AN-1205 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com 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