AD7376ARUZ10-R7 datasheet(12/20 Pages) AD

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AD7376

Rev. D | Page 12 of 20

THEORY OF OPERATION

PROGRAMMING THE VARIABLE RESISTOR

Rheostat Operation

The part operates in rheostat mode when only two terminals

are used as a variable resistor. The unused terminal can be left

floating or tied to the W terminal as shown in Figure 24.

Figure 24. Rheostat Mode Configuration

The nominal resistance between Terminals A and B, RAB, is

available in 10 kΩ, 50 kΩ, and 100 kΩ with ±30% tolerance and

has 128 tap points accessed by the wiper terminal. The 7-bit

data in the RDAC latch is decoded to select one of the 128

possible settings. Figure 25 shows a simplified RDAC structure.

0x01

0x7F

0x00

SWB

RDAC

LATCH

AND

DECODER

SWA

RS = RNOMINAL/128

SHDN

Figure 25. AD7376 Equivalent RDAC Circuit

The general equation determining the digitally programmed

output resistance between the W and the B terminals is







128

)

(

(1)

where:

is the decimal equivalent of the binary code loaded in the

7-bit RDAC register from 0 to 127.

RAB

is the end-to-end resistance.

is the wiper resistance contributed by the on resistance of

the internal switch.

The AD7376 wiper switches are designed with the transmission

gate CMOS topology, and the gate voltage is derived from the

VDD. Each switch’s on resistance, RW, is a function of VDD and

temperature (see Figure 13).

Contrary to the temperature coefficient of RAB, the temperature

coefficient of the wiper resistance is significantly higher because

the wiper resistance doubles with every 100° increase. As a result,

the user must take into consideration the contribution of RW on

the desirable resistance. On the other hand, each switch’s on

resistance is insensitive to the tap point potential and remains

relatively flat at 120 Ω typical at a VDD of 15 V and a

temperature of 25°C.

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

starts at the B terminal for programming code 0x00, where SWB

is closed. The minimum resistance between Terminals W and B

is therefore 120 Ω in general. The second connection is the first

tap point, which corresponds to 198 Ω (RWB = 1/128 × RAB + RW

= 78 Ω + 120 Ω) for programming code 0x01, and so on.

Each LSB data value increase moves the wiper up the resistor

ladder until the last tap point is reached at 10,042 Ω (RAB – 1 LSB +

). Regardless of which settings the part is operating with, care

should be taken to limit the current conducted between any A

and B, W and A, or W and B terminals to a maximum dc

current of 5 mA and a maximum pulse current of 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 W and A terminals 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 into the latch

increases in value. The general equation for this operation is









128

)

(

(2)

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