Circuit Note CN-0306 Devices Connected/Referenced Circuits from the Lab® reference designs 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/CN0306. AD7988-1 16-Bit, 100 kSPS PulSAR ADC AD8641 Low Power, Rail-to-Rail Output Precision Single JFET Op Amp ADR435 Ultralow Noise XFET® 5.0 V Voltage Reference 16-Bit, 100 kSPS, Low Power Data Acquisition System Optimized for Sub-Nyquist Input Signals up to 1 kHz EVALUATION AND DESIGN SUPPORT Drive amplifiers for high performance successive approximation ADCs are typically selected to handle a wide range of input frequencies. However, when an application requires a lower sampling rate, considerable power can be saved because reducing the sampling rate reduces the ADC power dissipation proportionally. Circuit Evaluation Boards CN-0306 Circuit Evaluation Board (EVAL-CN0306-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials To take full advantage of the power saved by reducing the ADC sampling rate, a low bandwidth, low power amplifier is required. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a 16-bit, 100 kSPS successive approximation analog-to-digital converter (ADC) system that has a drive amplifier that is optimized for a low system power dissipation of 7.35 mW for input signals up to 1 kHz and sampling rates of 100 kSPS. For instance, the 80 MHz ADA4841-1 op amp (12 mW at 10 V) is recommended for operation with the AD7988-1 16-bit successive approximation register (SAR) ADC (0.7 mW at 100 kSPS). The total system power dissipation including the ADR435 reference (4.65 mW at 7.5 V) is 17.35 mW at 100 kSPS. This approach is highly useful in portable battery powered or multichannel applications, or where power dissipation is critical. It also provides benefits in applications where the ADC is idle most of the time between conversion bursts. For input bandwidths up to 1 kHz and sampling rates of 100 kSPS, the 3 MHz AD8641 op amp (2 mW at 10 V) offers excellent signalto-noise ratio (SNR) and total harmonic distortion (THD) performance and reduces total system power from 17.35 mW to 7.35 mW, which is a 58% power savings at 100 kSPS. VCC = +7.5V 2 0.1µF VREF = +5V VOUT 6 VIN ADR435 GND VDD = +2.5V 4 VIO = +1.8V TO +5V VCC = +7.5V 0.1µF VIN+ 49.9Ω AD8641 2 1 7 634Ω 6 3 1 49.9Ω REF 3 IN+ 2.7nF 0.1µF 4 IN– 10 VIO SDI 9 AD7988-1 SDK 8 ADC SDO 7 GND 5 VSS = –2.5V 0.1µF 2 VDD CNV 6 3-WIRE INTERFACE 11095-001 VREF GND 0.1µF 22µF ≤1kHz Figure 1. System Circuit Diagram of Low Power AD8641 Amplifier Driving the AD7988-1 ADC (Simplified Schematic: All Connections Not Shown) Rev. A Circuits from the Lab® reference designs 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 toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) 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 ©2012–2013 Analog Devices, Inc. All rights reserved. CN-0306 Circuit Note CIRCUIT DESCRIPTION The circuit comprises the AD7988-1 ADC, AD8641 amplifier, and ADR435 reference. The AD7988-1 is a 16 bit, 100 kSPS SAR ADC whose low power is scalable with sampling rate and consumes 0.7 mW at 100 kSPS. Its low power also comes with industry-leading ac performance: SNR = 91 dB and THD = −114 dBc. The driving amplifier is the low power precision AD8641 that has a supply current of 200 µA and a gain bandwidth product of 3 MHz. The AD8641 can be driven with supplies ranging from 5 V to 26 V. The reference for the ADC is the ADR435, which is a high precision, low noise, 5 V XFET voltage reference. The ADR435 has a very low temperature coefficient of 3 ppm/°C at a low supply current of 620 µA. The total power for this circuit is 7.35 mW at 100 kSPS. The SNR is 88.5 dBFS, and the THD is −103 dBc with an input frequency up to 1 kHz. This compares favorably to the 16-bit distortion performance (THD less than −100 dBc) of the AD8641 for up to 1 kHz inputs. Distortion increases beyond 1 kHz so that it is not advisable to use this circuit with higher input frequencies or to use this amplifier in a multiplexed application due to the longer settling time. Note that the AD8641 requires at least 2 V of input headroom with respect to the positive supply voltage. The output stage is rail-to-rail. Performance Results The goal of this circuit is to deliver good ac performance at the lowest ADC driver power level possible for a given input frequency range up to 1 kHz and sampling rate of 100 kSPS. Figure 2 shows an FFT plot of the circuit performance with a 1 kHz input signal. An SNR of 88.5 dB, and a THD of −103 dB is achieved. The main reason for the reduction in SNR from the 91 dB specification of the AD7988-1 is the higher noise of the AD8641 of 28 nV/√Hz vs. 2 nV/√Hz for the ADA4841-1. The total system power is 7.35 mW: 0.7 mW for the ADC, 2 mW for the amplifier, and 4.65 mW for the reference. This represents a 58% reduction in power from using the ADA4841-1, which consumes 12 mW for a total system power of 17.35 mW. 11095-002 The AD8641 is configured as a unity-gain buffer and has an RC filter (634 Ω, 2.7 nF) with a 93 kHz cutoff frequency between it and the AD7988-1. The filter allows the use of a higher noise amplifier such as the AD8641 at 28 nV/√Hz while still getting the benefits of much lower power consumption. The tradeoff of higher noise for lower power causes a 2.5 dB reduction in the SNR performance of the system compared to the ADC specification. The higher value of R (634 Ω) relative to the recommended data sheet value (20 Ω) means the AD8641 can drive the large 2.7 nF input capacitor. The higher R value limits the maximum input bandwidth to 1 kHz for low distortion. Figure 2. System Circuit Performance with the AD8641 Amplifier Driving the AD7988-1 Rev. A | Page 2 of 4 Circuit Note CN-0306 Figure 3 shows how the system THD and SNR decrease with input frequencies beyond ~1 kHz. This is due to the amplifier distortion as can be seen in the THD+N vs. frequency plot shown in Figure 4. –85 COMMON VARIATIONS The AD8641 amplifier can be used to drive higher speed, pin compatible ADCs like the AD7988-5 and the AD7980 but only at lower sampling rates of up to 100 kSPS. The OP1177 amplifier can be used to drive the AD7988-1 at double the current (400 µA) with the benefits of improved distortion up to 4 kHz and better system SNR of 90 dB because of its lower noise. 88.6 88.4 88.2 –95 88.0 CIRCUIT EVALUATION AND TEST 87.8 Equipment Needed (Equivalents Can Be Substituted) 87.6 THD –100 87.4 SNR (dB) THD (dB) –90 The following equipment is needed: • • • SNR 87.2 –105 87.0 86.8 0 1 2 3 4 5 INPUT FREQUENCY (kHz) 6 7 • • 11095-003 86.6 –110 8 Figure 3. THD and SNR vs. Input Frequency for AD8641 Amplifier Driving the AD7988-1 Setup and Test 0.004 VSY = ±13V LOAD = 100kΩ GAIN = +1 THD + NOISE (%) 0.001 Install the 10-lead PulSAR software downloadable from the AD7988-1 product page on the Analog Devices website using the installation guide in the UG-340 user guide. The block diagram of the measurement setup is shown in Figure 5. 8V p-p INPUT 1V p-p INPUT Connect the 9 V wall power supply to the evaluation board power terminal. To measure the frequency response, connect the equipment as shown in Figure 5. Set the Audio Precision SYS-2522 signal generator for a 1 kHz frequency and a 5 V p-p sine wave with a 2.5 V dc offset. Record the data using the evaluation board software. The software analysis is part of the evaluation board software that allows the user to capture and analyze ac and dc performance. This software and its features are described in UG-340 user guide. 2V p-p INPUT 0.0001 4V p-p INPUT 0.00001 100 1k 10k FREQUENCY (Hz) 20k 11095-004 0.000001 1 The EVAL-CN0306-SDPZ evaluation board The System Demonstration Board (EVAL-SDP-CB1Z) A function generator/signal source, such as the Audio Precision SYS-2522 used in these tests The 9 V wall power supply included with the evaluation board A PC with a USB port, a USB cable, and the 10-lead PulSAR software installed Figure 4. THD+N vs. Input Frequency Performance of the AD8641 Amplifier 9V WALL SUPPLY +9V VIN+ AUDIO PRECISION SYS-2522 USB VIN– EVAL-CN0306-SDPZ SDP BOARD 120-PIN CONNECTOR Figure 5. Functional Diagram of Test Setup Rev. A | Page 3 of 4 PC WITH FFT ANALYSIS SOFTWARE CN-0306 Circuit Note LEARN MORE System Demonstration Platform (EVAL-SDP-CB1Z) CN0306 Design Support Package: http://www.analog.com/CN0306-DesignSupport AD7988-1 Data Sheet UG-340 User Guide, Evaluation Board for the 8-/10-Lead Family of 14-/16-/18-Bit PulSAR ADCs, Analog Devices. AD7980 Data Sheet EVAL-SDP-CB1Z System Demonstration Platform (SDP) MT-021 Tutorial, Successive Approximation ADCs, Analog Devices AD7988-5 Data Sheet ADR435 Data Sheet AD8641 Data Sheet OP1177 Data Sheet MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Analog Devices. ADA4841-1 Data Sheet MT-101 Tutorial, Decoupling Techniques, Analog Devices. 12/13—Rev. 0 to Rev. A Changes to Title .................................................................................1 Voltage Reference Selection and Evaluation Wizard, Analog Devices. Data Sheets and Evaluation Boards REVISION HISTORY 11/12—Rev. 0: Initial Version CN-0306 Circuit Evaluation Board (EVAL-CN0306-SDPZ) (Continued from first page) Circuits from the Lab reference designs 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 reference designs 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 reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab reference designs 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 reference designs at any time without notice but is under no obligation to do so. ©2012–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN11095-0-12/13(A) Rev. A | Page 4 of 4