AN-1169: Linear Gain Setting Mode: A Detailed Description (Rev. A) PDF

AN-1169
APPLICATION NOTE
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Linear Gain Setting Mode: A Detailed Description
by Miguel Usach
INTRODUCTION
Digipots are commonly used to digitally program the gain in an
amplifier or set the output voltage of a power supply regulator
as shown in Figure 1 and Figure 2.
If the resistances R1 and R2 are directly replaced with a digital
potentiometer, then the transfer function becomes logarithmic.
Figure 4 shows an example for the LDO.
4.5
ADP123
4.0
VOUT
R1
3.5
11057-001
OUTPUT VOLTAGE (V)
ADJ
R2
Figure 1. Adjustable Output Voltage LDO
VIN
AD8515
R1
2.0
1.5
0.5
0
CODE
Figure 2. Noninverting Amplifier
Figure 4. Logarithmic Transfer Function for the LDO
In both cases, the transfer equation depends on two different
variables, R1 and R2, as shown in Equation 1 for the LDO and
in Equation 2 for the noninverting amplifier.
 R
VOUT = 0.5 × 1 + 1
 R2
2.5
1.0
11057-002
R2
VOUT
3.0




(1)

R 
VOUT = VIN × 1 + 1 
R
2 

(2)
Using the digipot in potentiometer mode in these transfer
equations is not straightforward since both resistors strings,
RAW and RWB, are complementary; in other words,
RAW = RAB − RWB, as shown in Figure 3.
11057-004
VOUT
This logarithmic transfer function can be desirable in some
applications, such as light or audio control, because the human
body is not a linear receptor of those stimuli, but in many
electronic applications a linear transfer function is preferred.
LINEARIZING THE OUTPUT
There are three different methods to achieve a linear output
directly proportional to the code loaded in the digipot. These
three methods are described in detail in the sections that follow.
Use the Digipot in Rheostat Mode
The digipot can be used in rheostat mode, where only two
terminals are used, as shown in Figure 5.
A
A
RAW
W
W
B
11057-003
RWB
B
11057-005
RAB
Figure 5. Rheostat Mode
Figure 3. Potentiometer Resistance
Rev. A | Page 1 of 4
AN-1169
Application Note
This mode requires using a discrete resistor in conjunction with
the digipot. An example of the noninverting amplifier is shown
in Figure 6.
VIN
AD8515
R2
The consequences are similar to the previous approach, that
is, the adjustable output gain is reduced, but, in this case, the
settling time is reduced due to the lower RWB’ value, as defined
in Equation 4.
RWB ' =
VOUT
(4)
The overall parallel resistance value is smaller, thus the resistor
noise is lower than the series resistance approach.
11057-006
RWB
RHEOSTAT MODE
R2 × RWB
R2 + RWB
Figure 6. Noninverting Amplifier with Rheostat Control
The main benefit of using this solution is the simplicity in the
circuit, the wide output ranges, and the fast settling time. As
a trade-off, the overall output error may be quite high because
the typical tolerance error in a digipot is around ±20% maximum. Given that R2 is fixed, this can cause resistor mismatches.
Analog Devices, Inc., offers ±8% and ±1% resistor tolerance
error digipots to improve the performance in these
configurations as shown in the selection table.
As a precaution, remember that the digipot has internal leakage
current. If you choose the parallel resistor, R2, to be small
enough to force not enough current through the digipot, the
linearity errors, R-INL and R-DNL, could be considerable
higher than specified in the data sheet.
Linearize the Potentiometer
Configuring the digipot as a vernier DAC, as shown in Figure 9,
the voltages in the Terminal A and Terminal B are limited by
the placement of the in-series resistors, R1 and R2.
+IN
Additionally, the output error can be reduced by using a serial
resistor with the digipot, as shown in Figure 7, for the LDO.
RAB
A
R1
RAW
W
ADP123
R2
R1
ADJ
RWB’
–IN
R2
RHEOSTAT
MODE
RWB
11057-009
VOUT
Figure 9. Vernier DAC
RWB
11057-007
The idea of this approach is to reduce the output range resulting
in a more linear output, as shown in Figure 10, for two different
configurations.
Figure 7. Reduced Tolerance Error with Series Resistance
1.2
In this case, to assume the 20% tolerance error is negligible
R2 >> RWB; or, in other words, the output error can be improved
by reducing the adjustable output gain and increasing the
settling time. The final resistance is defined in Equation 3.
R1 = 1kΩ, R2 = 50kΩ
R1 = 1kΩ, R2 = 10kΩ
LINEAR (R1 = 1kΩ, R2 = 50kΩ)
LINEAR (R1 = 1kΩ, R2 = 10kΩ)
1.0
OUTPUT VOLTAGE (V)
RWB ' = R2 + RWB
(3)
A second way to reduce the error is by placing a parallel
resistance with the digipot as shown in Figure 8.
ADP123
0.8
0.6
0.4
VOUT
0
R1
ADJ
CODE
RWB’
RWB
11057-010
0.2
VOUT
Figure 10. LDO Voltage with a Vernier DAC
R2
This configuration provides lower linearity error than using the
digipot in rheostat mode and it results in a lower tempco.
11057-008
RHEOSTAT
MODE
OUT
B
VOUT
Figure 8. Reduced Tolerance Error with Parallel Resistance
In this case, the assumption is that R2 << RWB due to the
nominal end-to-end resistor values, 10 kΩ, 50 kΩ and 100 kΩ.
The final resistance between terminals is defined in Equation 5
and Equation 6.
Rev. A | Page 2 of 4
R1 ' = R1 + R AW
(5)
R2 ' = R2 + RWB
(6)
Application Note
AN-1169
Enable the Linear Gain Setting Mode
It is recommended to use a low resistor tolerance error digipot,
±8% and ±1%. Note that the higher the tolerance, the greater
the mismatch resistance error.
In linear gain setting mode, the internal resistors strings, RAW
and RWB, are dependant. The newly patented architecture implemented in the AD5144, AD5142, AD5124A, and AD5141
improves flexibility, allowing independent programming of the
value for each string, RAW and RWB, as shown in Figure 13.
In this case, using a typical 20% resistance tolerance error, a
parallel resistor should be used with the digipot to reduce the
overall error as shown in Figure 11.
+IN
RDAC
REGISTER
AW
R1
RAB
A
RAW
RAW
R3
OUT
RAB = RAW + RWB
–IN
RDAC
REGISTER
WB
Again, it is important to consider the effect of the leakage
current in this configuration. Selecting a low parallel value
could force the current through R3.
Figure 13. Linear Gain Setting Mode
Enabling this mode, the output voltage can be linear, fixing the
value of one resistor string, that is, RWB, and setting the other
string, RAW. The mode of operation is similar to using the
digipot in rheostat mode in conjunction with a discrete resistor,
but in this case the overall tolerance error is below 1% without
using any external parallel or series resistance combination.
To calculate the final resistance between terminals can be quite
complex, thus the best approach is to use a Y-Δ transform as
shown in Figure 12.
RAW
+IN
R3
+IN
RWB
RAB
R1
R4
OUT
This is achieved because the gain is set by the resistance ratios,
and the overall resistor tolerance error, as is common in both
string arrays, can be disregarded.
RAW’
R6
OUT
R5
R4
–IN
R6
R5
R2
RWB’
11057-012
R2
–IN
Figure 12. Y-Δ Transform
Figure 14 shows an example of sweeping RAW from zero scale to
full scale, fixing RWB at midscale for a 10 kΩ digipot. Analyzing
the plot in detail, at lower codes when the resistances, RAW or
RWB, are small, the mismatch becomes higher than ±1%. This is
due to an error added by the non-negligible effect in the
internal CMOS switches resistance.
where:
5
R AW xR3
R AB + R3
(7)
R5 =
RWB xR3
R AB + R3
(8)
R AW ' = R1 + R4
(9)
RBW ' = R2 + R5
(10)
4
MISMATCH ERROR (%)
R4 =
R6 should be connected to a high impedance input so that the
effect of this resistance can be considered negligible.
3
2
1
0
–1
0
50
100
150
200
RAW DECIMAL CODE
Figure 14. 10 kΩ Resistance Match Error
Rev. A | Page 3 of 4
250
11057-114
R3
11057-013
11057-011
RWB
B
Figure 11. Reduced Tolerance Error in Vernier DAC
R1
W
RWB
R2
AN-1169
Application Note
The switches effect can be cancelled by selecting codes higher
than quarter scale.
RWB at code 250 is −2 ppm/C
The gain error due to RAW is
Enabling linear setting mode, the maximum resistance between
Terminal A and Terminal B can be set to double the nominal
digipot resistance. In other words, if the RAB resistance is 10 kΩ
in potentiometer mode, in lineal setting mode when programming both string resistors at full scale, the RAB = 20 kΩ.
ErrorRAW =
The gain error due to RWB
ErrorRBW = 0.04%
Thus, the total error is defined as,
GAIN ERROR = ErrorRAW + ErrorRWB = 0.17%
75
Similar to the resistance match error, at lower codes the switch
resistance tempco is dominant, but the effect is minimizing at
higher codes.
55
35
If a lower error vs. temperature is required, a higher end-to-end
resistance value needs to be used as shown in Figure 17 for
100 kΩ. In this particular case, the tempco is much more flat in
all the code range, so the error expected should be smaller.
0
–5
0
50
100
150
200
CODE
255
20
TEMPERATURE COEFFICIENT (ppm/°C)
Figure 15. 10 kΩ Resistance Tempco
Take, for example, the circuit in Figure 16. Choosing a gain of 3,
the codes ratio is defined in equation 11.
AD5141
A
B
RAW
RWB
W
11057-117
VOUT
VIN
Gain = 1 +
RWB
⇒ 2 × R AW = RWB
R AW
(11)
Fixing the RWB code to 250, the RAW code is 125. As a rough
estimate, the overall error due tempco over the full temperature
range is defined as,
RAW at code 125 is 20 ppm/C
10
0
–10
–20
–30
–40
Figure 16. Noninverting Amplifier and AD5141 in Linear Gain Setting Mode
RWA
RWB
0
50
100
150
200
CODE
255
11057-017
15
11057-016
TEMPERATURE COEFFICIENT (ppm/°C)
=
9765.625

3 − 1 +

 4882.8125 + 9.765 
= 0.13%
=
3
TEMPCO RWB
TEMPCO RAW
95
Gain




RW B (250)

3 − 1 +
20 × R AW (125) × 100 

(
125
)
+
R


AW
1e6


=
=
3
Similar performance can be achieved by using a dual channel
digipot, but this solution increases the cost and size with
degradation in the settling time.
Another additional benefit of using this configuration is the
reduced temperature coefficient, as shown in Figure 14. In this
case, the importance is not the absolute tempco for each string
resistor, but the difference between the tempco for the specific
codes that define the ratio.
Gain − Gain RAW
Figure 17. 100 kΩ Resistance Tempco
REVISION HISTORY
8/13—Rev. 0 to Rev. A
Changes to Equation 2 ......................................................................1
12/12—Revision 0: Initial Version
©2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN11057-0-8/13(A)
Rev. A | Page 4 of 4
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