feature

128 position potentiometer replacement 10 kV, 50 kV, 100 kV; very low power: 40 max mA; up/down count control.

application

Mechanical potentiometer replacement; remote incremental adjustment application; instrumentation: gain, offset adjustment; programmable voltage current conversion; programmable filter, delay, time constant; line impedance matching; power supply adjustment.

General Instructions

The AD5220 provides a single-channel, 128-bit digitally controlled variable resistor (VR) device. The device performs the same electronic adjustment function as a potentiometer or variable resistor. These products are optimized for pushbutton applications in instrumentation and test equipment. Due to the wide choice of end-to-end resistance values, it is possible to choose between bandwidth or power consumption.

The AD5220 contains a fixed resistor with a wiper contact that taps the fixed resistor value at a point determined by a digitally controlled up/down counter. The resistance between the wiper and either end of the fixed resistor provides a constant resistance step equal to the end-to-end resistance divided by the number of positions (for example, RSTEP=10 kΩ/128=78Ω). The variable resistor provides a true adjustable resistance value between the a terminal and the wiper, or between the B terminal and the wiper. The nominal temperature coefficient of the 10 kΩ, 50 kΩ or 100 kΩ fixed A to B terminal resistance is 800 ppm/°C.

Chip select CS, count CLK, and U/D direction control inputs set the variable resistor position. These inputs that control the internal up/down counters can be easily generated by mechanical or push button switches (or other contact closure devices). The negative edge sensitive CLK pin requires an external demagnetization circuit. This simple digital interface eliminates the need for a microcontroller in front panel interface designs.

The AD5220 is available in surface mount (SO-8) and 8-lead plastic dip packages. For ultra-compact solutions, models are available in thin μSOIC packages. All parts are guaranteed to operate over the extended industrial temperature range of -40°C to +85°C. For 3-wire, SPI-compatible interface applications, see the AD7376/AD8400/AD8402/AD8403 products.

Typical performance characteristics

Parametric Test Circuit

operate

The AD5220 provides a 128-bit digitally controlled variable resistor (VR) device. VR settings are changed by pulsing the CLK pin when CS is active low. The direction of the increment is controlled by the U/D (up/down) control input pins. When the wiper hits the end of the resistor (terminal A or B), the extra CLK pulses no longer change the wiper setting. The wiper position is instantly decoded by the wiper decode logic that changes the wiper resistance. When using pushbutton switches to control the count sequence and count direction, a proper denoising circuit is required. The exact timing requirements are shown in Figure 3. The AD5220 is energized in the center position, showing that the resistances of RWA and RWB are nearly equal.

Digital interface operation

The AD5220 includes a three-wire serial input interface. The three inputs are Clock (CLK), CS and Up/Down (U/D). The negative edge sensitive CLK input requires clean transitions to avoid clocking multiple pulses into the internal up/down counter registers, see Figure 35. Standard logic families work fine. If a mechanical switch is used for product evaluation, a trigger or other suitable method should be used to remove the shock. When CS is active low, the clock starts to increment or decrement the internal up/down counter according to the state of the U/D control pin. The up/down counter value (D) starts 40 hours after the system is powered on. As long as the U/D pin is logic high, each new CLK pulse will increment the value of the internal counter by one LSB until the full-scale value of 3FH is reached. If the U/D pin is set to logic low, the counter will stop counting down at code 00H (zero scale). Additional clock pulses on the CLK pin will be ignored when the wiper is in the 00H position or the 3FH position.

All digital inputs (CS, U/D, CLK) are protected with series input resistors and parallel Zener ESD structures, as shown in Figure 36.

Variable Resistor Rheostat Operation Programming

The nominal resistance values of the RDAC between terminals A and B are 10 kΩ, 50 kΩ, and 100 kΩ, respectively. The last three characters of the part number determine the nominal resistance value, eg 10 kΩ=10; 50 kΩ=50; 100 kΩ=100. The nominal resistance (RAB) of the VR has 128 contacts connected through the wiper terminal and the B terminal contact. At power on, the resistance from the wiper to either terminal A or B is approximately equal. Timing the CLK pin will increase the resistance from wiper W to terminal B, adding one RS resistance unit (see Figure 38). Resistor RWB is determined by the number of pulses applied to the clock pin. Each segment of the internal resistor string has a nominal resistance value RS=RAB/128, which becomes 78Ω for the 10 kΩ AD5220BN10 product. It should be noted that the current between W and B is limited to a maximum of 5 mA in direct contact to avoid degradation or possible damage to the internal switch contacts.

Like the mechanical potentiometer that the RDAC replaced, it is fully symmetrical (see Figure 38). The resistance between wiper W and terminal A also produces a digitally controlled resistance RWA. When using these terminals, the B-terminal should be tied to the wiper.

The typical part-to-part distribution of RBA is process batch dependent with a ±30% variation. RBA has a temperature coefficient of 800 ppm/°C as a function of temperature.

Due to the large contribution rate of the wiper contact switch resistance (its temperature coefficient is 0.5%/°C), the RBA temperature coefficient increases when the wiper is programmed near the B terminal. Figure 14 shows the effect of wiper contact resistance as a function of code setting. Another performance factor affected by the switch contact resistance is the relative linearity error performance between the 10 kΩ and 50 kΩ or 100 kΩ versions. The switch contact resistance is the same for all three models. Therefore, the performance of the 50 kΩ and 100 kΩ devices with the least effect on the wiper switch resistance shows the best linearity error, see Figures 7 and 8.

Program Potentiometer Divider Voltage Output Operation

Digital potentiometers tend to generate an output voltage proportional to the input voltage applied to a given terminal. For example, connecting one terminal to +5 V and B to ground produces an output voltage at the wiper that can be anywhere from zero volts up to 1 LSB less than +5 V . Each LSB voltage is equal to the voltage applied on terminal AB divided by the 128 position resolution of the potentiometer divider. For any given input voltage applied across terminal AB, the general equation defining the output voltage with respect to ground is:

D represents the current content of the internal up/down counter.

The operation of the digital potentiometer in voltage divider mode allows for more precise operation when the temperature is too high. Here, the output voltage depends on the ratio of the internal resistance, not the absolute value, so the drift increases to 20ppm/°C.

application information

The negative edge sensitive CLK pin does not contain any internal demagnetization circuitry. This standard CMOS logic input responds to fast negative edges and needs to be de-noised externally with an appropriate circuit designed for the type of switch closure used. The CLK input pin performs well when the minimum slew rate for negative logic transitions is 1V/µs. A variety of standard circuits such as one-shot multivibrators, Schmitt trigger gates, cross-coupled flip-flops, or RC filters can be used to drive the CLK pin with a uniform negative edge. This will prevent the digital pot from skipping output codes due to the bounce of the switch contacts while counting.

Dimensions

Dimensions are in inches and (mm).