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
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. The general
equation defining the output voltage at VW with respect to
ground for any valid input voltage applied to Terminal A and
Terminal B is
For a more accurate calculation, which includes the effect of
wiper resistance, VW can be found as
Operation of the digital potentiometer in the divider mode
results in a more accurate operation over temperature. Unlike
the rheostat mode, the output voltage is dependent mainly on
the ratio of the internal resistors (RWA and RWB) and not the
absolute values. Therefore, the temperature drift reduces to
SPI-COMPATIBLE 3-WIRE SERIAL BUS
The AD5160 contains a 3-wire SPI-compatible digital interface
(SDI, CS, and CLK). The 8-bit serial word must be loaded MSB
first. The format of the word is shown in Table 6.
The positive-edge sensitive CLK input requires clean transitions
to avoid clocking incorrect data into the serial input register.
Standard logic families work well. If mechanical switches are
used for product evaluation, they should be debounced by a
flip-flop or other suitable means. When CS is low, the clock
loads data into the serial register on each positive clock edge
(see Figure 37).
The data setup and data hold times in the specification table
determine the valid timing requirements. The AD5160 uses an
8-bit serial input data register word that is transferred to the
internal RDAC register when the CS line returns to logic high.
Extra MSB bits are ignored.
All digital inputs are protected with a series input resistor and
parallel Zener ESD structures are shown in Figure 40 and
Figure 41. This applies to SDI, CLK, and CS, which are the
digital input pins.
Figure 40. ESD Protection of Digital Pins
Figure 41. ESD Protection of Resistor Terminals
Because the ESD protection diodes limit the voltage compliance
at the A, B, and W terminals, it is important to power VDD/GND
before applying any voltage to the A, B, and W terminals;
otherwise, the diode forward biases such that VDD is powered
unintentionally and may affect the rest of the user’s circuit. The
ideal power-up sequence is in the following order: GND, VDD,
digital inputs, and then VA/B/W. The relative order of powering
VA, VB, VW, and the digital inputs is not important as long as
they are powered after VDD/GND.
LAYOUT AND POWER SUPPLY BYPASSING
It is a good practice to employ compact, minimum lead length
layout design. Keep the leads to the inputs as direct as possible
with a minimum conductor length. Ground paths should have
low resistance and low inductance.
Similarly, it is also a good practice to bypass the power supplies
with quality capacitors for optimum stability. Bypass supply
leads to the device with disc or chip ceramic capacitors of
0.01 μF to 0.1 μF. To minimize any transient disturbance and
low frequency ripple, apply low ESR 1 μF to 10 μF tantalum or
electrolytic capacitors at the supplies (see Figure 42). To
minimize the ground bounce, join the digital ground remotely
to the analog ground at a single point.
C3 + C1
Figure 42. Power Supply Bypassing
Rev. C | Page 15 of 16