AS1500 datasheet(9/14 Pages) AMSCO

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AS1500

Data Sheet - Deta iled Description

Rheostat Operation

The digital potentiometer family AS150x offers nominal resistor values of 10k

Ω, 20kΩ, 50kΩ and 100kΩ. The resistor

has 256 contact points where the wiper can access the resistor. The 8-bit code word determines the position of the

wiper and is decoded through an internal logic. The lowest code 00h is related to the terminal B. The resistance is then

only determined by the wiper resistance (100

Ω). The resistance for the next code 01h is the nominal resistor RAB

(10k

Ω, 20kΩ, 50kΩ or 100kΩ) divided through 256 plus the wiper resistor. In case of AS1501 (10kΩ) the total

resistance is 39

Ω+100Ω=139Ω. Accordingly the resistor for code 02h is 78Ω+100Ω=178Ω. The last code 255h does

not connect to terminal A directly (see Figure 5). So the maximum value is 10000

Ω - 39Ω +100Ω = 10061Ω. The

general formula for the calculation of the resistance RWB is:

RWB (Dx)= (Dx)/256 • RAB + RW

(EQ 1)

where RAB is the nominal resistance between terminal A and B, RW is the wiper resistance and DX is the 8-Bit Code

word. In Table 7 the resistor values between the wiper and terminal B for AS1500 are given for specific codes DX. In

the zero-scale condition the wiper resistance of 100

Ω remains present.

The maximum current through the wiper and terminal B is 5mA. If the current exceeds this limit the internal switches

can degrade or even be damaged. As a mechanical potentiometer the resistances RWA and RWB are totally

symmetrical. The relation between them is shown in Figure 5.

Figure 5. RWA and RWB versa code

Table 7. RDAC-Codes WB

DX (Dec)

RWB (Ω)

Output State

255

10061

Full Scale

128

5100

Midscale

139

1 LSB

0100

Zero-Scale

(Wiper Contact Resistance)

CODE - Decimal

RWA

RWB

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