Transcript
Circuit Note CN-0040
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.
AD7352
Differential Input, Dual, 3 MSPS. 12-Bit SAR ADC
AD8138
Low Distortion Differential ADC Driver
OP177
Ultra-Precision Operational Amplifier
DC-Coupled, Single-Ended-to-Differential Conversion Using the AD8138 Low Distortion Differential ADC Driver and the AD7352 Dual, 3 MSPS, 12-Bit SAR ADC applying differential drive to the AD7352 is to use a differential amplifier such as the AD8138. This part can be used as a singleended-to-differential amplifier or as a differential-to-differential amplifier. The AD8138 also provides common-mode level shifting. Figure 1 shows how the AD8138 can be used as a single-ended-to-differential amplifier in a dc-coupled application. The positive and negative outputs of the AD8138 are connected to the respective inputs on the ADC through a pair of series resistors to minimize the loading effects of the switched capacitor inputs of the ADC. The architecture of the AD8138 results in outputs that are very highly balanced over a wide frequency range without requiring tightly matched external components. The single-ended-to-differential gain of the circuit in Figure 1 is equal to RF/RG, where RF = RF 1 = R F2 and RG = RG1 = RG 2.
CIRCUIT FUNCTION AND BENEFITS The circuit described in this document provides a dc-coupled, single-ended-to-differential conversion of a bipolar input signal to the AD7352 dual, 3 MSPS, 12-bit SAR ADC. This circuit is designed to ensure maximum performance of the AD7352 by providing adequate settling time and low impedance.
CIRCUIT DESCRIPTION Differential operation requires VINx+ and VINx− of the ADC to be driven simultaneously with two equal signals that are 180° out of phase and are centered around the proper common-mode voltage. Because not all applications have a signal preconditioned for differential operation, there is often a need to perform a single-ended-to-differential conversion. An ideal method of CF1
2.048V 1.024V 0V
RF1 +5V RG1 VOCM
+2.048V GND –2.048V
RG2
CF2
VDD
VDRIVE
AD7352 RS*
10kΩ 10µF
VINx+
AD8138 –5V RF2
+1.024V
R S*
+2.5V +2.5V TO +3.6V
VINx–
REFA/REFB
2.048V 1.024V 0V AGND AGND +5V
OP177
+2.048V
10µF
10kΩ
*MOUNT AS CLOSE TO THE AD7352 AS POSSIBLE. RS = 33Ω; RG1 = RG2 = RF1 = RF2 = 499Ω; C F1 = CF2 = 39pF.
08449-001
–5V
Figure 1. AD8138 as a DC-Coupled, Single-Ended-to-Differential Converter Driving the AD7352 Differential Inputs (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 ©2009 Analog Devices, Inc. All rights reserved.
CN-0040
Circuit Note
If the analog inputs source being used has zero impedance, all four resistors (RG1, RG2, RF 1, and RF 2) should be the same as shown in Figure 1. If the source has a 50 Ω impedance and a 50 Ω termination, for example, the value of RG 2 should be increased by 25 Ω to balance this parallel impedance on the input and thus ensure that both the positive and negative analog inputs have the same gain. This also requires a small increase in R F1 and RF 2 to compensate for the gain loss caused by increasing RG1 and RG 2. Complete analysis for the terminated source condition is found in the ADIsimDiffAmp interactive design tool and in MT-076 Tutorial. The AD7352 requires a driver that has a very fast settling time due to the very short acquisition time required to achieve 3 MSPS throughput with a serial interface. The track-and-hold amplifier on the front end of the AD7352 enters track mode on the rising edge of the 13th SCLK period during a conversion. The ADC driver must settle before the track-and-hold returns to hold (68 ns later for 3 MSPS throughput on the AD7352 using a 48 MHz SCLK). The AD8138 has a specified 16 ns settling time that satisfies this requirement. The voltage applied to the VOCM pin of the AD8138 sets up the common-mode voltage. In Figure 1, VOCM is connected to 1.024 V, which is a divided version of the internal 2.048 V reference on the AD7352. If the on-chip 2.048 V reference on the AD7352 is to be used elsewhere in a system (as illustrated in Figure 1), the output from REFA or REFB must first be buffered. The OP177 features the highest precision performance of any op amp currently available and is a perfect choice for a reference buffer. Note that the AD8138 operates on dual 5 V supplies whereas the AD7352 is specified for power supply voltages of 2.5 V to 3.6 V. Care must be taken to ensure that the input maximum input voltage limits of the AD7352 are not exceeded during transient or power-on conditions (see MT-036 Tutorial). 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 AD7352 evaluation board layout).
COMMON VARIATIONS The OP07D, an ultralow offset voltage op amp, is a lower cost alternative to the OP177. It offers similar performance with the exception of the offset voltage specification. Alternatively, the AD8628 or the AD8638 offers very high precision with very low drift with time and temperature.
LEARN MORE MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Analog Devices. MT-036 Tutorial, Op Amp Output Phase-Reversal and Input Over-Voltage Protection, Analog Devices. MT-074 Tutorial, Differential Drivers for Precision ADCs, Analog Devices. MT-075 Tutorial, Differential Drivers for High Speed ADCs Overview, Analog Devices. MT-076 Tutorial, Differential Driver Analysis, Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. John Ardizonni and Jonathan Pearson, "Rules of the Road" for High-Speed Differential ADC Drivers, Analog Dialogue, Volume 43, May 2009, Analog Devices. ADIsimDiffAmp (Differential Amplifier Tool), Analog Devices.
Data Sheets and Evaluation Boards AD7352 Data Sheet. AD7352 Evaluation Board. AD8138 Data Sheet. OP177 Data Sheet. OP07D Data Sheet.
REVISION HISTORY 11/09—Rev. 0 to Rev. A Updated Format .................................................................. Universal Changes to Circuit Note Title ..........................................................1 10/08—Revision 0: Initial Release
(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. Trademarks and registered trademarks are the property of their respective owners. CN08449-0-11/09(A)
Rev. A | Page 2 of 2
