THEORY OF OPERATION
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
PROGRAMMING THE VARIABLE RESISTOR
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
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.
Figure 39. Equivalent RDAC Circuit
The general equation determining the digitally programmed
output resistance between W and B is
D is the decimal equivalent of the binary code loaded in the
8-bit RDAC register.
RAB is the end-to-end resistance.
RW is the wiper resistance contributed by the on resistance of
the internal switch.
In summary, if RAB = 10 kΩ and the A terminal is open
circuited, the following output resistance RWB is set for the
indicated RDAC latch codes.
Table 7. Codes and Corresponding RWB Resistance
D (Dec.) RWB (Ω) Output State
Full Scale (RAB − 1 LSB + RW)
1 99 1 LSB
0 60 Zero Scale (Wiper Contact 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. The general equation for this operation is
For RAB = 10 kΩ and the B terminal is open circuited, the
following output resistance RWA is set for the indicated RDAC
Table 8. Codes and Corresponding RWA Resistance
255 99 Full Scale
Typical device-to-device matching is process lot dependent and
may vary by up to ±30%. Because the resistance element is
processed in thin film technology, the change in RAB with
temperature has a very low 45 ppm/°C temperature coefficient.
Rev. C | Page 14 of 16