Circuit Note CN-0254 Devices Connected/Referenced 16-Bit, 8-Channel, 250 kSPS PulSAR ADC AD7689 Circuits from the Lab reference designs are engineered and Precision CMOS Rail-to-Rail I/O Quad tested for quick and easy system integration to help solve todays AD8608 Op Amp analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0254. Precision CMOS Rail-to-Rail I/O Op Amp AD8605 ADuM3471 PWM Controller and Transformer Driver ADP3336 500 mA anyCAP Adjustable Low 16-Bit, 250 kSPS, 8-Channel, Single Supply, Isolated Data Acquisition System EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards The circuit shown in Figure 1 is a cost effective, highly integrated CN-0254 Circuit Evaluation Board (EVAL-CN0254-SDPZ) 16-bit, 250 kSPS, 8-channel data acquisition system that can System Demonstration Platform (EVAL-SDP-CB1Z ) digitize 10 V industrial level signals. The circuit also provides Design and Integration Files 2500 V rms isolation between the measurement circuit and the Schematics, Layout Files, Bill of Materials host controller, and the entire circuit is powered from a single isolated PWM controlled 5 V supply. +5VAI +5VAI +5VDI 8.2pF 47H VREF JA4631-BL 330 1.5A +5VD AGNDI 8 1 10k 10F 0.1F 0.1F 0.1F 7 2 49.9k 0.1F 0.1F +5VAI 47F 330 1.5A AD8605 0.1F CH0 6 3 22 4.99k AGNDI AGNDI +5VDI 5 4 47F 2.7nF AGNDI DGNDI AGNDI AGNDI 1/4 AD8608 REFIN VDD VDD VIO DGND 3.57k 0.1F AGNDI TEMP V X1 X2 REG V SENSOR REF 34k DD2 * AGNDI REG V AI0 IN0 AD7689 DD1 FB 0.1F PRIM 11.3k IN1 SEC GND CTRL 2 AI1 CTRL GND 1 SEQUENCER OC IN2 AI2 V DDA IN3 100k AI3 SCK C1 VOA VIA SCK IN4 8.2pF AI4 DIN VOB VIB SDI IN5 16-BIT SAR AI5 CNV 10k ADC VOC VIC R1 CNV IN6 49.9k AI6 +5VAI R5 SDO VID VOD CH7 ONE-POLE 22 SDO AI7 IN7 R3 LPF V IN 4.99k V OUT COM GND GND EPAD 2.7nF GND GND 2 ADuM3471 1 * 1/4 AD8608 3.57k AGNDI AGNDI AGNDI AGNDI AGNDI DGNDI DGND AGNDI *CONNECTS TO 3 OTHER + INPUTS OF QUADAD8608. Figure 1.16-Bit, 250 kSPS, 8-Channel Data Acquisition System (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. B 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) MUX SPI SERIAL INTERFACE 07353-001CN-0254 Circuit Note For a 0 V input signal, the output of the op amp should be at CIRCUIT DESCRIPTION midscale or 0.5 V . REF This circuit uses the AD7689 16-bit, 8-channel, 250 kSPS PulSAR ADC along with two AD8608 low cost precision quad op amps R1 + R2 R4 = 0.5 (3) to provide the entire signal conditioning and digitizing function R1 R3 + R4 in the data acquisition system. The only other op amp required Substituting Equation 1 in Equation 2 yields is the AD8605 that is used to buffer the reference voltage from R3 the AD7689. =1.4 (4) R4 The AD8605 and AD8608 are low cost single and quad rail-to- The common-mode voltage at the input of the op amp is rail input and output CMOS amplifiers. The AD8608 inverts, calculated by level shifts, and attenuates the 10 V input signal so that it matches the input range of the ADC, which is 0 V to +4.096 V R4 V = V (5) when using a +4.096 V reference and a +5 V single supply. CM REF R3 + R4 The AD8605 acts as an external reference buffer to provide For R3/R4 = 1.4 and VREF = 4.096 V, the common-mode voltage sufficient driving ability for level shifting. The AD8605 and of the op amp is 1.7 V. AD8608 feature very low offset voltage, low input voltage and There are four amplifiers in each AD8608, and the four current noise, and wide signal bandwidth, therefore making them noninverting inputs are shorted together and connected to a good choices for a wide variety of applications. The low current resistor divider, R3/R4. A second divider is used for the second and voltage noise of the AD8608 ensures that the resistor noise is AD8608. To cancel the op amp input bias currents, the dominant term in the output noise for high input impedance. In the circuit, the input impedance (equal to R1) is 50 k . R1 R2 = 4 (R3 R4) (6) The AD7689 16-bit, 8-channel, 250 kSPS PulSAR ADC contains The input impedance of the circuit is R1, which should ideally be all the necessary components for the multichannel low power data high. However, a resistor has thermal noise which is proportional acquisition system. It includes a 16-bit SAR ADC, an 8-channel to the square root of resistance, and therefore, the system noise low crosstalk multiplexer, a low drift reference and buffer, a performance degrades as this resistor value increases. A simple temperature sensor, a selectable one-pole filter, and a channel noise analysis is needed to determine the optimum value. sequencer. The sequencer is useful for continuously scanning Based on the Nyquist criteria, the maximum signal frequency channels, and a microcontroller or FPGA is not required to component should be less than half of the maximum sample control channel switching. The AD7689 is packaged in a tiny rate. The AD7689 250 kSPS sampling rate yields a Nyquist 20-lead 4 mm 4 mm LFCSP footprint, thereby minimizing cost frequency of 125 kHz. To minimize the attenuation of a signal and printed circuit board (PCB) area. Operation is specified in this bandwidth, the 3 dB cutoff frequency of the front end is from 40C to +85C. Power dissipation with a 5 V supply at designed to be about 12 times the Nyquist frequency or 1.5 MHz. 250 kSPS is 12.5 mW (typical). The noise model of this circuit is shown in Figure 2. There are The ADuM3471 is a quad-channel digital isolator with an three noise sources in this circuit: resistor noise, amplifier integrated PWM controller and transformer driver for an voltage noise, and amplifier current noise. The rms value of isolated dc-to-dc converter. The ADuM3471 provides the each noise source is shown in Table 1. See Application Note isolated 5 V, 2 W power for the circuit as well as isolates the AN-358, and the MT-047, MT-048, and MT-049 tutorials for digital signals at the SPI interface. more details about op amp noise. Analog Front End Design e R2 R2 Signal levels of up to 10 V are typical in process control and e e R1 N industrial automation systems. An inverting amplifier with R1 attenuation and level shifting is used in the Figure 1 circuit to V R3 I OUT N convert the 10 V signal into one that fits the range of the ADC. I N+ e AD8608 R3 R4 The equation of the circuit is as follows: e R4 R2 R1 + R2 R4 V = V + V (1) OUT IN REF R1 R1 R3 + R4 Figure 2. Noise Model for Inverting Configuration of the AD8608 The signal gain (R2/R1) of the front end is set to 0.2 so that The total rms noise in front of the ADC within the bandwidth the range of the signal into the ADC is 4 V peak-to-peak. This of interest should be less than 0.5 LSB so that the input signal fits the input range of 0 V to 4.096 V, which is equal to the can be digitized correctly by the ADC. reference voltage, VREF. R1 = 5 (2) R2 Rev. B Page 2 of 6 10379-002