AS1500
Data Sheet
- Detailed Description
Rheostat Operation
The digital potentiometer family AS150x offers nominal resistor values of 10kΩ, 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 (100Ω). The resistance for the next code 01h is the nominal resistor RAB
(10kΩ, 20kΩ, 50kΩ or 100kΩ) divided through 256 plus the wiper resistor. In case of AS1501 (10kΩ) the total
resistance is 39Ω+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 10000Ω - 39Ω +100Ω = 10061Ω. The
general formula for the calculation of the resistance R
WB
is:
R
WB
(Dx)= (Dx)/256 • R
AB
+ R
W
(EQ 1)
where R
AB
is the nominal resistance between terminal A and B, R
W
is the wiper resistance and D
X
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 D
X
. In
the zero-scale condition the wiper resistance of 100Ω remains present.
Table 7. RDAC-Codes WB
D
X
(Dec)
255
128
1
0
R
WB
(Ω)
10061
5100
139
100
Output State
Full Scale
Midscale
1 LSB
Zero-Scale
(Wiper Contact Resistance)
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 R
WA
and R
WB
are totally
symmetrical. The relation between them is shown in
Figure 5. R
WA
and R
WB
versa code
10
R
WA
, R
WB
- % of Nominal R
AB
R
WA
75
R
WB
50
25
0
0
64
12
19
25
CODE - Decimal
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