AN1080

AN1080
Understanding Digital Potentiometer Resistor Variations
Author:
Mark Palmer
Microchip Technology Inc.
INTRODUCTION
All semiconductor devices have variations over
process. In the case of digital potentiometer devices,
this process variation affects the device resistive
elements (RAB -> RS and RW). These resistive
elements also have variations with respect to voltage
and temperature, which will also be discussed.
This application note will discuss how process, voltage,
and temperature affect the Resistor Network’s
characteristics and specifications. Also, application
techniques will be covered that can assist in optimizing
the operation of the device to improve performance in
the application.
The process technology used also affects the
operational characteristics. We will focus on the
characteristics for devices fabricated in CMOS.
RBW - The total resistance from Terminal B to the Wiper
Terminal. This resistance equals:
RS * (Wiper Register value) + RW.
RAW - The total resistance from Terminal A to the Wiper
Terminal. This resistance equals:
RS * (Full Scale value - Wiper Register value) + RW.
Full Scale - When the Wiper is connected to the
closest tap point to Terminal A.
Zero Scale - When the Wiper is connected to the
closest tap point to Terminal B.
A
n = 256
(Full Scale) RW
RS
TERMINOLOGY
n = 255
To assist with the discussions in this application note,
the following terminology will be used. Figure 1
illustrates several of these terms.
Resolution - The number of unique wiper positions
that can be selected between Terminal B and Terminal
A.
RW
RS
W
n=2
RAB - The total resistance between the A Terminal and
the B Terminal.
Resistor Network - Is the combination of RS resistors
and RW resistor that create the voltage levels and current paths between the A Terminal, B Terminal, and
Wiper Terminal.
© 2007 Microchip Technology Inc.
RW
RBW
RS
RS - The Step resistance. This is the change in resistance that occurs between two adjacent wiper register
values. It is also the RAB resistance divided by the number of RS resistors (resolution) in the Resistor Ladder.
Resistor Ladder - Is the serial string of RS resistors
between Terminal B and Terminal A. The total resistance of this string equals RAB.
RW
RAB
Wiper Value - The value in the wiper register which
selects the one wiper switch to close so that the Wiper
Terminal is connected to the Resistor Network.
RW - The resistance of the analog switch that connects
the Wiper Terminal to the Resistor Ladder. Each analog
switch will have slightly different resistive characteristics.
RAW
n = 254
n=1
RW
RS
n=0
(Zero
Scale)
RW
B
FIGURE 1:
8-Bit Resistor Network.
DS01080A-page 1
AN1080
THE RAB RESISTANCE
The Step Resistance (RS)
The RAB resistance is the total resistance between
Terminal A and Terminal B. The RAB resistance is really
a resistor string of RS resistors. The RS resistors are
designed to be uniform, so they have minimal variation
with respect to each other. The RS resistors, and the
RAB resistance, will track each other over voltage, temperature, and process.
Microchip offers Digital Potentiometer devices with
typical RAB resistances of 2.1 kΩ, 5 kΩ, 10 kΩ, 50 kΩ
and 100 kΩ. These devices will either offer 6-bits or 8bits of resolution. The step resistance (RS) is the RAB
resistances divided by the number of wiper steps.
Many manufacturers specify the devices RAB resistance to be ±20% from the targeted (typical) value. This
specification is to indicate that from “device-to-device”
the resistance could range ±20% from the typical value.
This specification is NOT meant that a given devices
resistance will vary ±20% over voltage and
temperature.
The step resistance is important to understand when
you are using the device in a rheostat mode, or the
potentiometer is being windowed by resistors on the
Terminal A and/or on the Terminal B. Table 1 shows the
step resistances available for the different RAB values
available.
TABLE 1:
STEP RESISTANCE
Step Resistance
(RS) (Ω - typ.)
So, when the RAB resistance is +10% from the typical
value, then each RS resistor is also +10% from the
typical value.
RAB
Resistance
(kΩ - typ.)
The “device-to-device” RAB resistance could be off by
up to 40% of the typical value. This occurs if one device
has a resistance (RAB) that is -20% and the other
device is +20%.
2.1
33.33
—
5.0
79.37
—
10.0
158.73
39.06
Can trade off between
cost and Step Resistance (resolution)
50.0
793.65
195.31
Can trade off between
cost and Step Resistance (resolution)
100.0
—
390.63
Largest RAB resistance
6-Bit
Device
(63 RS)
Comment
8-Bit
Device
(256 RS)
RAB(MAX) = 12 kΩ
+20%
Δ40%
RAB(TYP) = 10 kΩ
-20%
RAB(MIN) = 8 kΩ
FIGURE 2:
RAB Variations.
So, naturally the RAB resistance may have some effect
in a Potentiometer configuration (voltage divider), but
this variation can have a real effect in a Rheostat
configuration (variable resistor).
In the Potentiometer configuration, if the A and B
terminals are connected to a fixed voltage, then this
variation should not effect the system. But, if either (or
both) the A or/and B terminals are connected through
resistors to the fixed voltage source, then the change in
RAB value could effect the voltage at the W terminal (for
a given wiper code value).
Smallest Step
resistance available
On a semiconductor device, a resistor can be made
with metal/poly/contact components. Designing a
structure from these components can be used to form
a resistive element (RS). Repeating this resistive
element into a string of resistors (RS) creates the RAB
resistance. The node between each RS resistor is a
contact point (source or drain) for the wiper switch.
In the Rheostat configuration, the RBW resistance value
will vary as RS varies. So, at full scale RBW
approximately equals RAB, and will have the same
±20% from the typical value.
DS01080A-page 2
© 2007 Microchip Technology Inc.
AN1080
Devices with Multiple Potentiometers
THE RW RESISTANCE
Some devices are offered that have two or more
independent potentiometers. Each potentiometer will
exhibit similar characteristics given similar conditions
(terminal voltages, wiper settings, …).
Figure 4 show the common way to illustrate the block
diagram. In this figure, the wiper resistance is
represented as a resistor. In actuality, the wiper is connected to each RS node with an analog switch (see
Figure 3). Each of these analog switches has a resistive property to them and will vary from switch to switch.
Also, the resistive nature of these analog switches is
more susceptible to process variations, voltage, and
temperature than the step resistors (RS) in the resistor
ladder.
The RAB variation between potentiometers on the
same silicon is relatively small. In dual potentiometer
devices, the variation is typically specified as a
maximum variation (RAB1-RAB2/RAB1 or RAB1-RAB2/
RAB2) of 1%. This is true even though from device-todevice, the RAB variation can be ±20% over process.
The RAB of both potentiometers (and therefore the
RSs) will track each other as the device conditions
change. It is assumed that the terminals of each
potentiometer are at the same voltages (and wiper
value). If not, then they may not track each other to the
same degree.
A
RW
N = 255
(FFh)
RW
RS
RAB vs. RBW Resistance
The RAB resistance is “constant” in that it is independent of the value in the wiper register. While the RBW
(or RAW) resistance is directly related to the value in the
wiper register. When the wiper register is loaded with
it’s maximum value, the RBW resistance is close to RAB
resistance. The “closeness” depends on the Resistor
Network implementation (see Figure 4), the RS resistance, and the wiper resistance (RW).
N = 256
(100h)
RS
W
RS
B
FIGURE 3:
N=1
(01h)
RW
N=0
(00h)
RW
Analog
Mux
RW Implementation.
The characteristics of the analog switch depends on
the voltages on the switch nodes (source, drain, and
gate). The characterization graphs shown in Figure 10
through Figure 13 had Terminal B to VSS and Terminal
A to VDD.
Within a voltage range, the change in resistance will be
linear relative to the device voltage. At some point as
the voltage decreases, the resistive characteristics of
the switches will become non-linear at increase
exponentially. This is related to the operational characteristics of the switch devices at the lower voltage.
All the wiper switches will start to increase non-linearly
at about the same voltage.
Temperature also effects the resistive nature of the
wiper switches greater than the RAB (RS) resistance.
The wiper resistance increases as the voltage delta
between the resistor network node and the voltage on
the analog mux switch becomes “small”, so that the
switch is not fully turned on. The wiper resistance curve
would look different if Terminal A was at VDD/2 while
Terminal B is at VSS. In this case, the higher value
wiper codes would have the higher wiper resistance
(RW).
© 2007 Microchip Technology Inc.
DS01080A-page 3
AN1080
Implementation B has 255 steps (28 - 1 steps) but 256
Step Resistors (RS). This allows the wiper register to be
8-bits wide, but now the Wiper (W) can no longer
connect to Terminal A, since there is one RS resistor
between the maximum wiper tap position and the
Terminal A connection.
The Resistor Network
Figure 4 shows three possible Resistor Network
implementations for an 8-bit resistor network. Each has
an advantage and a disadvantage. The system
designer needs to understand which implementation
the device uses to ensure the circuit meets the system
requirements.
Implementation C has 255 steps (28 - 1 steps) and 255
Step Resistors (RS). This allows the wiper register to be
8-bits wide, and to allow the selection of N = 255 (Full
Scale).
Implementation A has 256 steps (28 steps) and 256
Step Resistors (RS), but the wiper register must be 9bits wide to allow the selection of N = 256 (Full Scale).
This increases the complexity of the wiper decode logic
(increases cost), but this implementation allows the
Wiper (W) to be connected to Terminal A.
TABLE 2:
Note:
The possible nodes that the wiper can
connect to on the resistor ladder will
depend on the digital potentiometer
device.
IMPLEMENTATION DIFFERENCES
Implementation
“True”
Wiper
Full
Register
Scale
RAB =
RBW =
Comment
A
Yes
9-bits
256 RS 256 RS + RW Wiper can connect to the full range of taps from
Terminal A and Terminal B, but firmware must take
into account the extra addressing bit. The increased
complexity of the addressing decode adds cost to the
device.
B
No
8-bits
256 RS 255 RS + RW Wiper can not connect to the Terminal A tap. The
application design or the controller firmware may be
required to take this into account.
C
Yes
8-bits
255 RS 255 RS + RW Wiper can connect to the full range of taps from
Terminal A and Terminal B, but the controller firmware
would need to ensure it addressed that there are 255
RS resistors and not 256 RS resistors.
Implementation A
A
A
N = 256
(100h)
Implementation B
A
Implementation C
RW
RS
RS
N = 255
(FFh)
RS
N = 255
(FFh)
RW
RS
N = 255
(FFh)
RW
RS
W
W
N=1
(01h)
RS
N=0
(00h)
B
FIGURE 4:
DS01080A-page 4
RW
RW
N=1
(01h)
RS
N=0
(00h)
Analog
Mux
RW
B
RW
RW
W
N=1
(01h)
RS
N=0
(00h)
Analog
Mux
B
RW
RW
Analog
Mux
Possible 8-Bit Resistor Network Implementations.
© 2007 Microchip Technology Inc.
AN1080
THE RBW OR RAW RESISTANCE
The Floating Terminal, What to do?
When using a Digital Potentiometer device in a Rheostat configuration, should the variable resistor be
created from the Wiper to Terminal B (RBW) or from the
Wiper to Terminal A (RAW)?
When the Digital Potentiometer device is used in a
Rheostat configuration, the third terminal (let’s say Terminal A) is “floating”. So what should be done with it?
This question really depends on which Terminal (A or
B) that the Wiper connects to when the wiper register is
loaded with 0h (Zero Scale). For this discussion, we will
assume that the Wiper will connect to Terminal B.
1.
2.
“Tie” it to the W Terminal.
Leave it floating.
Method 1
Method 2
A
A
W
W
RW
RW
B
FIGURE 5:
RBW1 RAW1
RBW1 RAW1
In either case, you can load the wiper register to get the
desired resistance value, but if you recall Terminal B is
at Zero-Scale. So, that means when using the RBW
configuration, as the wiper register is incremented, the
resistance increases. Conversely, when using the RAW
configuration, as the wiper register is incremented, the
resistance decreases. Which configuration is used
depends more on any advantages that may occur in
the applications firmware algorithm for the control of
the resistance.
There are two possibilities:
B
Rheostat Configurations.
Method 1: “Tie” it to the W Terminal
In this case, the effective resistance of the wiper
resistance (RWEFF) will be RW || RAB1. This resistance
will always be less than RW, but it will vary over the
selected tap position. The RWEFF resistance can be
calibrated out of the system, but it becomes a much
more complicated controller firmware task.
Method 2: Leave it floating
This way, the wiper resistance remains “constant” over
the selected tap position. This becomes much easier
for the controller firmware to calibrate out of the
system.
© 2007 Microchip Technology Inc.
DS01080A-page 5
AN1080
There are two variations that occur over voltage and
temperature that we will look at. These are the
variations of the RAB resistance and the RW resistance.
The characterization graphs also show how these
variations effect the INL and DNL error of the device.
RAB Variation
4875
VDD = 5.5V
4850
4825
VDD = 2.7V
Table 3 shows the RAB data from the MCP402X Data
Sheet (DS21945D) Characterization Graphs at 5.5V
and 2.7V, and over temperature (@ -40°C, +25°C and
+125°C). The minimum and maximum resistance
values are also captured. This data was then analyzed
over this characterization range.
The RS value can be calculated by:
RAB / (# RS resistors in RAB)
AB)
2080
2060
AB)
AB)
10250
10230
10210
10190
10170
10150
10130
10110
10090
10070
10050
2040
0
20 40 60 80 100 120
Ambient Temperature (°C)
VDD = 5.5V
VDD = 2.7V
-40
-20
0
20 40 60 80 100 120
Ambient Temperature (°C)
FIGURE 8:
MCP402X 10 kΩ – Nominal
Resistance (Ω) vs. Ambient Temperature and
VDD.
49800
49600
49400
49200
49000
48800
48600
48400
48200
48000
VDD = 5.5V
VDD = 2.7V
-40
VDD = 5.5V
-20
0
20 40 60 80 100 120
Ambient Temperature (°C)
FIGURE 9:
MCP402X 50 kΩ – Nominal
Resistance (Ω) vs. Ambient Temperature and
VDD.
2020
VDD = 2.7V
2000
0
40
80
Ambient Temperature (°C)
Nominal Resistance (R
(Ohms)
Depending on the silicon implementation of the RS
resistors will determine the characteristic shape of the
resistance over temperature. For these devices, the RS
resistor was designed so that one part of the resistor
has a negative temperature coefficient and another
part of the resistor has a positive coefficient. That is the
reason why the resistance bows over the temperature
range. This is done to minimize the end-to-end change
in resistance, and in effect reduces the worst-case
delta resistance over temperature.
-20
FIGURE 7:
MCP402X 5 kΩ – Nominal
Resistance (Ω) vs. Ambient Temperature and
VDD.
AB)
2: For this characterization, Terminal A =
VDD and Terminal B = VSS.
Nominal Resistance (R
(Ohms)
4900
-40
Nominal Resistance (R
(Ohms)
Note 1: The MCP401X and MCP402X devices
have 6-bits of resolution (RAB = 63 RS).
-40
2.7V Vdd
5.5V Vdd
4925
4800
For this discussion, we will look at the characterization
graphs from the MCP402X Data Sheet (DS21945D).
These graphs are shown in Figure 6 through Figure 9.
These graphs are used to illustrate several points, but
the general characteristics will be seen on all digital
potentiometers.
Note:
4950
Nominal Resistance (R
(Ohms)
VARIATIONS OVER VOLTAGE AND
TEMPERATURE
120
FIGURE 6:
MCP402X 2.1 kΩ – Nominal
Resistance (Ω) vs. Ambient Temperature and
VDD.
DS01080A-page 6
© 2007 Microchip Technology Inc.
AN1080
From the analysis, it can be determined that the smaller
the RAB resistance, the greater the effect that voltage
and temperature has as a percentage of the target
resistance.
It is interesting to note that depending on the devices
target RAB value, either limiting the voltage of operation
or limiting the temperature range will lead to minimizing
the variation. In the case of the 2.1 kΩ device, if the
voltage is held constant, the variation is about 1%,
while the variation over temperature is about 2.2%. On
the 5.0 kΩ device, variation over temperature is about
the same as the variation over voltage. for the 10.0 kΩ
and 50.0 kΩ devices, the variation over voltage is much
larger than the variation over temperature.
Also, if the application is operating at a narrower
voltage or temperature window, the RAB variation will
be less than across the entire voltage/temperature
range.
TABLE 3:
RAB VALUES AND VARIATION OVER VOLTAGE AND TEMPERATURE
Max.
2053
2075
2052
2075
22
0.95%
2030
2010
2018
2007
2030
23
1.10%
Delta Resistance over Voltage
35
43
57
48
45
% (of Target Resistance: 2.1 kΩ)
1.67%
2.05%
2.71%
2.29%
2.14%
5.0 kΩ
5.5V
4895
4873
4920
4873
4920
47
0.94%
2.7V
4860
4825
4860
4824
4860
36
0.72%
Delta Resistance over Voltage
35
48
60
49
60
% (of Target Resistance: 5.0 kΩ)
0.70%
0.96%
1.20%
0.98%
1.20%
10.0 kΩ
5.5V
10223
10113
10152
10092
10223
131
1.31%
2.7V
10200
10073
10102
10050
10200
150
1.50%
Delta Resistance over Voltage
23
50
40
42
23
0.23%
0.50%
0.40%
0.42%
0.23%
5.5V
49590
48880
49220
48810
49590
780
1.56%
2.7V
49510
48880
49080
48790
49510
720
1.44%
% (of Target Resistance:
10.0 kΩ)
50.0 kΩ
Delta Resistance over Voltage
% (of Target Resistance:
50.0 kΩ)
80
0
140
20
80
0.16%
0.00%
0.28%
0.04%
0.16%
% (of Target
Resistance)
Min.
2065
Lowest Min.(1) to
Highest Max.(1)
+125°C
% (of Target
Resistance)
+25°C
5.5V
2.7V
Delta
-40°C
2.1 kΩ
Voltage
Device RAB
Characterization RAB Value
68
3.24%
96
1.92%
173
1.73%
800
1.6%
Note 1: The lowest Minimum is typically found at 2.7V and the highest Maximum is typically found at 5.5V.
See shaded cells.
© 2007 Microchip Technology Inc.
DS01080A-page 7
AN1080
This change in wiper resistance (RW) effects the INL of
the device much greater for devices with the smaller
RAB (and therefore RS) resistance value. This can be
seen in comparing the wiper resistance and INL error in
the graphs of Figure 11 and Figure 13.
Wiper Resistance
(Rw)(ohms)
100
25C Rw
25C INL
25C DNL
85C Rw
85C INL
85C DNL
125C Rw
125C INL
125C DNL
80
60
RW
20
0
0
8
16 24 32 40 48
Wiper Setting (decimal)
8
-2
16 24 32 40 48
Wiper Setting (decimal)
56
-40C Rw
-40C INL
-40C DNL
150
25C Rw
25C INL
25C DNL
85C Rw
85C INL
85C DNL
125C Rw
125C INL
125C DNL
0.15
0.1
INL
0.05
100
0
RW
50
-0.05
DNL
0
-0.1
0
8
16
24
32
40
48
56
Wiper Setting (decimal)
FIGURE 12:
MCP402X 50 kΩ Rheo
Mode – RW (Ω), INL (LSb), DNL (LSb) vs. Wiper
Setting and Ambient Temperature (VDD = 5.5V).
-40C Rw
-40C INL
-40C DNL
25C Rw
25C INL
25C DNL
85C Rw
85C INL
85C DNL
125C Rw
125C INL
125C DNL
1.5
1
RW
400
0.5
INL
300
0
DNL
200
-0.5
-0.2
100
-1
-0.4
0
56
FIGURE 10:
MCP402X 2.1 kΩ Rheo
Mode – RW (Ω), INL (LSb), DNL (LSb) vs. Wiper
Setting and Ambient Temperature (VDD = 5.5V).
DS01080A-page 8
0
DNL
RW
200
500
0
DNL
2
100
0.6
0.2
40
4
200
600
0.4
6
300
0.8
INL
8
FIGURE 11:
MCP402X 2.1 kΩ Rheo
Mode – RW (Ω), INL (LSb), DNL (LSb) vs. Wiper
Setting and Ambient Temperature (VDD = 2.7V).
Wiper Resistance
(Rw)(ohms)
-40C Rw
-40C INL
-40C DNL
Error (LSb)
120
10
125C Rw
125C INL
125C DNL
Error (LSb)
Depending on the configuration of the digital potentiometer in the application (VDD, VA, VB, and wiper
code), the wiper resistance may show waveform over
wiper code.
85C Rw
85C INL
85C DNL
INL
0
Wiper Resistance
(Rw)(ohms)
The variation of the wiper resistance is also influenced
by the wiper code selected and the voltages on
Terminal A and Terminal B.
400
25C Rw
25C INL
25C DNL
0
2: For this characterization, Terminal A =
VDD and Terminal B = VSS.
When the device is at 5.5V, the wiper resistance is
relatively stable over the wiper code settings. As the
device voltage drops, the wiper resistance increases.
Then, at some threshold voltage, the middle codes of
the wiper will start to have the highest resistance (see
Figure 11). This is due to the resistive characteristics of
the analog switch with respect to the voltages on the
switch nodes (source, drain, and gate).
-40C Rw
-40C INL
-40C DNL
Error (LSb)
Note 1: The MCP401X and MCP402X devices
have 6-bits of resolution (RAB = 63 RS).
500
Wiper Resistance
(Rw)(ohms)
For this discussion, we will look at the characterization
graphs from the MCP402X Data Sheet (DS21945D).
These graphs are shown in Figure 10 through
Figure 13. These graphs are used to illustrate several
points, but the general characteristics will be seen on
all digital potentiometers.
Error (LSb)
RW Variation
-1.5
0
8
16
24
32
40
48
56
Wiper Setting (decimal)
FIGURE 13:
MCP402X 50 kΩ Rheo
Mode – RW (Ω), INL (LSb), DNL (LSb) vs. Wiper
Setting and Ambient Temperature (VDD = 2.7V).
© 2007 Microchip Technology Inc.
AN1080
Table 4 shows the relationship of the Step resistance
(RS) to the Wiper Resistance. This is important to
understand when the resistor network is being used in
a Rheostat configuration, since the variation of the
wiper resistance (RW) has a direct effect on the RBW (or
RAW) resistance. The system can be designed to
calibrate these variations as long as the system is
capable of measuring the digital potentiometer device
voltage and the system temperature.
TABLE 4:
TYPICAL STEP RESISTANCES AND RELATIONSHIP TO WIPER RESISTANCE
RW / RS (%) (1)
Resistance (Ω)
RW = Typical
RW = Max @ 5.5V
RW = Max @ 2.7V
RW = Typical
RW = Max @ 5.5V
RW = Max @ 2.7V
Wiper (RW) (3)
Typical
225.0%
375.0%
975.0%
3.57%
5.95%
15.48%
325
94.5%
157.5%
409.5%
1.5%
2.50%
6.50%
325
47.25%
78.75%
204.75%
0.75%
1.25%
3.25%
125
192.0%
256.0%
320.0%
0.75%
1.0%
1.25%
0.65%
8-bit Device
(256 resistors)
33.33
—
75
125
325
5000
79.37
—
75
125
10000
158.73
—
75
125
—
39.06
75
100
2100
50000
100000
(4)
6-bit Device
(63 resistors)
Step (RS)
Total
(RAB)
Typical
RW / RAB (%) (2)
Max @ Max @
5.5V
2.7V
793.65
—
75
125
325
9.45%
15.75%
40.95%
0.15%
0.25%
—
195.31
75
100
125
38.4%
51.2%
64.0%
0.15%
0.20%
0.25%
—
390.63
75
100
125
19.2%
25.6%%
32.0%
0.08%
0.10%
0.13%
Note 1: RS is the typical value. The variation of this resistance is minimal over voltage.
2: RAB is the typical value. The variation of this resistance is minimal over voltage.
3: RW values are taken from the MCP402X Data Sheet (6-bit devices) and the MCP41XXX/MCP42XXX Data
Sheet (8-bit devices).
4: MCP41XXX and MCP42XXX devices.
© 2007 Microchip Technology Inc.
DS01080A-page 9
AN1080
THE A AND B TERMINALS
TABLE 5:
The voltage on the A and B terminals (VA and VB) can
be any voltage within the devices power supply rails
(VSS and VDD). Lets call the voltages at these nodes,
VA and VB.
This allows a less precise (lower cost) device to be
used for more precise circuit tuning over a narrower
voltage range. Table 5 shows the effective resolution of
the digital potentiometer relative to the system voltage
and the VA - VB voltage.
V1
R1
6-bit Device
(63 RS)
8-bit Device
(256 RS)
VAB
(V)
5.0
79.4
19.5
6-bits
8-bits
VAB = VDD
2.5
39.7
9.8
7-bits
9-bits
VDD = 5.0V,
VAB = VDD/2
1.25
1.98
4.9
8-bits
10-bits VDD = 5.0V,
VAB = VDD/4
W
POT1 (RAB)
Some devices support a “shutdown” mode. The
purpose of this mode is to reduce system current. A
common implementation is to disconnect either Terminal A or Terminal B from the internal resistor ladder.
This creates an open circuit and eliminates the current
from Terminal A (or Terminal B) through the RS
resistors to Terminal B (or Terminal A). The current to/
from the wiper depends on what the device does with
the W Terminal in shutdown. The MCP42XXX device
forces the W Terminal to connect to Terminal B (Zero
Scale).
A
SHDN
B
VB
R2
V2
FIGURE 14:
N = 256
(100h)
RW
N = 255
(FFh)
RW
RS
RS
W
Windowed Trimming.
There is no requirement for a voltage polarity between
Terminal A and Terminal B. This means that VA can be
higher or lower then VB.
Comment
Shutdown Mode
VA
A
Effective
Resolution
8-bit Device
(256 RS)
This means that the potentiometer can be used to trim
a voltage set point within a defined voltage window (see
Figure 14). So, if the digital potentiometer is 8-bits (256
steps) and the delta voltage between VA and VB is 1V,
then each step of the digital potentiometer results in a
change of 1/256 V, or 3.9 mV. If the device needed to
have this resolution over an entire 5V range, then the
digital potentiometer would require 1280 steps, which
is over 10-bits of accuracy.
Step Voltage
(VS) (mV)
6-bit Device
(63 RS)
The voltage across the resistor RAB (VAB) is | VA - VB |.
In the circuit shown in Figure 14, as the VAB voltage
becomes smaller relative to the voltage range, the
effective resolution of the device increase, though the
resolution is limited to between the VA and VB voltages.
HOW THE VAB VOLTAGE
EFFECTS THE EFFECTIVE
RESOLUTION
RS
SHDN
B
N=1
(01h)
RW
N=0
(00h)
RW
Analog
Mux
FIGURE 15:
Disconnecting Terminal A (or
Terminal B) from the Resistor Ladder.
DS01080A-page 10
© 2007 Microchip Technology Inc.
AN1080
IMPLEMENTING A MORE PRECISE
RHEOSTAT
12 kΩ
The RAB (RS) value of a typical digital potentiometer
can vary as much as ±20% from device to device. This
variation can have a great effect on a circuit that is
using the RBW resistance for tuning and this variation
for the rheostat value may not be desirable.
8 kΩ
If you want to make your variable resistor more precise
for system calibration and tuning, the following
technique may be useful.
To create a circuit with greater accuracy, the system
needs to be able to calibrate the digital potentiometer
to make a precise rheostat. This is at a cost of the
resolution of the digital potentiometer.
At the system manufacturing test, a method needs to
be present to measure the resistance of the RAB value.
This could be done by measuring the current through
RAB. This value (RAB(CAL)) would be saved on the
embedded systems non-volatile memory. The embedded systems controller could use this information to
calibrate the rheostat value (RBW), where:
RBW = ((RAB(CAL)/Resolution) * Wiper Value) + RW
For this discussion, we will use a digital potentiometer
with a typical RAB resistance of 10 kΩ. That means that
the RAB resistance could be as small as 8 kΩ
(RAB(MIN)) or as large as 12 kΩ (RAB(MAX)). Figure 16 a
graphic representation of the variations of RAB
resistance by showing the minimum and maximum
resistances verses the wiper code value.
Table 6 shows the actual calculations for each step for
the typical RAB resistance (10 kΩ) and worst-case RAB
resistances (8 kΩ and 12 kΩ). When the RAB (RBW)
resistance is 12 kΩ, the RBW = 8 kΩ crossover occurs
at wiper value 171 (decimal).
Very few devices will actually be the 8 kΩ value, but
every device will have a wiper register value that will be
close to this 8 kΩ resistance. The circuit should
assume that the resistance is the minimum. That is
because all devices can have a wiper value which
“creates” this resistance value.
The embedded systems controller firmware would take
the calibration value and ensure that the digital potentiometer wiper value did not exceed the desired
resistance (8 kΩ). For a system that had a “typical”
device (10 kΩ), that would mean the wiper value would
not exceed 205 (decimal), while for a “+20% “device
(12 kΩ) the wiper value would not exceed 171
(decimal). These values give the closest resistance
value to the desired rheostat target value of 8 kΩ.
The calibration information could be represented as the
maximum wiper value code or as the actual RAB or RS
value. The embedded systems controller firmware then
would calculate the appropriate wiper values for the
desired RBW resistance. Voltage and temperature
calibration information could also be stored.
© 2007 Microchip Technology Inc.
Zero
Scale
(0)
FIGURE 16:
RBW Resistance
10 kΩ
Full
Scale
(256)
171 205
Wiper Code
RAB Variation.
Here we have designed the application circuit where
this rheostat only operates from 0Ω to 8 kΩ and all
digital potentiometer devices (over process) will meet
this requirement. This means that we have reduced the
resolution of the digital potentiometer since we no
longer have the full 256 steps. Looking at the worstcase resistance (+20%), there are a maximum of 171
steps. This means that the worst-case step accuracy is
1/171 (~0.58%). This represents a resolution of
approximately 7.4-bits. We have a trade-off between a
precise variable resistor and the resolution (number of
steps) that the variable resistor can support.
The error from the 8 kΩ target will be no greater than
±RS(MAX)/2 (or ± 23.5Ω) which is ≤ 0.29%. Where:
RS(MAX) = RAB(MAX)/Resolution
= 12000/256 = 46.875Ω.
Any RBW resistance ≤ 8 kΩ can be selected for the
variable resistor range. Choosing a lower resistance
does not necessarily affect the accuracy, but does
affect the number of steps available for the resistor.
Let’s say that we select a 5 kΩ resistance, the wiper
values would range from 107 (+20%) to 160 (-20%).
The worst-case (minimum) number of steps is 107,
which gives an step accuracy of 1/107 (~0.93%) and an
error from target resistance ≤ 0.47%. This is still in line
with systems designed using 1% resistors, but still
requires a fixed voltage and temperature. Additional
calibration values can be used to correct for the change
of the wiper resistance (RW) over temperature and
voltage.
Additional embedded systems controller firmware
calibration can be done to take into account the change
in RS and RW resistance over temperature and voltage.
DS01080A-page 11
AN1080
Referring to Table 3, for the 10 kΩ (typical) device, the
RAB variation over the specified voltage range is
~0.4%. The RAB variation for a given device over
temperature is ~1.4%. Other system techniques could
be used to calibrate out the effect of these variations.
A precise variable resistor can be implemented in a
system, if each system’s digital potentiometer is
calibrated.
Table 6 shows the calculations for a 10 kΩ device, over
process. The calculation is based on an 8-bit device
that has 256 step resistors (RS) and 257 steps. When
the Wiper code value is “01”, that shows the step resistance (RS).
TABLE 6:
Min. (-20%)
Typical
0.00
0.00
31.25
39.0625
46.875
02
62.50
78.125
93.75
0.00
:
:
:
106
3312.50
4140.625
4868.75
107
3343.75
4179.6875
5015.625
108
3375.00
4218.75
5062.50
:
:
:
159
4968.75
6210.9375
7453.125
160
5000.00
6250.00
7500.00
161
5031.25
6289.0625
7546.875
:
:
:
170
5312.50
6640.625
7968.75
171
5343.75
6679.6875
8015.625
172
5375.00
6718.75
8062.50
:
:
:
204
6375.00
7968.75
9562.50
205
6406.25
8007.8125
9609.375
206
6437.50
8046.875
9656.25
:
:
:
:
:
Comment
Max (+20%)
01
:
Using some of these calibration techniques, it was
shown how a precise rheostat (variable resistor) can be
implemented in a system.
RBW RESISTANCE AT WIPER CODE - 10 kΩ (TYPICAL) 8-BIT (256 RS’S) DEVICE
00
:
We have discussed how the components of the resistor
network (RAB, RS, and RW) can vary over process,
voltage, temperature, and wiper code. Understanding
these variations allows you to understand the
implications in your application and if required use
techniques to compensate or calibrate for these
variations to optimize the application operation.
RBW Resistance (Ω) (1)
Wiper
Code
:
SUMMARY
254
7937.50
9921.875
11906.25
255
7968.75
9960.9375
11953.125
256
8000.00
10000.00
12000.00
This indicates the RS resistance value
This Wiper Code makes a +20% device have the closest
resistance to the 5 kΩ target.
This Wiper Code makes a -20% device have the closest
resistance to the 5 kΩ target.
This Wiper Code makes a +20% device have the closest
resistance to the 8 kΩ target.
This Wiper Code makes a typical device have the closest
resistance to the 8 kΩ target.
8 kΩ resistance is the maximum resistance that is
supported by ALL 10 kΩ (typical) devices (over process)
Note 1: RBW resistance assume a wiper resistance (RW) of 0Ω.
DS01080A-page 12
© 2007 Microchip Technology Inc.
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•
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•
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•
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DS01080A-page 13
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DS01080A-page 14
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