls a set of switches, and these switches divide the reference voltage through a series of matched resistors. For the R-2R ladder architecture, the positive reference voltage is divided by switching each resistor to generate a controlled current. The current is sent to the output amplifier, the voltage output DAC converts this current into a voltage output, and the current output DAC buffers the R-2R ladder current through the amplifier and outputs it. If you choose a DAC, you must also consider specific indicators, such as serial port/parallel port, resolution, number of input channels, current/voltage output, cost, etc. For systems that focus on speed, you can choose a parallel interface; if you focus on cost and size, you can choose a 3-wire or 2-wire serial port. This kind of device has fewer pins, which can significantly reduce costs, and some 3-wire interfaces can reach 26 MHz communication rate, the 2-wire interface can reach a rate of 3.4 MHz. Another indicator of DAC is resolution, 16-bit or 18-bit DAC can provide microvolt level control. For example, an 18-bit, 2.5V reference DAC, each LSB corresponds to 9.54μV, high resolution is extremely important for industrial control (such as robots, engines) products. At present, the highest resolution digital potentiometer can provide is 10 bits or 1024 taps. Another advantage of the digital-to-analog converter is the ability to integrate multiplexers in a single chip. For example, the MAX5733 has 32 built-in DACs, each of which can provide 16-bit resolution. The current digital potentiometer can only provide 6 channels at most, such as DS3930.

The DAC can source or sink current, providing designers with greater flexibility. For example, the MAX5550 10-bit DAC can provide up to 30mA of output drive through an internal amplifier, P-channel MOSFET, and pull-up resistor. The MAX5547 10-bit DAC combined with the amplifier, N-channel MOSFET and pull-down resistor can provide 3.6 mA sink current. In addition to the current output, some DACs can also be connected to external amplifiers to provide additional output control. Because digital/analog converters usually have built-in amplifiers, the cost is higher than digital potentiometers. But as the size of the new DAC shrinks, the cost difference is getting smaller and smaller.

Digital potentiometer

It has been mentioned that the digital potentiometer can control the resistance through the digital input. The 3-terminal digital potentiometer in Figure 1a is actually an adjustable resistor divider with a fixed end-to-end resistance. The digital potentiometer can be configured as a 2-terminal variable resistor by connecting the center tap of the potentiometer to the high-end or low-end or floating the high-end or low-end. Unlike digital/analog converters, digital potentiometers can connect the H terminal to the highest or lowest voltage terminal. When selecting a digital potentiometer, users also need to consider specific indicators: linear or logarithmic adjustment, number of taps, number of tap levels, non-volatile memory, cost, etc. The control interface has increment/decrement, button, SPI, and I2C.

Like digital/analog converters, digital potentiometers communicate via serial ports, including I2C and SPI. In addition, the digital potentiometer also provides 2-wire increment and decrement interface control. Generally, the significant difference between a DAC and a digital potentiometer is that there is an output amplifier inside the D/A converter. The output amplifier can drive low impedance loads.

DAC/potentiometer selection

In many applications, users can easily choose between DAC and potentiometer. For motor control, sensor or robot systems requiring high resolution, DAC is required. In addition, high-speed applications, such as base stations, meters, etc., require high speed and resolution, and even DACs with parallel interfaces are required. The linear characteristic of the potentiometer facilitates the realization of the amplifier feedback network. Compared with digital/analog converters, logarithmic potentiometers are more suitable for volume adjustment.

Figure 4. DAC or digital potentiometer is used to control the MAXl553 LED driver

But in many current applications, the choice between the DAC and the digital potentiometer is hard to be made. Both the DAC and the digital potentiometer in Figure 4 can be used to control the MAXl553 LED driver. MAXll53 brightness (BRT) input DC voltage and current-sense resistance determine the LED current.