rev. 0.2 2/13 copyright ? 2013 by silicon laboratories AN614 AN614 a s imple a lternative to a nalog i solation a mplifiers 1. introduction analog circuits sometimes require linear (analog) signal isolation for safety, signal level shifting, and/or ground loop elimination. linear signal is olation is typically difficult to implem ent, costly, and often exhibits mediocre performance. while the design community thirsts for a flexib le and inexpensive linear isolator solution, it is the analog isolation amplifier (isoamp) th at most often captures the socket. isoamps are hybridized devices that co ntain linear input and output circuits separated by an internal isolation barrier. isoamps typically modulate the linear input signal and transmit the resulting digital information across the isolation barrier to a demodulator where it is conver ted back to analog (figure 1). isoamps typically employ transformers and high-voltage capacitors or optocouple rs in the isolation barrier design. while isoamps are a convenient single-package linear isolat ion solution, the industry's limited dev ice offerings often force the designer to make difficult trade-offs or add external circuitry for a complete solution. in addition, transformer-based isoamps are susceptible to signal corruption from external field interference and errors due to poor common-mode transient immunity (cmti). optocoupler-based isoamp versions also often suffer from poor linearity and even worse cmti. this application note provides insigh t into a robust linear is olator reference design based on the silicon labs si86xx family of cmos digital isolators. figure 1. isoamp block diagram modulator or driver isolation barrier demodulator or conditioner input ? amp output ? amp input output
AN614 2 rev. 0.2 2. silicon labs isolinear reference design overview silicon labs offers the isolinear reference design (si86isolin), a simple linea r isolation circuit that is enabled by the si86xx isolator family, offering i ndustry-leading integration, operating performance, and reliability. the si86xx isolators are available in ul/vde/csa certified isolation ratings of 2.5 or 5 kv. the isolinear reference design architecture is shown in figure 2. a self-oscillating pulse width modulator (pwm) converts the linear input voltage into a series of fixed-frequency, variable duty, cycle puls es in which the width of each pulse is proportional to the sampled input signal amplitude as shown in the waveform diagrams at the bottom of figure 2. this modulated (digital) signal is passed through the cmos digital isolat or and then restored to analog format by a fourth-order analog filter. unlike isoamps, the isolinear reference design gives the designer flexibility to craft low-cost, linear isolators that meet the needs of the end application. for example, the us er may elect to relax output ripple requirements to use a lower order (lower cost) output filter; or modulator perf ormance can be enhanced if higher slew rate op-amps and/ or low-delay comparators are used, and so on. figure 2. isolinear reference design schematic and operation self-oscillating modulator r int c int r in r f r ref1 r ref2 v dd v ref v dd v dd v ss r fb v in si86xx isolator 4 th ? order ? filter ? (demodulator) r in1 c f1 r1 c1 v out r in2 c f2 r2 c2 v dd filter output filter output t0 t1 t2 t3 t4 t5 t0 t1 t2 t3 t4 t5 modulator input modulator output modulator samples
AN614 rev. 0.2 3 figure 3. six-channel linear isolator block diagram in addition, a single si86xx isolator can host multiple linear isolators by adding additional modulator/filter circuits to a multi-channel si86xx digital isolator, as shown in figure 3. in this example, an si8663 six-channel digital isolator provides three forward and three reverse linear isolator channels, amortizing the isolator across all six channels. figure 4. isolinear reference design evaluation board figure 4 shows the board included in the isolinear refere nce design kit. this boar d contains reference design configurations for 9-bit, 10-bit, and 12-bit linear isolator pe rformance, enabling the user to optimize isolator cost/ performance trade-offs. (for detailed circuit isolinear re ference design schematics, see the isolinear reference design users guide available for download at www.silabs.com/isolation .) si8663 filter modulator filter modulator filter modulator modulator filter modulator filter modulator filter isolator ? channel isolator ? channel isolator ? channel isolator ? channel isolator ? channel isolator ? channel amp amp amp amp amp amp amp amp amp amp amp amp ? 12 \ bit ? version (circuit ? #1) 10 \ bit ? version (circuit ? #2) 9 \ bit ? version (circuit ? #3)
AN614 4 rev. 0.2 figure 5 shows total harmonic distortion and nonlinearity measurements from the evaluation board of figure 4. waveform a shows the isolinear input an d output oscilloscope waveforms. th e fft of waveform a reveals a total harmonic distortion (thd) of 0.096% (waveform b). the pe rformance of this design rounds to 12-bit performance with 0.125% nonlinearity. (note: for more informat ion about the isolinear reference design, please see application note an559: isolating analog signals using th e si86xx cmos isolator fam ily. this application note provides detailed technical in formation for modifying the stock isolinear reference design, including modulator and filter design equations, design guidelin es and more. the isolinear reference design kit (si86isolin)is available for purchase at www.silabs.com/isolation .) figure 5. isolinear reference design performance example table 1 compares the isolinear reference design performance against several commercially-available isoamps. the isolinear reference designs offer higher signal-to-nois e (sans the 9-bit version); higher cmti, greater isolation rating flexibility and lower cost compar ed to competing isoamps. (note: a higher cost, faster comparator can be used to reduce nonlinearit y to less than 0.1%.) input output ab c fft 2.25 2.35 2.45 2.55 2.65 2.75 2.85 - 0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 vin (v) linearity 0.125% nonlinearity 0.096% thd vout (v)
AN614 rev. 0.2 5 table 1. isolinear vs. analog isolation amplifiers device snr (db) bw (khz ) linearity isolation rating (kv) cmti (kv/s) price at 1 ku ($) silicon labs isolinear (12-bit reference design) 71.8 100 0.125 2.5 or 5.0 35 (min), 50 (typ) 2.50 to 3.50 silicon labs isolinear (10-bit reference design) 62 250 0.125 2.5 or 5.0 35 (min), 50 (typ) 2.00 to 2.50 silicon labs isolinear (9-bit reference design) 50.8 500 0.125 2.5 or 5.0 35 (min), 50 (typ) 1.75 to 2.25 avago hcpl-7840 not specified 100 0.1 2.5 15 3.05 avago acpl-790 60 200 0.05 5.0 15 6.63 avago acpl-780 not specified 100 0.004 5.0 15 7.30 avago acpl-c79a 60 200 0.05 5.0 15 10.00 avago acpl-c790 ( ??? ) 60 200 0.05 5.0 15 4.65 ti iso-124 not specified 50 0.01 1.5 not specified 8.30 adiad202 not specified 5 0.05 1.0 not specified 29.82
AN614 6 rev. 0.2 3. application examples the electrocardiograph (ecg) application shown in figure 6 requires safety isolation to protect the patient from dangerous leakage currents. medical applications of this ty pe typically use conductive gels at sensor sites, which lower human body impeda nce to the extent that a few milliamps of le akage current can cause injury or death. to mitigate this issue, ecgs typically ha ve multiple stages of isolation to prevent downstream leakage current from flowing into the patient. the ecg of figure 6 shows the patient connected to an inst rumentation amplifier powered by a low-current floating supply, v1. the isolinear circuit (s hown as a functional block in the center of the diagram) galvanically isolates the instrument ation amplifier input side from potential down-stream leakage currents generated by the voltage source, v2. the lower voltage, hi gher-current dsp circuit is also isolated from the adc/ filter circuit by a separate si86xx digital isolator, again to ensure no leakage current paths into the ecg inputs. figure 6. ecg (safety isolation) application v2 filter(s) si86xx isolator instrumentation ? amplifier adc to dsp right ? leg ? driver v1 isolinear ? reference ? design fg fg v3 modulator si86xx isolator demodulator
AN614 rev. 0.2 7 figure 7. ac line current monitor (safety isolation and level shift) application figure 7 shows an ac line current monitor that demonstrates bo th safety isolation and leve l shifting. safety isolation galvanically isolates the 110 vac line from the low-voltage, ground-based circui ts. this circuit uses resistive shunt r1 to sense ac current. the high-voltage interface circuit is referenced to the ac neutral (white) wire in a two-wire (non-earth grounded) single-phase ac service. isolinear input-side bias voltage is line-derived and us es a 3 v linear regulator. the low-voltage output-side circuits are biased by a ground-referenced supply. because there is no earth ground in this system, an ac line perturbation can potentially cause high voltage to appear on the neutral (white) wire. this elevated neutral line common-mode voltage is rejected by the si86xx isolator's high cmti of 35 kv/s minimum, 50 kv/s typical. for more isolated level shifti ng application examples, see silicon labs ap plication note an598: high-speed level shifting using si8xxx isolators. figure 8 shows a common ground loop in the transmission pa th of two linear circuits (a common scenario in test and measurement, audio and other applications that use cable interconnects). the ground loop in the top diagram circulates between the connector grounds while parasiti c inductance (z) causes ringing that generates output noise. the bottom circuit of figure 8 inserts the isolinear circuit between the signal source and receiver, breaking the ground loop and dramatically reducing the local gr ound path lengths and associated parasitic inductance. r1 c1 u2 3.0v ldo c4 c5 isolinear ref design input vdda gnda white black d1 output vddb gndb 3.3v output side bias supply 1.5v r2 r3 r5 1.5v r12 r13 r14 r15 single-phase ac line r4 modulator isolation demodulator 3.3v mixed-signal circuits \ 1,000v +1,000v common mode transient
AN614 8 rev. 0.2 figure 8. ground loop elimination circuit z signal ? source signal ? receiver connector connector cable signal ? source signal ? receiver isolinear ? reference ? design modulator si86xx ? isolator demodulator vdd1 vdd2 vdd1 gnd1 gnd2 vdd2 in out ground loop noisy output signal clean output signal
AN614 rev. 0.2 9 4. summary many analog systems require isolation to provide safety, le vel shifting or to mitigate ground noise. isoamps are a common but expensive solution that lack flexibility and fo rce the designer to compro mise and/or add supplemental circuitry to a design. silicon labs' isolinear reference design (si 86isolin-evb) offers a lower-cost, more flexible linear isolation solution. this design is enabled by th e silicon labs si86xx industry-leading digital isolator and requires only four operational amplifiers and some passive components in addition to the isolator. the isolinear reference design benefits the user with a robust, competitive solution and the opportunity to create an optimum solution for the application at hand at a fracti on of the price of an isoamp. 4.1. related documents ? an559: isolating analog signals using the si86xx cmos is olator family, silicon labs, 2011 ? isolinear users guide, silicon labs, 2011 ? an598: high-speed level shifting using si8xxx isolators
disclaimer silicon laboratories intends to provide customers with the latest, accurate, and in-depth documentation of all peripherals and modules available for system and software implementers using or intending to use the silicon laboratories products. characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific device, and "typical" parameters provided can and do vary in different applications. application examples described herein are for illustrative purposes only. silicon laboratories reserves the right to make changes without further notice and limitation to product information, specifications, and descriptions herein, and does not give warranties as to the accuracy or completeness of the included information. silicon laboratories shall have no liability for the consequences of use of the information supplied herein. this document does not imply or express copyright licenses granted hereunder to design or fabricate any integrated circuits. the products must not be used within any life support system without the specific written consent of silicon laboratories. a "life support system" is any product or system intended to support or sustain life and/or health, which, if it fails, can be reasonably expected to result in significant personal injury or death. silicon laboratories products are generally not intended for military applications. silicon laboratories products shall under no circumstances be used in weapons of mass destruction including (but not limited to) nuclear , biological or chemical weapons, or missiles capable of delivering such weapons. trademark information silicon laboratories inc., silicon laboratories, silicon labs, silabs and the silicon labs logo, cmems?, efm, efm32, efr, energy micro, energy micro logo and combinations thereof, "the worlds most energy friendly microcontrollers", ember?, ezlink?, ezmac?, ezradio?, ezradiopro?, dspll?, isomodem ?, precision32?, proslic?, siphy?, usbxpress? and others are trademarks or registered trademarks of silicon laboratories inc. arm, cortex, cortex-m3 and thumb are trademarks or registered trademarks of arm holdings. keil is a registered trademark of arm limited. all other products or brand names mentioned herein are trademarks of their respective holders. http://www.silabs.com silicon laboratories inc. 400 west cesar chavez austin, tx 78701 usa smart. connected. energy-friendly products www.silabs.com/products quality www.silabs.com/quality support and community community.silabs.com
|
