Circuit Note CN-0055 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. AD5450/AD5451/ AD5452/AD5453 8-/10-/12-/14-Bit Multiplying DAC AD8065 High Performance FastFET TM Amplifier Programmable Gain Element Using the AD5450/AD5451/AD5452/AD5453 Current Output DAC Family CIRCUIT FUNCTION AND BENEFITS temperature coefficients of the DAC. This approach is recommended in circuits where gains of greater than 1 are required. In applications where the DAC output voltage range is required to be larger than the input voltage, a programmable gain circuit can be used. This circuit provides a programmable gain function using a multiplying DAC, the AD5450/AD5451/ AD5452/AD5453, and a fast, low offset operational amplifier, the AD8065. The maximum gain value and the temperature coefficient are set by external resistors, and the resolution of the programmable gain is set by the resolution of the DAC. VOUT = –Gain × VIN × D 2N (1) where D is the digital word loaded to the DAC. D = 0 to 255 (8-bit AD5450), D = 0 to 1023 (10-bit AD5451), D = 0 to 4095 (12-bit AD5452), D = 0 to 16383 (14-bit AD5453); N is the number of bits. The key benefit of this circuit is its ability to overcome gain TC errors using resistor matching. The TC of the external resistors needs to match each other but do not need to match that of the DAC internal ladder resistance. CIRCUIT DESCRIPTION The circuit shown in Figure 1 is the recommended method of increasing the gain of the circuit. R1, R2, and R3 should all have similar temperature coefficients, but they need not match the VDD = +5V VIN R1 VREF R2 × R3 R1 = R2 + R3 AD5450/ AD5451/ AD5452/ AD5453 4.7pF VDD1 = +12V RFB IOUT1 VOUT AD8065 VSS = –12V SYNC SCLK SDIN GND R3 GAIN = µCONTROLLER VSS VDD 10µF AGND 0.1µF 10µF R2 R2 + R3 R2 VDD1 0.1µF 10µF 0.1µF 08620-001 VDD Figure 1. Programmable Gain Circuit Using a Current Output DAC (Simplified Schematic: Decoupling and All Connections 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 ©2009 Analog Devices, Inc. All rights reserved. CN-0055 Circuit Note Resistor R1 is required because R1 plus the input impedance of the DAC must equal the total feedback resistance which is RFB plus R2||R3. The input impedance of the DAC is RFB, so R1 + RFB = RFB + R2||R3 (2) R1 = R2||R3 (3) The values of R1 and R2 must be chosen such that the output voltage does not exceed the output range of the operational amplifier for the given supply voltage. Also note that the bias current of the operational amplifier is multiplied by the total feedback resistance (RFB + R2||R3) to give an associated offset. Thus, the values of R1 and R2 cannot be too large or they will have a significant effect on the overall offset voltage. The AD5450/AD5451/AD5452/AD5453 products are designed on a 5 V CMOS process and operate from a VDD1 power supply of 2.5 V to 5.5 V. The output amplifier is driven from a dual power supply voltage (VDD/VSS), which needs to be large enough to accommodate the analog output range of the circuit. Generally, ±12 V supplies are sufficient. The 4.7 pF capacitor is used to prevent ringing or instability in the closed-loop application. The input offset voltage of an op amp is multiplied by the variable noise gain (due to the code-dependent output resistance of the DAC) of the circuit. A change in this noise gain between two adjacent digital codes produces a step change in the output voltage due to the amplifier’s input offset voltage. This output voltage change is superimposed on the desired change in output between the two codes and produces a differential linearity error, which if large enough, could cause the DAC to be non-monotonic. The AD8065 benefits from both a low input offset voltage and low bias currents to overcome this issue. COMMON VARIATIONS LEARN MORE ADIsimPower Design Tool. Analog Devices. Kester, Walt. The Data Conversion Handbook. Chapter 3, 7. Analog Devices. 2005. MT-015 Tutorial, Basic DAC Architectures II: Binary DACs. Analog Devices. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND.” Analog Devices. MT-033 Tutorial, Voltage Feedback Op Amp Gain and Bandwidth. Analog Devices. MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues. Analog Devices. MT-101 Tutorial, Decoupling Techniques. Analog Devices. Voltage Reference Wizard Design Tool. Analog Devices. Data Sheets AD5450 Data Sheet AD5451 Data Sheet AD5452 Data Sheet AD5453 Data Sheet AD8065 Data Sheet OP1177 Data Sheet REVISION HISTORY 11/09—Rev. 0 to Rev. A Updated Format .................................................................. Universal 1/09—Revision 0: Initial Version The OP1177 is another excellent op amp candidate for the I-V conversion circuit. It also provides low offset voltage and ultralow bias current. For the selection of the reference, the input voltage is restricted by the rail-to-rail voltage of the operational amplifier selected and also the gain set up by the resistors R2 and R3. (Continued from first page) "Circuits from the Lab" 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" 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". Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" 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" at any time without notice, but is under no obligation to do so. Trademarks and registered trademarks are the property of their respective owners. ©2009 Analog Devices, Inc. All rights reserved. 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