Circuit Note CN-0292 Devices Connected/Referenced 24-Bit, 250 kSPS, - ADC with Precision, Selectable Gain, Fully AD7176-2 AD8475 20 s Settling Differential Funnel Amplifier Circuits from the Lab reference designs are engineered and Ultralow Noise, High Accuracy, Isolated Switching Regulators ADR4550 ADuM3471 tested for quick and easy system 5.0 V Voltage Reference (3/1 Channel Directionality) integration to help solve todays 30 V, Micropower, Overvoltage 20 V, 300 mA, Low Noise, ADA4096-4 ADP7102 analog, mixed-signal, and RF Protection, RRIO, Quad Op Amp CMOS LDO Regulator design challenges. For more Low Capacitance, Low Charge High Voltage, Low Noise, information and/or support, visit ADG1204 ADA4898-1 Low Distortion, Unity Gain Injection, iCMOS, 15 V/12 V, www.analog.com/CN0292. 4:1 Multiplexer Stable, High Speed Op Amp 50 mA, High Voltage, 28 V, 200 mA, Low Noise, ADP1720 ADP7182 Micropower Linear Regulator Linear Regulator Completely Isolated, Robust, 4-Channel, Multiplexed Data Acquisition System for Industrial Level Signals switching is less than 15 ppm FS (less than 90 dB) because of EVALUATION AND DESIGN SUPPORT the unique selection of fast settling components in the multiplexed Circuit Evaluation Boards signal chain. CN-0292 Circuit Evaluation Board (EVAL-CN0292-SDZ) The circuit acquires and digitizes standard industrial signal System Demonstration Platform, SDP-B (EVAL-SDP-CB1Z) Design and Integration Files levels of 5 V, 10 V, 0 V to 10 V, and 0 mA to 20 mA. The Schematics, Layout Files, Bill of Materials input buffers also provide overvoltage protection, thereby eliminating the leakage errors associated with conventional CIRCUIT FUNCTION AND BENEFITS Schottky diode protection circuits. The circuit in Figure 1 is a completely isolated, robust, industrial, Applications for the circuit include process control (PLC/DCS 4-channel data acquisition system that provides 16-bit, noise modules), battery testing, scientific multichannel instrumentation, free code resolution and an automatic channel switching rate and chromatography. of up to 42 kSPS. The channel to channel crosstalk at 42 kSPS Rev. 0 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 2014 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) CN-0292 Circuit Note +16.76V TRANSFORMER +V ISO 100k SCHOTTKY +V ISO +15V REG DIODE VIN VOUT FULLWAVE 8.06k RECTIFIER 11.3k ADP7102 ADJ V ISO 1k GND V X1 REG +15V REG +1.25V 10V FB X2 ADA4096-4 INPUTS V ISO +15V REG 15V REG +15V REG +5V REG ADG1204 V1 10 +5V REG +5V EXT VIN VOUT IN OUT V V DD2 DD1 47 11.3k ADP7182 ADP7182 100k 10nF V CH1 DDA OC ADJ 47nF +15V REG GND EXT 1k GND GND 2 1 V V IN OUT GND GND V2 10 GND GND 2 1 ADR4550 +5V REG 47 GND 10nF CH2 +5V REG +15V REG AD8475 47nF +5V 1.8k ADuM3471 270pF 1k 2.5k AVDD1, REF+ V3 10 68pF AVDD2 10 +IN CS V V 47 OA IA AIN1 0.4 10nF SCLK V V OB IB CH3 680pF AD7176-2 15V REG SPI 47nF AIN0 DIN V V IN OC IC ADA4898-1 10 0.4 DOUT V V ID OD V4 10 DGND AVSS 270pF 2.5k 1k 47 +15V REG 10nF VOCM CH4 47nF 10k 15V REG +2.5V EN ISOLATED PROCESSOR A0 SIDE SIDE A1 15V REG Figure 1. Functional Block Diagram of 4-Channel Data Acquisition System (Simplified Schematic: All Connections and Decoupling Not Shown) V IN CIRCUIT DESCRIPTION CH1 R Signal Path +FS C Four channels of input signals are buffered by the ADA4096-4, V OUT V IN a quad, rail-to-rail, input/output op amp featuring overvoltage CH2 +FS C D protection against phase reversal or latch-up for inputs of up to R FS FS 32 V above or below the 15 V supply rails, which eliminates C the need for additional overvoltage protection circuitry. Figure 2. RC Kickback Isolation Circuit The inputs are designed for typical low frequency, industrial signals of 10 V. The input buffers provide a high impedance The ADG1204 multiplexer, featuring low drain capacitance to the sources and isolate the inputs from the multiplexer switching (<4 pF), minimizes the kickback charge. transients. The output of the multiplexer is buffered by an ADA4898-1 op amp The RC networks on the inputs of the buffers (10 /10 nF) to prevent loading errors due to the on resistance of the have a bandwidth of 1.6 MHz and provide high frequency noise switches. The ADA4898-1 is unity-gain stable, settles to 0.1% in filtering. less than 85 ns, and has only 0.9 nV/Hz of input voltage noise. The worst-case input signal to the buffer is a 10 V, 21 kHz The RC networks on the outputs of the ADA4096-4 (47 /47 nF) isolate the buffers from the multiplexer switching transients. square wave when two adjacent channels have full-scale positive Figure 2 shows an equivalent circuit. The drain capacitor, C , and full-scale negative voltages on their respective inputs. D must be charged by the input voltage before switching to the The RC network on the input of the ADA4898-1 (1.8 k/68 pF) next channel. There can be as much as 20 V between channels, has a bandwidth of 1.3 MHz and acts as a wideband noise filter. and a transient current is generated when the multiplexer switches The time constant of this filter is 122 ns, and the 16-bit settling to the next channel. time is achieved in approximately 1.34 s (~11 time constants). Rev. 0 Page 2 of 7 REFOUT GPIO0 GPIO1 10976-002 10976-001