Circuit Note CN-0058 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. ADG1211 Quad SPST Switch with Minimal Charge Injection AD8672 Dual, Precision, Very Low Noise Operational Amplifier Sample-and-Hold Circuit Using the ADG1211 Switch CIRCUIT FUNCTION AND BENEFITS only 1 pC compared to the next best solutions on the market, which have 10 pC to 20 pC of charge injection. The ADG1211 also has low capacitance and leakage current, making it an ideal choice for this circuit. This circuit is a sample-and-hold amplifier (SHA) function, which is basic to the data acquisition and analog-to-digital conversion process. In some programmable logic controller (PLC) output modules, a sample-and-hold in each channel stores the output samples from a single high performance DAC. The sample-and-holds are often implemented using analog switches or multiplexers and buffers. The circuit shown in Figure 1 provides a precise, fast sample-and-hold circuit capable of processing industrial-level signals of up to ±10 V. CIRCUIT DESCRIPTION This is a precise, fast sample-and-hold circuit. During the sample mode, SW2 is closed, and the output, VOUT, follows the input signal, VIN. In the hold mode, SW2 is opened, and the signal is held by the hold capacitor, CH. Due to switch and capacitor leakage current, the voltage on the hold capacitor decays (droops) with time. The ADG1211 minimizes this droop due to its low leakage specifications. The ADG1211 has 20 pA leakage typically at 25°C and 100 pA maximum. The droop rate is further minimized by the use of a polystyrene hold capacitor. Switches and multiplexers are useful in data acquisition applications where instantaneous analog values must be captured. The input signal from the analog switch charges a hold capacitor that is connected to the input of an op amp. Ideally, the closed switch conveys nothing but the input signal to the capacitor. However, the switch also delivers a packet of charge when it switches, called charge injection. The ADG1211 quad SPST switch has excellent charge injection performance of A second switch, SW1, which operates in parallel with SW2, is included in this circuit to reduce pedestal error. Since both +15V +5V 13 12 2200pF +15V SW1 VIN +15V AD8672 S D 1000pF 75Ω SW2 S D CH 2200pF –15V CC RC AD8672 VOUT –15V 5 4 –15V 08625-001 ADG1211 Figure 1. High Precision Fast Sample-and-Hold Circuit (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-0058 Circuit Note switches are at the same potential, they have similar transients, which act as a common-mode signal to the op amp, thereby minimizing charge injection effects. Pedestal error is also reduced by the compensation network RC and CC. This compensation network also reduces the hold-time glitch, while optimizing the acquisition time. The circuit in Figure 1 gives the following results: droop rate of 2 mV/ms, pedestal error of less than 0.5 mV, and acquisition time of 3 ms. The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see Tutorial MT-031 and Tutorial MT-101). LEARN MORE MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND." Analog Devices. MT-088 Tutorial, Analog Switches and Multiplexers Basics. Analog Devices. MT-090 Tutorial, Sample-and-Hold Amplifiers. Analog Devices. MT-101 Tutorial, Decoupling Techniques. Analog Devices. Data Sheets ADG1211 Data Sheet AD8672 Data Sheet REVISION HISTORY 11/09—Rev. 0 to Rev. A Updated Format .................................................................. Universal 1/09—Revision 0: Initial Version (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. 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