Data Sheet AS1500/1/2/3
DX (Dec)
Output State
RWB (Ω)
A
255
10061
Full Scale
RS
RS
RS
D7
D6
D5
D4
D3
D2
D1
D0
128
1
5100
139
Midscale
1 LSB
Zero-Scale
(Wiper Contact Resistance)
0
100
W
Table 7: RDAC-Codes WB
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
potentiometer the resistance RWA and RWB are totally
symmetrical. The relation between them is shown in Figure
4.
RDAC
LATCH
AND
a mechanical
RS
DECODE
B
RS=RNOMINAL / 256
Figure 5: Equivalent RDAC Circuit
10
Voltage Output Operation
The AS150x family can easily used in an voltage output
mode, where the output voltage is proportional to an
applied voltage to a given terminal. When 5V are applied to
terminal A and B is set to ground the ouput voltage at the
wiper starts at zero volts up to 1LSB less then 5V. One
LSB of voltage corresponds to the voltage applied at
terminal AB divided through 256 steps of possible wiper
settings. The formula is given by
RW
RW
75
50
25
0
VW (Dx)= (Dx)/256⋅VAB + VB
where VAB is the voltage applied between terminal A and B,
VW is the voltage at the wiper, DX is the 8-Bit Code word
and VB is the voltage at terminal B. The temperature drift is
significant better than in Rheostat mode, since the
temperature coefficient is determined by the internal
resistor ratio. Therefore the temperature drift is only
15ppm/°C.
0
64
12
19
25
CODE -
Figure 4: RWA and RWB versa Code
The resistance RWA is the complimentary resistor to RWB
and can be controlled digitally as well. RWA starts at the
maximum value of the nominal resistance and is reduced
with increasing 8-Bit code words. The formula to calculate
RWA is given below:
Applications
The digital potentiometer can replace in many applications
the analog trimming potentiometer. The digital
potentiometer is not sensitive to vibrations and shocks. It
has an extremely small form-factor and can be adjusted
very fast (e.g. AS1500 has an update rate of 600kHz)
Furthermore the temperature drift, resolution and noise are
RWA (Dx)= (256 - Dx)/256⋅RAB + RW
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 8 the resistor values between the wiper and
terminal B for AS1501 are given for specific codes DX.
significant better and cannot be achieved with
a
mechanical trimming potentiometer. Due to the
programmability the resistor settings can be stored in the
system memory, so that after a power down the exact
settings can be recalled easily.
DX (Dec)
Output State
Full Scale
Midscale
1 LSB
Zero-Scale
RWA (Ω)
89
5050
10011
10050
255
128
1
All analog signals must remain within 0 to VDD range. For
standard potentiometer applications the wiper output can
be used directly. In the case of a low impedance load a
buffer shall be used.
0
Table 8: RDAC-Codes WA
Revision 1.0, Oct 2004
Page 6 of 8
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