Circuit Note CN-0261 Devices Connected/Referenced Circuits from the Lab reference designs are engineered and AD7691 18-Bit 1.5 LSB INL, 250 kSPS PulSAR ADC tested for quick and easy system integration to help solve todays Ultralow Distortion, Ultralow Noise analog, mixed-signal, and RF design challenges. For more AD8597 Amplifier information and/or support, visit www.analog.com/CN0261. ADR435 Ultralow Noise XFET 5 V Reference 18-bit, 250 kSPS, Data Acquisition System Optimized for AC Performance EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards Choosing complementary products for high performance CN-0261 Circuit Evaluation Board (EVAL-CN0261-SDPZ) ADCs can be a challenge. The circuit in Figure 1 shows a System Demonstration Platform (EVAL-SDP-CB1Z) complete front end solution for the 18-bit, 250 kSPS PulSAR Design and Integration Files ADC, which is optimized for ac performance. Schematics, Layout Files, Bill of Materials ADR435 5V VREF VDD = 8V VIN VOUT 6 2 0.1F 1F GND 4 VDD = 8V VIO = 0.1F 1.8V TO 5V VDD = 5V VCM = VREF 2 AD8597 = 2.5V 49. 9 2 7 22F 0.1F VIN+ 180 6 3 1 1 2 10 2.7nF 0.1F VCM 590 REF VDD VIO 3 9 49. 9 0.1F SDI VREF = 5V 0.1F 590 IN+ 8 VIN+ SCK AD7691 VCM = 2.5V 7 VIN IN SDO 3-WIRE INTERFACE VOLTAGE DIVIDER VSS = 2V 6 GND FOR BIASING PURPOSES GND CNV 4 VDD = 8V 5 0.1F VCM = VREF 2 AD8597 18-BIT, 1.5LSB INL = 2.5V 49. 9 2 250kSPS DIFFERENTIAL VIN 7 180 PulSAR ADC 3 1 2.7nF VCM 590 49. 9 0.1F 0.1F 590 VSS = 2V VOLTAGE DIVIDER FOR BIASING PURPOSES Figure 1. High Performance, 18-Bit ADC Front End (Simplified Schematic: All Connections and Decoupling 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 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) 10467-001CN-0261 Circuit Note The circuit centers on the AD7691, which is a low power ADC The ADR435 is available in either an 8-lead MSOP or an 8-lead (1.35 mW 2.5 V and 100 kSPS) from the PulSAR family. narrow SOIC package. The ADC is driven directly from the AD8597 ultralow The dynamic performance of the above configuration is shown distortion, ultralow noise amplifier, and the ADCs reference is in Figure 2 and Figure 3 and is summarized below: the ultralow noise 5 V ADR435. The circuit achieves 101 dB SNR = 101.02dB SNR and 118 dB THD with a 1 kHz input tone. THD = 118.44 dB CIRCUIT DESCRIPTION SINAD = 100.94 dB The heart of this circuit is the AD7691, an 18-bit, 250 kSPS Dynamic Range = 101.5 dB charge redistribution, successive approximation, analog-to- digital converter (ADC) that operates from a single power supply. It contains a low power, high speed, 18-bit sampling ADC with no missing codes, an internal conversion clock, and a versatile serial interface port. On the CNV rising edge, it samples the voltage difference between the IN+ and IN pins. The voltages on these pins swing in opposite phases between 0 V and REF. The reference voltage, REF, is applied externally and can be set up to the supply voltage. The AD7691 power scales linearly with throughput. For the experiments carried out for this circuit note, the AD7691was interfaced to the SDP (System Demonstration Platform board, EVAL-SDP-CB1Z), and the ADC SPI- compatible serial interface was connected to the DSP SPORT Figure 2. Evaluation Board Software Output Screen Capture interface VIO supply. The AD7691 is housed in a 10-lead MSOP or a 10-lead QFN (LFCSP). The ADC is driven from the AD8597 (4.8 mA/amplifier), which 0 is a low noise, low distortion operational amplifiers ideal for use FUNDAMENTAL = 1kHz, 0.3dB FROM FS as an input buffer. The low noise of 1.1 nV/Hz and low SAMPLING FREQUENCY = 250kSPS 50 SNR = 101.02dB harmonic distortion of less than 120 dB at audio frequencies THD = 118.44dB SINAD = 100.94dB give the AD8597 the wide dynamic range necessary for DYNAMIC RANGE = 101.5dB preamplifiers in audio, medical, and instrumentation 100 applications. The excellent slew rate of 14 V/s and 10 MHz gain bandwidth product make it highly suitable for medical 150 applications. The AD8597 can be operated on supply voltages up to 15 V. In 200 the circuit, supply voltages of +8 V and 2 V were chosen in order to minimize power dissipation. 250 The AD8597 is available in 8-lead SOIC and LFCSP packages. 0 20 40 60 80 100 120 The 180 resistors and the 2.7 nF capacitors form a single -pole FREQUENCY (kHz) 327 kHz low-pass filter to further reduce noise. Figure 3. FFT Details for 1 kHz Tone Sampling at 250 kSPS The voltage reference used in this application is the ADR435, which is one of a family of XFET voltage references featuring low noise, high accuracy, and low temperature drift performance. Using patented temperature drift curvature correction and XFET (eXtra implanted junction FET) technology, voltage change vs. temperature is minimized. The ADR43x family can source up to 30 mA of output current and sink up to 20 mA. It also has a trim terminal to adjust the output voltage over a 0.5% range without compromising performance. Rev. A Page 2 of 4 AMPLITUDE (dB) 10467-002 10467-003