Circuit Note CN-0044 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. AD7995 4-Channel, 10-Bit ADC AD8599 Ultralow Distortion, Ultralow Noise Op Amp AD780 Ultrahigh Precision Band Gap Voltage Reference Using the AD8599 Op Amp as an Ultralow Distortion Driver for the AD7995 4-Channel, 10-Bit ADC CIRCUIT FUNCTION AND BENEFITS CIRCUIT DESCRIPTION The circuit described in this document provides an ultralow distortion driver circuit for the AD7995 10-bit, 4-channel ADC, which is designed to achieve optimum ac and dc performance. The circuit uses the ultralow distortion, ultralow noise AD8599 dual-supply op amp and ultrahigh precision AD780 band gap voltage reference to ensure that the maximum AD7995 performance is achieved, by providing a low impedance driver with adequate settling time and a highly accurate reference voltage. The AD8599 is a dual operational amplifier that operates with supplies from ±4.5 V to ±18 V. The AD7995 has an I2 C-compatible serial interface and is offered in an 8-lead SOT-23 package. It is always recommended to buffer analog input signals before applying them to ADCs with switched capacitor inputs such as the AD7995. This is particularly important in applications where the signal source has high source impedance and where low distortion and high signal-to-noise-ratio is important. The circuit illustrated in Figure 1 shows how the AD8599, an ideal choice for high accuracy designs, can be used to buffer the analog input channels. The AD7995 can be operated as a 4-channel input device using VDD as a reference (the input voltage range is 0 V to VDD) or as a 3-channel input device with the fourth channel used as an external reference input, VREF (the input range is 0 V to VREF). These options are programmable via the I2C-compatible interface. +5V + 10µF RP AD8599 –5V VIN0 1µF +2.5V + SDA VIN2* SCL 2-WIRE SERIAL INTERFACE MICROCONTROLLER/ MICROPROCESSOR VIN3/VREF GND +5V VDD VIN1* RP AD7995 AD780 *CONNECTED TO AD8599 OP AMPS SIMILARLY TO VIN0 08483-001 ANALOG INPUT 1/2 5V SUPPLY + 0.1µF Figure 1. AD7995 ADC with the AD8599 Low Distortion Driver and AD780 Ultrahigh Precision Reference (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, youare 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 fromthe 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 ©2008-2009 Analog Devices, Inc. All rights reserved. CN-0044 Circuit Note The AD780 is a 2.5 V/3 V ultrahigh precision, band gap voltage reference and is recommended for use with AD7995. A 1 μF decoupling capacitor is recommended on the VIN3/VREF signal for best performance. Care must be taken to ensure that the analog input signal to the ADC does not exceed the supply rails by more than 300 mV. If the signal does exceed this level, the internal ESD protection diodes become forward-biased and start conducting current into the substrate. Each diode can conduct a maximum current of 10 mA without causing irreversible damage to the part. The MT-036 Tutorial discusses methods to protect the input circuits of op amps and ADCs against such damage. In addition, 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 MT-031 Tutorial, MT-101 Tutorial, and the AD7995 evaluation board layout). COMMON VARIATIONS If single-supply op amp operation is required for buffering the input signal, the AD8605 is a suitable choice. Note that the output of the AD8605 operating on a single +5 V supply can only go to approximately 20 mV above ground; therefore, the AD7995 input range from 0 V to +20 mV cannot be exercised (see MT-035 Tutorial). LEARN MORE MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND." Analog Devices. MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues. Analog Devices. MT-036 Tutorial, Op Amp Output Phase-Reversal and Input Over-Voltage Protection. Analog Devices. MT-101 Tutorial, Decoupling Techniques. Analog Devices. Data Sheets and Evaluation Boards AD7995 Data Sheet. AD8599 Data Sheet. AD8605 Data Sheet. AD780 Data Sheet. AD7995 Evaluation Board. REVISION HISTORY 09/09—Rev. 0 to Rev. A Updated Format .................................................................. Universal 10/08—Revision 0: Initial Version The AD7995 can accept a reference input voltage from 1.2 V to VDD; therefore, different voltage reference sources can be used. (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. ©2008-2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08483-0-9/09(A) Rev. A | Page 2 of 2