CN0253

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Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0253. Devices Connected/Referenced ADG5408/ ADG5409 High Voltage Latch-Up Proof, 4-/8-Channel Multiplexers AD8226 Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier A Robust, Low Power, Battery Monitoring Circuit Front End Rev. A Circuits from the Lab™ circuits 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 Circuits from the Lab circuits. (Continued on last page) Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2012 Analog Devices, Inc. All rights reserved. EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards CN-0253 Circuit Evaluation Board (EVAL-CN0253-SDPZ) System Demonstration Platform (EVAL-SDP-CS1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a robust battery monitoring front end designed for environments where transients are likely to occur, such as in industrial or process automation environments. The circuit uses the ADG5408 8-channel CMOS multiplexer followed by the AD8226 instrumentation amplifier to provide accurate voltage monitoring of individual cells at low power and low cost, and requires no additional external transient protection circuitry. Transient overvoltage conditions may cause traditional CMOS switches to experience latch up. In junction isolation technology, the N- and P-wells of the PMOS and NMOS transistors form a parasitic silicon-controlled rectifier (SCR) circuit. An overvoltage condition triggers this SCR, causing a significant amplification of current that, in turn, leads to latch-up. Latch-up is an undesirable, high current state that can lead to device failure and can persist until the power supply is turned off. Latch-up can occur if either the input or the output pin voltage exceeds the supply rail by more than a diode drop, or by improper power supply sequencing. If a fault occurs on the channel, and the signal exceeds the maximum rating, the fault can trigger the latch-up state in an typical CMOS part. During circuit power up, it is also possible for voltages to occur on inputs before power is applied to the CMOS switch, especially if multiple supplies are used to power the circuit. This condition may exceed the maximum rating of the device and trigger a latch-up state. The two multiplexers and the instrumentation amplifier (IA) used in this design have robust inputs. The ADG5408 is a high voltage 8:1 multiplexer that is latch-up proof. The trench isolation technology used in the fabrication of the ADG5408 prevents the latch-up state and reduces the need for external protection circuitry. Latch-up proof does not guarantee overvoltage protection and only means the switch does enter the high current SCR mode. The ADG5408 also has an electrostatic discharge (ESD) rating of 8 kV human body model (ANSI/ESDA/JEDEC JS-001-2010). The AD8226 is a low cost, low power, instrumentation amplifier with robust inputs and can handle input voltages up to 40 V from the opposite supply rail, while restricting the output to within the rails. For instance, with ±18 V supplies, the positive or negative input of the AD8226 can swing between ±22 V with no damage. All inputs of the AD8226 are protected against ESD with internal diodes. CIRCUIT DESCRIPTION Battery monitoring systems (BMS) require the individual voltage across each battery in a battery stack to assess the state of charge (SOC) and state of health (SOH) of the battery. By multiplexing the terminals of a stack of batteries with two multiplexers, as shown in Figure 1, the voltage across each battery can be assessed. One multiplexer is used for the positive terminal and another for the negative terminal. This differential multiplexing allows the use of a single instrumentation amplifier for up to eight channels. The amplifier then removes the common-mode voltage from each of the batteries for use by the BMS. The ADG5408 has a low on-resistance per channel, typically 13.5 Ω, and a maximum of 22 Ω over temperature. With a maximum of 2 nA input offset current, there is a maximum of 44 nV error voltage across the channel resistances.

Figure 1. Robust Battery Monitoring Circuit Simplified Schematic (All Connections and Decoupling Not Shown)

Rev. A | Page 4 of 4 Setup With the power output of the supply off, connect a +18 V power supply to the J3-1 pin (VDD_EXT), a −18 V power supply to the J3-3 pin (VSS_EXT), and the ground connection to the J3-2 pin (GND_EXT). Attach the test battery cells to the battery connections. Ensure that the link headers are retained on the battery connections that do not have batteries connected; that is, if only using four batteries, the remaining four battery connections should remain connected. If computer control of the board is required, it is important to remove the link headers: EN, A0, A1, and A2. If using the E VA L-SDP-CS1Z, connect the E VA L-SDP-CS1Z to the E VA L- CN0253-SDPZ using the 120-pin connector. Secure the connection using the Nylon hardware. Test Apply power to the ±18 V supply. Use the EN link on the board to enable the outputs from the ADG5408 multiplexers. Use the A0, A1, and A2 links on the board to select the battery for testing. The SMB connector, VOUT, can be used to connect to a separate ADC evaluation board, such as the E VA L-AD7298SDZ or manually tested using a digital voltmeter. If computer control is required, connect the E VA L-SDP-CS1Z to the PC using the USB cable. Launch the CN-0253 evaluation software. The battery voltage can be tested as per the manual test. An additional 5 V power supply pin is provided if using the E VA L-SDP-CS1Z. LEARN MORE CN-0253 Design Support Package: http://www.analog.com/CN0253-DesiignSupport Ardizzoni, John. A Practical Guide to High-Speed Printed- Circuit-Board Layout, Analog Dialogue 39-09, September 2005. Redmond, Catherine, Winning the Battle Against Latchup in CMOS Analog Switches, Analog Dialogue Volume 35, Number 5, October, 2001, Analog Devices. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND”, Analog Devices. MT-069 Tutorial, In-Amp Input Overvoltage Protection, Analog Devices. MT-088 Tutorial, Analog Switches and Multiplexers Basics, Analog Devices. MT-092 Tutorial, Electrostatic Discharge (ESD), Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Data Sheets and Evaluation Boards CN-0253 Circuit Evaluation Board (EVAL-CN0253-SDPZ) System Demonstration Platform (EV AL-SDP-CB1Z) ADG5408 Data Sheet and Evaluation Board ADG5409 Data Sheet and Evaluation Board AD8226 Data Sheet and Evaluation Board

REVISION HISTORY

5/12—Rev. 0 to Rev. A Changed 4-Channel to 8-Channel in Circuit Function and 4/12—Revision 0: Initial Version (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 other wise under any patents or other intellectu al 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. ©2012 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN10374-0-5/12(A)