Datasheet

MCP40D17/18/19
DS22152B-page 44 © 2009 Microchip Technology Inc.
6.3 Wiper Resistance
Wiper resistance is the series resistance of the analog
switch that connects the selected resistor ladder node
to the Wiper Terminal common signal (see Figure 6-1).
A value in the volatile wiper register selects which
analog switch to close, connecting the W terminal to
the selected node of the resistor ladder.
The resistance is dependent on the voltages on the
analog switch source, gate, and drain nodes, as well as
the device’s wiper code, temperature, and the current
through the switch. As the device voltage decreases,
the wiper resistance increases (see Figure 6-4 and
Table 6-4).
The wiper can connect directly to Terminal B or to
Terminal A. A zero scale connections, connects the
Terminal W (wiper) to Terminal B (wiper setting of
000h). A full scale connections, connects the
Terminal W (wiper) to Terminal A (wiper setting of 7Fh).
In these configurations the only resistance between the
Terminal W and the other Terminal (A or B) is thaΩt of
the analog switches.
The wiper resistance is typically measured when the
wiper is positioned at either zero scale (00h) or full
scale (3Fh).
The wiper resistance in potentiometer-generated
voltage divider applications is not a significant source
of error.
The wiper resistance in rheostat applications can
create significant nonlinearity as the wiper is moved
toward zero scale (00h). The lower the nominal
resistance, the greater the possible error.
In a rheostat configuration, this change in voltage
needs to be taken into account. Particularly for the
lower resistance devices. For the 5.0 kΩ device the
maximum wiper resistance at 5.5V is approximately
3.2% of the total resistance, while at 2.7V it is
approximately 6.5% of the total resistance.
In a potentiometer configuration, the wiper resistance
variation does not effect the output voltage seen on the
W pin.
The slope of the resistance has a linear area (at the
higher voltages) and a non-linear area (at the lower
voltages). In where resistance increases faster than the
voltage drop (at low voltages).
FIGURE 6-4: Relationship of Wiper
Resistance (R
W
) to Voltage.
Since there is minimal variation of the total device
resistance over voltage, at a constant temperature (see
Figure 2-11, Figure 2-29, Figure 2-47, or Figure 2-65),
the change in wiper resistance over voltage can have a
significant impact on the INL and DNL error.
TABLE 6-4: TYPICAL STEP RESISTANCES AND RELATIONSHIP TO WIPER RESISTANCE
R
W
V
DD
Note: The slope of the resistance has a linear
area (at the higher voltages) and a non-
linear area (at the lower voltages).
Resistance (?) R
W
/ R
S
(%)
(1)
R
W
/ R
AB
(%)
(2)
Typical Wiper (R
W
)
R
W
=
Typical
R
W
= Max
@ 5.5V
R
W
= Max
@ 2.7V
R
W
=
Typical
R
W
= Max
@ 5.5V
R
W
= Max
@ 2.7V
Total
(R
AB
)
Step
(R
S
)
Typical
Max @
5.5V
Max @
2.7V
5000 39.37 100 170 325 254.00% 431.80% 825.5% 2.00% 3.40% 6.50%
10000 78.74 100 170 325 127.00% 215.90% 412.75% 1.00% 1.70% 3.25%
50000 393.70 100 170 325 25.40% 43.18% 82.55% 0.20% 0.34% 0.65%
100000 787.40 100 170 325 12.70% 21.59% 41.28% 0.10% 0.17% 0.325%
Note 1: R
S
is the typical value. The variation of this resistance is minimal over voltage.
2: R
AB
is the typical value. The variation of this resistance is minimal over voltage.