Circuit Note CN-0255 Devices Connected/Referenced AD7988-1 16-Bit, 100 kSPS PulSAR ADC Circuits from the Lab reference designs are engineered and ADA4841-1 Low Power, Low Noise Amplifier tested for quick and easy system integration to help solve todays analog, mixed-signal, and RF design challenges. For more AD8032 Dual-Channel, Rail-to-Rail Buffer Amplifier information and/or support, visit www.analog.com/CN0255. AD8031 Single-Channel, Rail-to-Rail I/O Op Amp ADR4525 2.5 V Low Noise Voltage Reference 16-Bit, 100 kSPS, Single-Supply, Low Power Data Acquisition System This circuit uses the AD7988-1, a low power (350 A) PulSAR EVALUATION AND DESIGN SUPPORT analog-to- digital converter (ADC), driven directly from the Evaluation Boards ADA4841-1 high performance, low voltage, low power op amp. CN-0255 Circuit Evaluation Board (EVAL-CN0255-SDPZ) This amplifier was chosen for its excellent dynamic performance System Demonstration Platform (EVAL-SDP-CB1Z) and its ability to operate with a single-supply voltage and to Design and Integration Files provide a rail-to-rail output. In addition, the input common- Schematics, Layout Files, Bill of Materials mode voltage range includes the negative rail. CIRCUIT FUNCTION AND BENEFITS The AD7988-1 ADC requires an external voltage reference In most systems, there are tradeoffs between performance and between 2.4 V and 5.1 V. In this application, the voltage reference low power. The focus of this circuit design is to explore a few of chosen was the ADR4525 precision 2.5 V reference. these tradeoffs and still achieve low power (8 mW typical) and high performance in a 16-bit, 100 kSPS data acquisition system. VDD = 4V VREF = 2.5V 2 6 V V IN OUT 0.1F ADR4525 GND VDD = 2.5V VIO = 1.8V TO 5V 4 VDD = 4V 0.1F 22F 0.1F VREF ADA4841-1 1 2 10 0.1F 49. 9 2 VIN+ 7 9 3 22 REF VDD VIO 6 IN+ SDI 3 GND 8 1 AD7988-1 2.7nF SDK ADC 7 SDO 3-WIRE INTERFACE TO SDP BOARD IN 49. 9 6 GND CNV 4 5 Figure 1. Basic Single-Ended, Low Voltage, Low Power, 16-Bit, 100 kSPS ADC Solution 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Fax: 781.461.3113 20122013 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 10382-001CN-0255 Circuit Note voltage range extends from the negative supply rail to within 1 V of CIRCUIT DESCRIPTION the positive rail. This necessitates 1 V headroom to accommodate The heart of this circuit is the AD7988-1, a 16-bit, 100 kSPS the signal range of interest (0 V to 2.5 V) therefore, in this circuit, a successive approximation, ADC that operates from a single 4 V rail was used. The ADA4841-1 is available in a 6-lead SOT-23 VDD power supply. It contains a low power, high speed, 16-bit or an 8-lead SOIC. sampling ADC and a versatile serial port interface (SPI). On the CNV rising edge, it samples an analog input, IN+, between 0 V The 2.5 V voltage reference used in this application is the to REF with respect to the ground sense pin, IN. The reference ADR4525 from the ADR45xx series of references, providing high precision, low power, low noise, and featuring 0.01% initial voltage, REF, is applied externally and can be set independent of accuracy, excellent temperature stability, and low output noise. the supply voltage, VDD. System performance is improved by the ADR4525 low thermally For the experiments carried out for this circuit note, the AD7988-1 induced output voltage hysteresis and low long-term output evaluation board was interfaced to the System Demonstration voltage drift. A maximum operating current of 700 A and a Platform (SDP), EVAL-SDP-CB1Z, where the ADC SPI-compatible low dropout voltage of 500 mV maximum make the device serial interface was connected to the DSP SPORT interface. The optimum for use in portable equipment. ADCs SPI interface features the ability to daisy-chain several ADCs on a single 3-wire bus. It is compatible with 1.8 V, 2.5 V, Each of the three products used in the circuit are rated for operation over the full industrial temperature range, 40C 3 V, or 5 V logic, using the separate VIO supply pin. to +125C. The AD7988-1 is housed in a 10-lead MSOP or a 10-lead QFN Performance Expectations (LFCSP). This board uses the MSOP package for convenience. Because power is key in this application, an analysis of each of The ADC input is buffered and driven from the ADA4841-1, a the component s contribution is necessary to ensure appropriate unity-gain stable, low noise and low distortion, rail-to-rail output selection among the many available products. The first step was amplifier that operates with a quiescent current of 1.1 mA reviewing the different supply currents for the three devices typically. This amplifier offers a low wideband voltage noise of selected. 2.1 nV/Hz and 1.4 pA/Hz current noise, along with excellent spurious-free dynamic range (SFDR) of 105 dBc at 100 kHz. The typical calculated and measured supply currents are shown To maintain a low noise environment at lower frequencies, the in Table 1 for each of the contributing components. The VIO amplifiers have low 1/f noise of 7 nV/Hz and 13 pA/Hz at 10 Hz. supply from the digital interface of the ADC is not included here because it is negligible. The measured currents compare A key feature of the ADA4841-1, making it ideally suited to single- favorably with the calculated values the small discrepancy can supply applications, is that in this application it can operate from a be attributed to the passive components and slight variations in single rail, with the negative rail tied to ground. The amplifier supply currents from that of the typical data sheet specifications. output can swing to within 50 mV of the ground level, which is acceptable for this application. Note that the input common-mode Table 1. Calculated and Measured Supply Current Contributions Calculated Measured Load Description Supply Current Supply Voltage Power Supply Current Supply Voltage Power ADC AD7988-1 150 A 2.5 V 375 W 148 A 2.5 V 370 W Driver ADA4841-1 1.1 mA 4 V 4.4 mW 1.95 mA 4 V 7.8 mW Reference ADR4525 700 A 4 V 2.8 mW Reference ADC ref current 60 A 4 V 240 W load Total 7.81 mW 8.17 mW Rev. A Page 2 of 7