Circuit Note CN-0146 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/circuits. ADG1611 ±5 V Quad SPST Switch with 1 Ω On Resistance AD620 Precision Instrumentation Amplifier AD8221 Precision Instrumentation Amplifier Low Cost Programmable Gain Instrumentation Amplifier Circuit Using the ADG1611 Quad SPST Switch and AD620 Instrumentation Amplifier CIRCUIT FUNCTION AND BENEFITS Data acquisition systems with wide dynamic range often need some method for adjusting the input signal level to the analog-todigital converter (ADC). In order to get the most from an ADC, the maximum input signal should match its full-scale voltage. This is achieved by implementing a programmable gain amplifier circuit. This circuit provides a programmable gain function using a quad SPST switch (ADG1611) and a resistor-programmable instrumentation amplifier (AD620). +IN The gain values are set by controlling the external gain setting resistor value, RG, with the four SPST switches, which are connected to four precision resistors. Low switch on resistance is critical in this application, and the ADG1611 has the industry’s lowest RON (1 Ω typical) and is available in the smallest package, a 16-lead, 4 mm × 4 mm LFCSP. The combination of the industry-standard low cost AD620 and the ADG1611 quad switch yields unmatched performance in this circuit and provides all the benefits of a precision instrumentation amplifier, along with the programmable gain feature. 100mV p-p, 10kHz SINE WAVE FOR TESTS +5V S1 R1 49.3Ω IN1 D1 S2 IN2 FROM SWITCH CONTROL LOGIC ADG1611 D3 S4 IN4 R2 98.8Ω D2 S3 IN3 +5V +IN RG R3 RG 499Ω –IN R4 1010Ω 3 8 1 2 7 +VS AD620 4 6 VOUT OUT 5 REF –VS –5V –5V –IN GND GND FOR TESTS 08841-001 D4 NOTES 1.R1, R2, R3 AND R4 ARE 0.1%. Figure 1. Programmable Gain Instrumentation Amplifier Circuit (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. 0 “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 Analog Devices, Inc. All rights reserved. CN-0146 Circuit Note CIRCUIT DESCRIPTION 10 Figure 1 shows the programmable gain instrumentation amplifier circuit, which is made up of an ultralow RON, ±5 V quad SPDT, the industry standard AD620 instrumentation amplifier, and four 0.1% standard resistors. RON MAX AT 125°C GAIN ERROR (%) 1 The ultralow on resistance of the ADG1611 makes it an ideal solution for gain switching applications, where low on resistance and distortion are critical. The AD620 is a low cost, high accuracy instrumentation amplifier that requires only one external resistor, RG, across pins 1 and 8 to set gain between 1 and 10,000. 08841-002 1 10 1000 Figure 2. Gain Error Due to Switch RON Variation Over Temperature The circuit tested in the lab included an automatic switching mode, which allows the automatic switching of the gain circuit from 1 to 50 to 100 to 500 to 1000 and back to 1 again. The waveforms in Figure 3 capture the circuit switching through gains from 50 to 1000. +1 For any arbitrary gain, RG is RG = 100 CALCULATED GAIN The gain is easily calculated by RG RON MAX AT 25°C 0.01 0.0001 The gain is changed by closing combinations of switches S1, S2, S3, and S4 to change RG. There are 16 possible gain settings controlled through the ADG1611 parallel interface. The AD620’s gain is resistor-programmed by the resistance between pins 1 and 8. The AD620 is designed to offer accurate gains using 0.1% to 1% tolerance resistors. 49.4 kΩ 0.1 0.001 Combining the ADG1611 and AD620 allows the designer to control the gain of the AD620 by switching in different gain setting resistors for RG. This circuit provides a low power, low cost programmable gain instrumentation amplifier solution. G= RON MAX AT 85°C GAIN = 500 49.4 kΩ GAIN = 50 G −1 The circuit in Figure 1 was setup where gains of 1, 50, 100, 500, and 1000 were required. Table 1 shows the ADG1611 control pins, IN1 through IN4, which control the resistance that appears between pins 1 and 8 of the ADG620. Standard 0.1% resistors were used to achieve the gain settings below. The table also shows the resulting gain by adding the on resistance of the ADG1611 in the signal chain and how the gain is affected by temperature. The ultralow on resistance of the ADG1611 switches makes it ideal because the RON is much less than RG, and the variation of RON over temperature is quite small. Figure 2 shows the gain error due to ADG1611 RON over temperature. GAIN = 100 08841-003 GAIN = 1000 CH2 2.00V M250µs CH3 Figure 3. AD620 Output Showing Gain Switching from 50 to 1000 Table 1. ADG1611 Calculated Gain Settings and % Error at 85°C IN1 0 0 0 0 1 1 IN2 0 0 0 1 0 1 IN3 0 0 1 0 0 1 IN4 0 1 0 0 0 1 Resistor Value (Ω) ∞ 1010 499 98.8 49.3 29.9 Gain Setting (No Switch) 1 49.91 100 501 1003 1653 Total Resistance Including Switch (Ω) ∞ 1011 500 99.8 50.3 30.3 Rev. 0 | Page 2 of 3 Gain Setting with ADG1611 1 49.85 99.8 496 983 1631 % Error Drift Due to Switch RON @ 85°C 0 0.039 0.079 0.394 0.773 1.331 Circuit Note CN-0146 The combination of the ADG1611 and AD620 provides a low cost, high accuracy solution for a programmable gain instrumentation amplifier with 16 levels of programmable gain. LEARN MORE Excellent layout, grounding, and decoupling techniques must be used to achieve the desired performance from the circuits discussed (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-061 Tutorial, Instrumentation Amplifier Basics, Analog Devices. COMMON VARIATIONS Although the circuit tested uses the industry-standard AD620, other instrumentation amplifiers are also suitable. The AD8221 is available in a low cost 8-lead SOIC and 8-lead MSOP, both of which offer the industry’s best performance. The MSOP requires half the board space of the SOIC, making it ideal for multichannel or space-constrained applications. The AD8220 is a single-supply, JFET input instrumentation amplifier also available in an MSOP package. Both the AD8220 and the AD8221 utilize the same gain-setting resistor values as the AD620. A Designer's Guide to Instrumentation Amplifiers (3rd Edition). Analog Devices. MT-063 Tutorial, Basic Three Op Amp In-Amp Configuration. Analog Devices. MT-088 Tutorial, Analog Switches and Multiplexers Basics. Analog Devices. Data Sheets and Evaluation Boards AD620 Data Sheet AD620 Evaluation Board AD8220 Data Sheet AD8220 Evaluation Board AD8221 Data Sheet AD8221 Evaluation Board ADG1611 Data Sheet REVISION HISTORY 4/10—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. 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. ©2010 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08841-0-4/10(0) Rev. 0 | Page 3 of 3