Circuit Note CN-0151 Devices Connected/Referenced Circuit Designs Using Analog Devices Products Apply these product pairings quickly and with confidence. For more information and/or support call 1-800-AnalogD (1-800-262-5643) or visit www.analog.com/circuit. AD5446/ AD5543 14-/16-Bit High Bandwidth DACs with Serial Interface OP1177/ AD8510 Precision, Low Noise, Low Input Bias Current Op Amps ADR425/ ADR512 Precision, Low Noise 5 V/1.2 V References Versatile High Precision Programmable Current Sources Using DACs, Op Amps, and MOSFET Transistors CIRCUIT FUNCTION AND BENEFITS CIRCUIT DESCRIPTION Digitally controlled current sources are critical functions in a variety of applications, such as power management, solenoid control, motor control, impedance measurement, sensor excitation, and pulse oximetry. Here we describe three current sources with serial interface digital control using DACs, op amps, and MOSFET transistors. All three circuits require a single 5 V supply for the DACs and ±15 V supplies for the op amps. Some circuits may need an accurate external voltage reference (see MT-087 Tutorial). Each circuit contains two stages. The first stage is the input stage, composed of the DAC and an op amp. The second stage is an N-channel MOSFET transistor output stage (Figure 1 and Figure 2), which supplies the current in response to the digital word sent to the system. The DACs selected are high resolution (14- or 16-bit), low power CMOS with standard serial interfaces. The AD5543 16-bit DAC is packaged in ultracompact (3 mm × 4.7 mm) 8-lead MSOP and 8-lead SOIC packages. The AD5446 14-bit DAC is available in a small 10-lead MSOP package. The two DACs are both compatible with most DSP interface standards and also SPI, QSPI, and MICROWIRE. The external reference voltage input allows many output level variations, up to 10 V. The input stage of the circuit, shown in Figure 1, is composed of a current output DAC (AD5446) and its op amp (AD8510). It provides the conversion of the command word and drives the transistor. It also modulates the voltage applied to the single resistor. The command word is sent via an SPI interface. The output stage is composed of an N-channel MOSFET transistor (NTE4153N), which can provide more current than the output of the op amp and a single resistor. The single resistor, R1, produces the current with the voltage applied to its pins. The transistor regulates this current. The combination of parts represents industry-leading small PC board area, low cost, and high resolution. The three designs offer low risk solutions and use industry-standard parts. +5V 4 SCLK AD5446 DIN 5 SDIN RFB IOUT1 SYNC SDO 6 7 SYNC SDO OUT VDD 8 IOUT2 GND +15V 3pF ILOAD 10 1 2 2 D − 7 AD8510 3 + 3 VREF 9 VIN SEE TEXT AGND 6 NTE4153NT1G G S 4 −15V R1 100Ω AGND 08977-001 SCLK Figure 1. Current Source Using a Current Output DAC (All Connections and Decoupling Not Shown) Rev. A “Circuits from the Lab” from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any“Circuit from the Lab”. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2010-2011 Analog Devices, Inc. All rights reserved. CN-0151 Circuit Note The load current is − D × V IN R1 × (1 + R1/R DAC ) The op amp used in this case is the OP1177. It is a high precision and very low offset device (60 µV maximum). Low offset voltage is essential when the DAC is used in voltage output mode because of the reduced signal swing. where D is the fractional representation of the digital word loaded into the DAC. However, RDAC >> R1, (RDAC is nominally 9 kΩ); therefore, the load current can be approximated as I LOAD = − D × V IN R1 The N-channel MOSFET transistor in conjunction with the op amp makes a high current output follower circuit. The negative feedback from the source pin of the transistor to the op amp input regulates the value of the current through the R1 resistor. With R1 = 100 Ω and VI N = −5 V, ILOAD is programmable from 0 mA to 50 mA with a resolution of 3 µA (1 LSB at 14 bits). The output compliance voltage is approximately 20 V and is limited by the breakdown voltage of the MOSFET transistor. The ADR425 is an ideal 5 V low power precision reference for this circuit, but its output must be inverted with an additional op amp to generate the −5 V reference. The load current is I LOAD = With R1=10 Ω and VIN = 1.2 V, ILOAD can be programmed from 0 mA to 120 mA with a resolution of 7 µA (1 LSB at 14 bits). The circuit shown in Figure 2 also uses the AD5446 DAC. However, in this case the DAC is used in the reverse or voltage mode, which provides a voltage output by using a 1.2 V voltage reference such as the ADR512. 4 SCLK SCLK AD5446 SYNC SYNC 7 SDO RFB IOUT1 6 OUT VDD 8 5 SDIN DIN IOUT2 GND SDO VREF V IN × D R1 +5V +15V ILOAD 10 1 2 VIN = 1.2V SEE TEXT 3 2 OP1177 + 6 NTE4153NT1G G S 4 3 9 D 7 − −15V R1 10Ω AGND 08977-002 I LOAD = The DAC output voltage range on Pin 9 varies from 0 V to 1.2 V. See the AD5446 data sheet for more details on the reverse voltage mode of operation. AGND Figure 2. Current Source Using a Current Output DAC Connected in the Reverse Voltage Mode (All Connections and Decoupling Not Shown) +5V 7 VDD SDI 1 SCLK 2 SDI RFB IOUT 3 +15V 3pF 5 2 7 AD8510 CS 8 CS VREF 4 VIN SEE TEXT 3 6 R2 15kΩ R1 150kΩ 10pF 4 AD5543 GND 6 AGND +15V −15V 2 AD8510 3 AGND 6 −15V R1' 150kΩ R3 50Ω 4 7 R3' 50Ω R2' 15kΩ OUT ILOAD AGND 08977-003 SCLK Figure 3. Bipolar Current Source Based on the Howland Current Source (All Connections and Decoupling Not Shown) Rev. A | Page 2 of 3 Circuit Note CN-0151 LEARN MORE The third circuit, shown in Figure 3, uses an AD5543 16-bit DAC as the input and a Howland current pump circuit as the output stage. Howland current pumps have two advantages over MOSFET outputs: high output impedance and the ability to provide bipolar output currents. Usually, to improve stability, the circuit is symmetrical. Therefore R1 = R1', R2 = R2', and R3 = R3'. MT-015 Tutorial, Basic DAC Architectures II: Binary DACs. Analog Devices. The load current is (see the AN-843 Application Note for derivation) MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Analog Devices. I LOAD = MT-087 Tutorial, Voltage References. Analog Devices. V IN × D × (R2 + R3) R1× R3 MT-101 Tutorial, Decoupling Techniques. Analog Devices. Voltage Reference Selection and Evaluation Wizard. Output impedance is Z OUT = Brennan, Sean. AN-843 Application Note, Measuring a Loudspeaker Impedance Profile Using the AD5933, Analog Devices. Data Sheets and Evaluation Boards R1' R3 × (R1 + R2) R1 × (R2'+R3' ) – R1' × (R2 + R3) AD5446 Data Sheet With R1 = 150 kΩ, R2 = 15 kΩ, R3 = 50 Ω, and VIN = 10 V, ILOAD is programmable from 0 mA to 20 mA with a resolution of 300 nA (1 LSB at 16 bits), and the circuit has a very high output impedance. Excellent layout and grounding and decoupling techniques must be used in all three circuits to separate correctly DACs and op amps and to achieve the desired performances (see the MT-031 and MT-101 tutorials). COMMON VARIATIONS In both circuits, other voltage references can be used to get more or less current output range (see Voltage Reference Selection and Evaluation Wizard). Note that a positive reference voltage input generates a negative output current because of the DAC architecture. Although a wide variety of DACs can be used to optimize the design for speed, precision, and so on, CMOS current output DACs such as the AD5543 and AD5446 give more flexibility and provide low risk solutions. Regarding the op amps, if you have a relatively small output signal range, CMOS amplifiers should work fine. If you want high input impedance, FET input op amps are good choices. In either case, precision amplifiers are required to maintain 14-bit to 16-bit precision. AD5446 Evaluation Board AD5543 Data Sheet AD5543 Evaluation Board AD5553 Data Sheet AD5553 Evaluation Board AD8510 Data Sheet ADR425 Data Sheet ADR512 Data Sheet OP1177 Data Sheet REVISION HISTORY 4/11—Rev. 0 to Rev. A Changes to Circuit Description Section......................................... 2 Changes to Figure 3 .......................................................................... 2 Changes to Learn More Section ...................................................... 3 4/10—Revision 0: Initial Release (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2010-2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08977-0-4/11(A) Rev. A | Page 3 of 3