Circuit Note CN-0185 Devices Connected/Referenced AD7400A Isolated Sigma-Delta Modulator ADuM5000 Isolated DC-to-DC Converter AD8646 Dual, 24 MHz, Rail-to-Rail, I/O Op Amp Circuits from the Lab reference circuits are engineered and 150 mA, Low Quiescent Current, CMOS tested for quick and easy system integration to help solve todays ADP121 Linear Regulator analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0185. High Accuracy, 100 mA, Low Dropout ADP3301 Linear Regulator ADG849 3 V/5 V CMOS 0.5 SPDT/2:1 Mux in SC70 Ultralow Noise, LDO XFET 3.0 V Voltage ADR443 Reference A Novel Analog-to-Analog Isolator Using an Isolated Sigma-Delta Modulator, Isolated DC-to-DC Converter, and Active Filter The circuit is based on the AD7400A, a second-order, sigma-delta EVALUATION AND DESIGN SUPPORT (-) modulator with a digitally isolated 1-bit data stream output. Circuit Evaluation Boards The isolated analog signal is recovered with a fourth-order active CN-0185 Circuit Evaluation Board (EVAL-CN0185-EB1Z) filter based on the dual, low noise, rail-to-rail AD8646 op amp. Design and Integration Files With the ADuM5000 as the power supply for the isolated side, Schematics, Layout Files, Bill of Materials the two sides are completely isolated and use only one power CIRCUIT FUNCTION AND BENEFITS supply for the system. The circuit has 0.05% linearity and benefits from the noise shaping provided by the modulator of the AD7400A The circuit shown in Figure 1 is a complete low cost and the analog filter. The applications of the circuit include implementation of an analog-to-analog isolator. The circuit motor control and shunt current monitoring, and the circuit is provides isolation of 2500 V rms (1 minute per UL 1577). also a good alternative to isolation systems based on optoisolators. +5V ISO +5V POWER IN V V OUT IN ISO DD1 +5.5V TO +12V GND GND 1 ISO GND SD RC IN V SEL RC OUT ADP3301-5 RC SEL V +3V ISO V REF DD1 GND GND ISO 1 ADuM5000 OUT IN VOUT VIN GND ISO GND GND EN ADR443 ADP121-3.3 22 V + IN V V DD1 DD2 1% +3V REF V MDAT IN+ 0.1F ADG849 V MCLKOUT 100pF 100pF V IN 24k 51k IN 5% 5% 1% 1% GND GND 1 2 22 1% AD7400A 24k 51k 1/2 1/2 GND ISO +5V 1% 1% AD8646 AD8646 V OUT 68pF 8.2pF 5% 5% 22 22 1% 1% Figure 1. Analog Isolator Using AD7400A (Simplified Schematic: All Connections and Decoupling Not Shown Rev. B 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 20112013 Analog Devices, Inc. All rights reserved. 09499-001CN-0185 Circuit Note Analog Active Filter Design CIRCUIT DESCRIPTION The cutoff frequency of the low-pass filter mostly depends on A block diagram of the circuit is shown in Figure 1. The analog the desired bandwidth of the circuit. There is a trade-off between input is sampled at 10 MSPS by the AD7400A - modulator. the cutoff frequency and noise performance, and there is more The 22 resistors and 0.1 F capacitor form a differential input noise if the cutoff frequency of the filter increases. This is especially noise reduction filter with a cutoff frequency of 145 kHz. The true in this design because the - modulator shapes the noise output of the AD7400A is an isolated 1-bit data stream. The and moves a large portion into the higher frequencies. The quantization noise is shaped by a second-order - modulator, cutoff frequency in this design is 100 kHz. which shifts the noise to the higher frequencies (see the MT-022 Tutorial). For a given cutoff frequency, the smaller the transition band of the filter is, the smaller the noise is that passes through the filter. To reconstruct the analog input signal, follow the data stream by Of all the filter responses (Butterworth, Chebyshev, Bessel, and an ADG849 switch connected to a 3 V ADR443 reference to so on), the Chebyshev filter was chosen for this design because it stabilize the peak-to-peak output of the MDAT. has a smaller transition band for a given filter order. However, The signal is then filtered by an active filter whose order is higher this smaller transition band comes at the expense of a slightly than the order of the modulator. A fourth-order Chebyshev filter is worse transient response. used for better noise attenuation. Compared to other filter The fourth-order filter is made up of two second-order filters, responses (Butterworth or Bessel), the response of the with a Sallen-Key structure. The Analog Filter Wizard and the Chebyshev provides the steepest rolloff for a given filter order. NI Multisim were used to design the filter. The parameters used The filter is implemented using the dual AD8646, a rail-to-rail, include the following: input and output, low noise, single-supply op amp. Filter type = low-pass, Chebyshev, 0.01 dB ripple The ADuM5000 is an isolated dc-to-dc converter based on Order = 4 Analog Devices, Inc., iCoupler technology. It is used for the fC = 100 kHz, Sallen-Key (updated format for clarity) power supply to the isolated side of the circuit containing the AD7400A. The isoPower technology of the ADuM5000 uses The recommended values generated by the program were high frequency switching elements to transfer power through a used with the exception of the feedback resistors, which were chip-scale transformer. reduced to 22 . The circuit must be constructed on a multilayer printed circuit Measurements board (PCB) with a large area ground plane. Proper layout, The circuit has a gain of 4.6875 and an output offset voltage of grounding, and decoupling techniques must be used to achieve 1.5 V. A differential signal of 0 V results in a digital bit stream of optimum performance (see the MT-031 Tutorial, Grounding Data 1s and 0s, where each occurs 50% of the time. The ADR443 output Converters and Solving the Mystery ofAGN andDGND the is 3.0 V therefore, after filtering, there is a 1.5 V dc offset. A MT-101 Tutorial, Decoupling Techniques, and the ADuC7060/ differential input of 320 mV ideally results in a stream of all 1s, ADuC7061 evaluation board layout). Ensure that the PCB layout which after filtering, yields a 3.0 V DC output. Therefore, the meets the emissions standards and isolation requirements between effective gain of the circuit is the two isolated sides (see the AN-0971 Application Note). GAIN = (3.0 1.5)/0.32 = 4.6875 In order not to overdrive the AD8646, the input signal must be From the measurements, the actual measured offset is 1.504V , and lower than the power supply (5 V). The output of the AD7400A the gain is 4.69. The dc transfer function of the system is shown is a stream of 1s and 0s with an amplitude equal to the in Figure 2. Linearity was measured as 0.0465%. AD7400A VDD2 supply voltage. Therefore, the VDD2 digital supply is 3.3 V supplied by the ADP121 linear regulator. 2.9 Alternatively, if a 5 V supply is used for V , attenuate the DD2 2.7 digital output signal before connecting to the active filter. In either case, well regulate the supply because the final analog 2.5 output is directly proportional to VDD2. 2.3 The 5 V supply for the circuit in Figure 1 is supplied from an ADP3301 5 V linear regulator, which accepts an input voltage 2.1 of 5.5 V to 12 V. 1.9 1.7 1.5 0 50 100 150 200 250 INPUT VOLTAGE (mV) Figure 2. System DC Transfer Function Rev. B Page 2 of 4 OUTPUT VOLTAGE (V) 09499-002