AN-1205: 30 V Low Cost DAC Using the AD5292 Digital Potentiometer (Rev. B) PDF

AN-1205
APPLICATION NOTE
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30 V Low Cost DAC Using the AD5292 Digital Potentiometer
This circuit shown in Figure 1 employs the AD5292 digital
potentiometer in conjunction with the ADR512 reference and
ADA4091-2 op amp, providing a 10-bit, low cost, high voltage
DAC. The circuit guarantees monotonicity, ±1 LSB DNL, and
has an integral nonlinearity of ±2 LSB typical.
CIRCUIT FUNCTION AND BENEFITS
This circuit shown in Figure 1 provides a low cost, high voltage
unipolar DAC using the AD5292 digital potentiometer in
conjunction with the dual ADA4091-2 op amp and ADR512
voltage reference. This circuit offers 10-bit resolution over an
output voltage range of 0 V to 30 V and is capable of delivering
up to ±20 mA output current. The AD5292 is programmable
over an SPI-compatible serial interface.
The high voltage regulator consists of a low voltage reference
followed by a noninverting amplifier whose gain is set by the
ratio of R1 to R2. The ADR512 1.200 V voltage reference has
low temperature drift, high accuracy, and ultralow noise
performance.
The ±1% resistor tolerance of the AD5292 allows it to be placed
in series with external divider resistors R3 and R4, as shown in
Figure 2 and Figure 5, to create a vernier DAC with 10-bit
resolution over a reduced VOUT range. This serves to increase
the sensitivity of the DAC, similar to adding resistance in series
with a potentiometer. In addition, the AD5292 has an internal
20-times programmable memory that allows a customized VOUT
at power-up.
The maximum resistor that ensures an ADR512 minimum
operating current is defined in Equation 1.
RBIAS 
The ADA4091-2 is an op amp that offers a low offset voltage
and rail-to-rail output. The ADR512 in combination with
the ADA4091 offer a low tempco and noise output voltage.
CIRCUIT DESCRIPTION
The resistors R1 and R2 adjust the gain in the amplifier. The
V1 output voltage of U1A defines the maximum VOUT range
of the DAC. Equation 2 is used to calculate the resistor
values.
Table 1. Devices Connected/Referenced
ADA4091-2
Description
Digital potentiometer, 10 bits, 1% resistor
tolerance
Low noise, precision 1.200 V reference
V1  1.2  (1 
Micropower, overvoltage protected (OVP)
rail-to-rail op amp
VDD
+30V
RBIAS
ADR512
U1A
VDD
+30V
12kΩ
10nF
R1
)
R2
1/2
V1
ADA4091-2
VDD +30V
U1B
AD5292
R2
1kΩ ± 1%
20kΩ
R1
22.1kΩ ± 1%
SERIAL
INTERFACE
VSS
2/2
VOUT
ADA4091-2
08415-001
ADR512
(1)
1 mA
In Figure 1 and Figure 2 the RBIAS resistor is 12 kΩ, which sets
the bias current of the ADR512 at 2.4 mA.
The circuit provides accurate, low noise, and low tempco output
voltage capability and is well suited for digital calibration
applications.
Product
AD5292
VDD – 1.2 V
Figure 1. Unipolar DAC Configuration (Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. B | Page 1 of 3
(2)
AN-1205
Application Note
VDD
+30V
RBIAS
ADR512
12kΩ
U1A
10nF
VDD
+30V
1/2
V1
ADA4091-2
R3
4.99kΩ ± 1%
VDD
R2
1kΩ ± 1%
+30V
U1B
R1
AD5292
22.1kΩ ± 1%
20kΩ
2/2
VOUT
ADA4091-2
SERIAL
INTERFACE
VSS
08415-002
R4
4.99kΩ ± 1%
Figure 2. Increasing the Accuracy by Reducing the Output Voltage Range and Using a Vernier DAC
(Simplified Schematic: Decoupling and All Connections Not Shown)
In Figure 1 the resistors are chosen to provide a gain of 23.1
and a V1 value of 27.72 V. This voltage can be used to power
other circuits with a maximum output current of 17 mA.
Typical INL and DNL plots are shown in Figure 3 and Figure 4.
In the configuration of Figure 1, the AD5292 operates
ratiometrically, which means that variation in the total
resistor tolerance does not affect the performance.
0
–1
0
500
1023
08415-003
INL (LSBs)
1
CODE (DEC)
Figure 3. INL vs. DAC Code
To improve circuit accuracy, the voltage reference across the
AD5292 can be reduced by using two external resistors, as
shown in Figure 5, which gives the full 10-bit resolution over a
limited voltage range (vernier DAC). Traditionally, digital
potentiometers have a ±20% end-to-end resistor tolerance error,
which affects the circuit accuracy because of mismatch error
between the digital potentiometer and the external resistors.
The industry-leading ±1% resistor tolerance performance of the
AD5292 helps to overcome the mismatch resistance error.
0.15
V1
R3
33.2kΩ ± 1%
V'1
24.85V
–0.05
AD5292
–0.25
–0.35
0
500
1023
VOUT
V'2
23.11V
R4
267kΩ ± 1%
08415-005
20kΩ
SERIAL
INTERFACE
–0.15
08415-004
DNL (LSBs)
0.05
27.72V
CODE (DEC)
Figure 5. INL Improvement Using Reduced Reference Voltages
to Create a Vernier DAC (Simplified Schematic: Decoupling
and All Connections Not Shown)
Figure 4. DNL vs. DAC Code
Rev. B | Page 2 of 3
Application Note
AN-1205
In this case:
V’1 = 24.85 V
(3)
V’2 = 23.11 V
(4)
1 LSB over the reduced range can be calculated:
1 LSB =
V'1 – V' 2
1024
= 1.69 mV
(5)
Equivalent resolution of the vernier DAC relative to the full
reference voltage V1:
 V1 
 = 14 bits
Resolution = log 2 
 1.69 mV 


Table 2. Typical Specifications for the Unipolar DAC in Figure 1
Parameter
Power Supply
Output Voltage
Output Current
DNL
INL
Settling Time
Min
30
0
–1
–2
0.2
Max
33
30
±20
+1
+2
2
Unit
V
V
mA
LSB
LSB
µs
LEARN MORE
(6)
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND", Analog Devices.
Figure 6 shows the INL (referenced to V1) using the vernier
DAC circuit of Figure 5.
MT-032 Tutorial, Ideal Voltage Feedback (VFB) Op Amp, Analog
Devices.
The AD5292 has a 20-times programmable memory, which
allows presetting the output voltage in a specific value at
power-up.
MT-087 Tutorial, Voltage References, Analog Devices.
MT-091 Tutorial, Digital Potentiometers, Analog Devices.
Excellent layout, grounding, and decoupling techniques must be
utilized in order to achieve the desired performance from the
circuits discussed in this note (see Tutorial MT-031 and Tutorial
MT-101). As a minimum, a 4-layer PCB should be used with one
ground plane layer, one power plane layer, and two signal layers.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Voltage Reference Wizard Design Tool
Data Sheets and Evaluation Boards
AD5291 Data Sheet
AD5292 Data Sheet
COMMON VARIATIONS
AD5292 Evaluation Board
The AD5291 (8-bit with 20-times programmable power-up
memory) and AD5293 (10-bit, no power-up memory) are both
±1% tolerance digital potentiometers that are suitable for this
application.
AD5293 Data Sheet
The ADR5044 low cost 4.096 V reference is another possibility
for the reference. The R1/R2 ratio can be adjusted appropriately
for the different reference voltage.
ADA4091-2 Data Sheet
ADA4091-4 Data Sheet
ADR5044 Data Sheet
ADR512 Data Sheet
REVISION HISTORY
INL (LSBs)
0.010
0.005
4/13—Rev. A to Rev. B
Changed Document Title from CN-0111 to
AN-1205 .............................................................................. Universal
0
3/10—Rev. 0 to Rev. A
Changes to Circuit Function and Benefits Section....................... 1
9/09—Revision 0: Initial Version
–0.010
0
200
400
600
800
1023
08415-006
–0.005
CODE (DEC)
Figure 6. INL of Vernier DAC Referenced to V1.
©2009–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN08415-0-4/13(B)
Rev. B | Page 3 of 3
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