Circuit Note CN-0194 Devices Connected/Referenced AD7685 16-Bit, 250 kSPS PulSAR ADC Circuits from the Lab reference circuits are engineered and ADuM1402 Quad-Channel Digital Isolator tested for quick and easy system integration to help solve todays 2.5 V, Micropower, Low Noise Precision analog, mixed-signal, and RF design challenges. For more ADR391 Voltage Reference with Shutdown information and/or support, visit www.analog.com/CN0194. Precision, 20 MHz, CMOS Rail-to-Rail AD8615 Input/Output Op Amp Galvanically Isolated, 2-Channel, 16-Bit, Simultaneous Sampling, Daisy-Chained Data Acquisition System EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards This circuit, shown in Figure 1, provides galvanic isolation for CN-0194 Circuit Evaluation Board (EVAL-CN0194-SDPZ) high speed, high accuracy, simultaneous sampling analog-to- System Demonstration Platform (EVAL-SDP-CB1Z) digital conversion applications. The 16-bit AD7685 PulSAR Design and Integration Files ADC is versatile and allows monitoring of multiple channels Schematics, Layout Files, Bill of Materials 4.5V REF 5V SDP BOARD 5V V , V V , V 3.3V DD1 E1 DD2 E2 10F 100nF R1 R2 100nF 100nF 4.42k 1k GND GND 1 2 10V INPUT AIN1 V V 5V IA OA REF VDD VIO SDO DATA 33 SCK U4 IN+ AD7685 CNV V V IB OB 1.84V REF 2.7nF RSCLK SDI IN GND AD8615 V V OC IC TSCLK V V OD ID TFS 4.5V REF 5V ADuM1402C 10F 100nF R20 R19 4.42k 1k 5V V , V V , V 3.3V 10V INPUT DD1 E1 DD2 E2 AIN2 100nF 100nF 5V REF VDD VIO SDO GND GND 1 2 33 SCK V V U14 IN+ AD7685 IA OA CNV RFS 1.84V REF 2.7nF SDI IN GND V V AD8615 IB OB R5 R4 V V OC IC 1.24k 1k 5V V V OD ID 4.5V REF AD8615 U13 ADR391 ADuM1402C 1k 5V IN OUT 1.84V REF R3 GND 2.80k R6 10F 100nF Figure 1. Galvanically Isolated, 2-Channel, Simultaneous Sampling, 16-Bit Analog-to-Digital Conversion System with Daisy Chain (Simplified Schematic: All Connections and Decoupling Not Shown) Rev.0 Circuits from the Lab circuits 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2011 Analog Devices, Inc. All rights reserved. 09612-001CN-0194 Circuit Note through daisy chaining. An input circuit based on the AD8615 bandwidth, and low input voltage and current noise. The part op amp level shifts, attenuates, and buffers a 10 V industrial uses a patented DigiTrim trimming technique that achieves signal to match the input requirements of the ADC. The flexible superior precision without laser trimming. The AD8615 is fully circuit includes a precision ADR391 reference and two quad- specified to operate from 2.7 V to 5 V single supplies. channel ADuM1402 digital isolators to provide a compact and The combination of greater than 20 MHz bandwidth, low cost effective solution to a popular industrial data acquisition offset, low noise, and low input bias current makes this application. amplifier useful in a wide variety of applications. Filters, integrators, photodiode amplifiers, and high impedance sensors CIRCUIT DESCRIPTION all benefit from the combination of performance features. The AD7685 is a 16-bit, charge redistribution successive AC applications benefit from the wide bandwidth and low approximation (SAR) analog-to-digital converter (ADC) that distortion. The AD8615 offers the highest output drive operates from a single VDD power supply from 2.3 V to 5.5 V. It capability of the DigiTrim family, which is excellent for audio contains a low power, high speed, 16-bit sampling ADC with no line drivers and other low impedance applications. missing codes, no pipeline delay, an internal conversion clock, Applications for the parts include portable and low powered and a versatile serial interface port. The ADC also contains a instrumentation, audio amplification for portable devices, low noise, wide bandwidth, short aperture delay, track-and-hold portable phone headsets, barcode scanners, and multipole circuit. On the CNV rising edge, it samples an analog input (IN+) between 0 V to VREF with respect to the ground sense filters. The ability to swing rail-to-rail at both the input and output enables designers to buffer CMOS ADCs, DACs, ASICs, (IN). The reference voltage, VREF, is applied externally and is and other wide output swing devices in single-supply systems. set from 0.5 V to VDD. The circuit of Figure 1 uses a 4.5 V reference voltage. The ADR391 micropower, low dropout voltage reference provides a stable output voltage from a minimum supply of Power dissipation of the AD7685 scales linearly with sampling rate. Power dissipation is 15 mW maximum for VDD = 5 V and 300 mV above the output. Its advanced design eliminates the need for external capacitors, which further reduces board space a sampling rate of 250 kSPS. The AD7685 is housed in a 10-lead and system cost. The combination of low power operation, MSOP or a 10-lead QFN (LFCSP) with operation specified small size, and ease of use makes the ADR391 precision voltage from 40C to +85C. The SPI-compatible serial interface also reference ideally suited for battery-operated applications. Using features the ability, using the SDI input, to either daisy chain patented temperature drift curvature correction techniques several ADCs on a single 3-wire bus or provide an optional from Analog Devices, the ADR391 reference achieves a low BUSY indicator. It is compatible with 1.8 V, 2.5 V, 3 V, or 5 V 9 ppm/C of temperature drift in a TSOT package. logic using the separate VIO supply pin. The ADuM1402 is a quad-channel digital isolator based on The complete analog signal chain runs on a single 5 V supply. Analog Devices iCoupler technology. Combining high speed The ADR391 low dropout 2.5 V reference and the U13 rail-to- CMOS and monolithic air core transformer technology, this rail CMOS AD8615 op amp develop the 4.5 V reference voltage for the ADCs. A 4.5 V reference gives 0.5 V headroom at the isolation component provides outstanding performance character- output of U13 so that the op amp remains in the linear region istics superior to alternatives such as optocoupler devices. for nominal variations in the 5 V supply. The ADR391 2.5 V By avoiding the use of LEDs and photodiodes, iCoupler devices output is amplified by the noise gain of U13, which is 1 + R4/R5. remove the design difficulties commonly associated with opto- For the R4 and R5 values chosen, the noise gain is 1.8, and the couplers. The typical optocoupler concerns regarding uncertain reference voltage is 1.8 2.5 V = 4.5 V. current transfer ratios, nonlinear transfer functions, and The U4 and U14 AD8615 op amps provide a signal gain of temperature and lifetime effects are eliminated with the simple 0.225 (set by the ratio of R1 to R2 and R19 to R20), which iCoupler digital interfaces and stable performance characteristics. reduces the amplitude of the 20 V p-p input signals to 4.5 V p-p The need for external drivers and other discrete components is at the inputs of the ADCs. A 2.25 V offset is required at the eliminated with these iCoupler products. Furthermore, outputs of U4 and U14 for a 0 V input. This requires a common- iCoupler devices consume one-tenth to one-sixth of the power mode voltage of 1.84 V at the noninverting input of U4 and of optocouplers at comparable signal data rates. U14. This voltage is developed by the resistor divider R3-R6. The ADuM1402 isolators provides four independent isolation The R-C network on the output of U4 and U14 (33 , 2.7 nF) channels with 2/2 directionality and multiple data rates up to forms a single-pole noise filter with a bandwidth of 1.8 MHz. 90 Mbps for the C grade (see the data sheet Ordering Guide). The AD8615 is a CMOS rail-to-rail input and output, single- All models operate with the supply voltage on either side ranging from 2.7 V to 5.5 V, providing compatibility with lower supply amplifier featuring very low offset voltage, wide signal Rev. 0 Page 2 of 5