AD5160BRJZ5-R2 datasheet(14/16 Pages) AD

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AD5160

Rev. B | Page 14 of 1

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

at power-up.

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

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.

RDAC

LATCH

AND

DECODER

Figure 39. Equivalent RDAC Circuit

The general equation determining the digitally programmed

output resistance between W and B is

256

)

(

(1)

where:

is the decimal equivalent of the binary code loaded in the

8-bit RDAC register.

RAB

is the end-to-end resistance.

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

255

9961

Full Scale (RAB − 1 LSB + RW)

128

5060

Midscale

1 LSB

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

−

256

)

(

(2)

For RAB = 10 kΩ and the B terminal is open circuited, the

following output resistance RWA is set for the indicated RDAC

latch codes.

Table 8. Codes and Corresponding RWA Resistance

D (Dec.)

RWA (Ω)

Output State

255

Full Scale

128

5060

Midscale

9961

1 LSB

10,060

Zero 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.

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