DESCRIPCIÓN GENERAL

The AD5160 provides a compact 2.9 mm × 3 mm packaged solution for 256-position adjustment applications. These devices perform the same electronic adjustment function as mechanical potentiometers1 or variable resistors but with enhanced resolution, solid-state reliability, and superior low temperature coefficient performance.

The wiper settings are controllable through an SPI-compatible digital interface. The resistance between the wiper and either end point of the fixed resistor varies linearly with respect to the digital code transferred into the RDAC latch.

Operating from a 2.7 V to 5.5 V power supply and consuming less than 5 μA allows for usage in portable battery-operated applications.

 

CARACTERÍSTICAS

256-position

End-to-end resistance: 5 kΩ, 10 kΩ, 50 kΩ, 100 kΩ

Compact SOT-23-8 (2.9 mm × 3 mm) package

SPI-compatible interface Power-on preset to midscale Single supply: 2.7 V to 5.5 V Low temperature coefficient: 45 ppm/°C

Low power, IDD = 8 μA Wide operating temperature: –40°C to +125°C

Evaluation board available

 

APLICACIONES

Mechanical potentiometer replacement in new designs

Transducer adjustment of pressure, temperature, position,chemical, and optical sensors

RF amplifier biasing

Gain control and offset adjustment

 

TEORÍA DE FUNCIONAMIENTO

The AD5160 is a 256-position digitally controlled variable resistor (VR) device.

An internal power-on preset places the wiper at midscale during power-on, which simplifies the fault condition recovery at power-up. PROGRAMMING THE VARIABLE RESISTOR

Funcionamiento del reóstato

The nominal resistance of the RDAC between Terminal A and Terminal B is available in 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ. The final two or three digits of the model number as listed in the Ordering Guide section determine the nominal resistance value, for example, in model AD5160BRJZ10, the 10 represents 10 kΩ; and in AD5160BRJZ50, the 50 represents 50 kΩ.

The nominal resistance (RAB) of the VR has 256 contact points accessed by the wiper terminal, plus the B terminal contact. The 8-bit data in the RDAC latch is decoded to select one of the 256 possible settings.

Assuming a 10 kΩ part is used, the first connection of the wiper starts at the B terminal for Data 0x00. Because there is a 60 Ω wiper contact resistance, such connection yields a minimum of 60 Ω resistance between Terminal W and Terminal B.

The second connection is the first tap point, which corresponds to 99 Ω (RWB = RAB/256 + RW = 39 Ω + 60 Ω) for Data 0x01.

The third connection is the next tap point, representing 138 Ω (2 × 39 Ω + 60 Ω) for Data 0x02, and so on. Each LSB data value increase moves the wiper up the resistor ladder until the last tap point is reached at 9961 Ω (RAB − 1 LSB + RW). Figure 39 shows a simplified diagram of the equivalent RDAC circuit where the last resistor string is not accessed; therefore, there is 1 LSB less of the nominal resistance at full scale in addition to the wiper resistance.

Note that in the zero-scale condition, a finite wiper resistance of 60 Ω is present. Take care to limit the current flow between W and B in this state to a maximum pulse current of no more than 20 mA. Otherwise, degradation or possible destruction of the internal switch contact can occur.

Similar to the mechanical potentiometer, the resistance of the RDAC between the Wiper W and Terminal A also produces a digitally controlled complementary resistance (RWA). When these terminals are used, the B terminal can be opened. Setting the resistance value for RWA starts at a maximum value of resistance and decreases as the data loaded in the latch increases in value.

PROGRAMMING

THE POTENTIOMETER DIVIDER

Voltage Output Operation

The digital potentiometer easily generates a voltage divider at wiper-to-B and wiper-to-A proportional to the input voltage at A-to-B. Unlike the polarity of VDD to GND, which must be positive, voltage across A to B, W to A, and W to B can be at either polarity.

If ignoring the effect of the wiper resistance for approximation, connecting the A terminal to 5 V and the B terminal to ground produces an output voltage at the wiper-to-B starting at 0 V up to 1 LSB less than 5 V. Each LSB of voltage is equal to the voltage applied across Terminal A and Terminal B divided by the 256 positions of the potentiometer divider.