AD5247BKSZ100-R2 datasheet(14/20 Pages) AD

14 / 20 page

AD5247

Data Sheet

Rev. F | Page 14 of 20

THEORY OF OPERATION

The AD5247 is a 128-position, digitally-controlled variable

resistor (VR) device. An internal power-on preset places the

wiper at midscale during power-on, which simplifies the

default condition recovery at power-up.

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The nominal resistance (RAB) 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 part number determine the nominal

resistance value; for example, 10 kΩ = 10 and 50 kΩ = 50. The

RAB of the VR has 128 contact points accessed by the wiper

terminal, plus the B terminal contact. The 7-bit data in the

RDAC latch is decoded to select one of the 128 possible settings.

Assuming a 10 kΩ part is used, the wiper’s first connection starts

at the B terminal for Data 0x00. Because there is a 50 Ω wiper

contact resistance, such a connection yields a minimum of 100 Ω

(2 × 50 Ω) resistance between Terminal W and Terminal B. The

second connection is the first tap point, corresponding to 178 Ω

(RWB = RAB/128 + RW = 78 Ω + 2 × 50 Ω) for Data 0x01. The third

connection is the next tap point, representing 256 Ω (2 × 78 Ω

+ 2 × 50 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 10,100 Ω (RAB + 2 × RW).

Figure 35 shows a simplified diagram of the equivalent RDAC

circuit where the last resistor string is not accessed.

RDAC

LATCH

AND

DECODER

Figure 35. AD5247 Equivalent RDAC Circuit

The general equation determining the digitally programmed

output resistance between W and B is

(D)

128

(1)

where:

D is the decimal equivalent of the binary code loaded in the

7-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 Terminal A is open-circuited,

the output resistance RWB, shown in Table 9, is set for the indicated

RDAC latch codes.

Table 9. Codes and Corresponding RWB Resistance

D (Decimal)

RWB (Ω)

Output State

127

10,072

Full scale (RAB + 2 × RW)

5150

Midscale

228

1 LSB

150

Zero scale (wiper contact resistance)

Note that in the zero-scale condition, a finite resistance of

100 Ω between Terminal W and Terminal B is present. Care

should be taken 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 Wiper W and Terminal A also produces a

digitally controlled complementary resistance, RWA. When

these terminals are used, the Terminal B can be opened. Set the

resistance value for RWA to start at a maximum value of resistance

and to decrease the data loaded in the latch increases in value.

The general equation for this operation is

(D)

−

128

(2)

If RAB = 10 kΩ and the B terminal is open-circuited, the output

resistance, RWA, shown in Table 10, is set for the indicated RDAC

latch codes.

Table 10. Codes and Corresponding RWA Resistance

D (Decimal)

RWA (Ω)

Output State

127

228

Full scale

5150

Midscale

10,071

1 LSB

10,150

Zero scale

Typical device-to-device matching is process lot dependent

and can 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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