circuit note cn- 0313 circuits from the lab? reference circuits are engineered and tested for quick and easy system integration to help solve todays analog, mixed - signal, and rf design challenges. for more i nformation and/or support , visit www.analog.com/cn0313 . devices connected /referenced adm3485e 15 kv esd - protected, 3.3 v,12 mbps, eia rs - 485 transceiver emc compliant rs - 485 transceiver protection circuits 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 suitability and applicability for your use and application. accordingly, in no event shall analog devices 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) one technology way , p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2013 analog devices, inc. all rights reserved. evaluation and desig n support circuit evaluation boards cn - 0313 circuit evaluation board (eval - cn0313 - sdpz) design and integration files schematics, layout files, bill of materials circuit function and benefits the circuits shown in figure 1 demonstrate proven and tested electromagnetic compatibility (emc) compliant solutions for three protection levels for popular rs - 485 communication ports using the adm3485e transceiver. each solution was tested and characterized to ensure that the dynamic interaction between the transceiver and the protection circuit components function s correctly together to protect against the electrostatic discharg e (esd), electrical fast transients (eft), and surge immunity specified in iec 61000 - 4 - 2, iec 61000 - 4 - 4, and iec 61000 - 4 - 5, respectively. the circuits offer proven protection for rs - 485 interfaces using the adm3485e to the esd, eft, and surge levels often encountered in harsh environments. figure 1. three emc compliant adm3485e protection circuits (simplified schematic, all connections not shown) di de ro re v cc adm3485e b a tvs cds0t23-sm712 120? protection scheme 1. tvs di de ro re v cc adm3485e b a tvs cds0t23-sm712 tbu tbu-ca065-200-wh tbu tbu-ca065-200-wh tisp tisp4240m3bjr-s protection scheme 2. tvs/tbu/tisp gdt 2038-15-sm-rplf 11250-001 di de ro re v cc adm3485e protection scheme 3. tvs/tbu/gdt b a tvs cds0t23-sm712 120? 120?
cn- 0313 circuit note rev. 0 | page 2 of 5 circuit description the rs - 485 bus standard is one of the most widely used physical layer bus designs in industrial and instrumentation applications . rs - 485 offers differential data transmission between multiple systems often over very long distances. applications for rs - 485 include process control networks; industrial automation; remote terminals; building automation, such as heating, ventilation, ai r conditioning (hvac) and security systems; motor control; and motion control. in these real systems, lightning strikes, power source fluctuations , inductive switching, and electrostatic discharge can cause damage to communications ports by generating larg e transient voltages. designers must ensure that equipment does not just work in ideal conditions , but it must also work in real - world conditions . t o ensure that these designs can work in electrically harsh environments , emc regulations must be met. many e mc problems are not simple or obvious and must be considered at the start of the product development cycle. proper solutions and protection circuits must be part of the total design effort and not left to the last minute. the protection circuits must incor porate the input and output structure of the specific transceiver manufacturer as part of the design. the iec 61000 specifications define the set of emc immunity requirements . within this set of specifications , designers must be concerned with the followin g three types of high voltage transients for data communication lines: ? iec 61000 - 4 - 2 electrostatic discharge (esd) ? iec 61000 - 4 - 4 electrical fast transients (eft) ? iec 61000 - 4 - 5 surge immunity esd and eft have similar rise times, pulse widths , and energy levels. the surge transient has longer rise times and pulse widths; as a result , the surge transient energy can be three to four orders of magnitude larger than the energy in an esd or eft transient. due to the similarities between esd and eft transients, the design of the circuit protection can be similar . h owever , due to the high energy associated with s urge transients, they must be dealt with differently. each solution protects data ports to esd voltages of 8 kv contact and 15 kv air discharge , and eft voltages of 2 kv. the different solutions provide an increased level of s urge protection up to 6 kv. protection levels for the ci rcuits are summarized in table 1 . table 1 . p rotection levels offered by e ach of the three protection circui ts in figure 1 protection scheme esd eft (kv) surge (kv) 1. tvs 8 kv c ont act , 15 kv a ir discharge 2 1 2. tvs/tbu/tisp 8 kv c ont act , 15 kv a ir discharge 2 4 3. tvs/tbu/gdt 8 kv c ont act , 15 kv a ir discharge 2 6 figure 2 shows a photo of the eva l - cn0313 - sd p z board. there are three adm3485e devi ces on the board, one for each protection scheme . each protection scheme provides esd and eft protection as described and increasing levels of surge protection. a complete design support package for the e va l - cn0313 - sdpz board including schematics, layout files, and bill of materials can be found at www.analog.com/cn0313 - designsupport . figure 2. eval - cn0313 - sdpz board 1 1250-002
circuit note cn- 0313 rev. 0 | page 3 of 5 the adm3485e is a 3.3 v, low power data transceiver suita ble for half - duplex communication on multipoint transmission lines . it has a data rate up to 12 mbps with a common - mode rang e on the bus pins (a and b) of ? 7 v to + 12 v . data transmit s on the di pin , and it is received on the ro pin. both the driver and receiver outputs can be enabled or disabled , that is, put into a high impedance state, by changing the logic levels on the de and re pins , respectively. power and ground are connected via a screw - wire connector ( v cc and gnd). this connec tor supplies all three adm3485e devices. logic inputs de and re are set using lk1 to lk6 . for each adm3485e , lk2, lk4, and lk6 relate to de and lk1 , lk3, and lk5 relate to re . for each link , p osition a connects the logic pin to v cc , p osition b connects the logic pin to gnd , and p osition c connects the logic pin to the four terminal , side screw - wire connector. the i nput di and output ro pins are connected directly to the four t erminal screw connector. the e va l - cn0313 - sd p z is also compatible with the analog device s, inc., ezlinx ? board ( ezlinx - iiide - ebz ) and the system development platform ( eva l - sdp - cb1z ). connector j8 link s the uart and gpio interface s on the sdp or the ezl inx board to the logic i/o of the adm3485e devices . the i/o connections and jumper configurations are shown in table 2 . table 2 . e z linx and sdp i/o connections and jumper configurations adm3485e i/o pin sdp/ezlinx connecto r selection tvs r o uart_rx lk7 (a) re gpio_0 lk1 (c) de gpio_ 3 lk2 (c) d i uart_tx lk8 (a) tvs/tbu/tisp ro uart_rx lk 7 (b) re gpio_ 1 lk3 (c) de gpio_ 4 lk4 (c) di uart_tx lk8 (b) tvs/tbu/gdt ro uart_rx lk7 (c) re gpio_ 2 lk5 (c) de gpio_ 5 lk6 (c) di uart_tx lk8 (c) the adm3485e transmitter and receiver share the same differential bus pins (a and b). the protection circuitry is used to protect these bus pins. in the first protection circuit , shown in figure 1 as tvs , it uses one component , the bourns cdsot23 - sm712. this is the transient voltage suppressor ( tvs ) array shown on the e va l - cn0313 - sd p z . it consists of two bidirectional tvs diodes optimized to protect rs - 485 systems with minimal overstress while allowing the full range of the rs - 485 signal and common - mode excursions . the tvs has hi gh impedance to ground in normal operating conditions. when an overvoltage occurs , the tvs enters into avalanche breakdown mode and clamps the pin voltage to a safe predetermined level . it then diverts the transient current away from the adm3485e to ground. this protection scheme offers up to 8 kv contact and 15 kv air discharge esd, 2 kv eft , and 1 kv surge protection. as described in the cdsot23 - sm712 data sheet, the part is designed specifically for rs - 485 devices . the next two protection schemes add to the cdsot23 - sm712 to provide higher levels of circuit protection against surge. in the second scheme, shown in figure 1 as tv s/tbu/tisp , the cdsot23 - sm712 tvs provides secondary protection , and bourns tisp4240m3bjr - s provides the primary protection. the tisp4240m3bjr - s is a totally integrated surge protector ( tisp ). the tisp is a solid - state thyristor . w hen its pre determined voltage is exceeded , the tisp provides a low impedance path to ground , diverting a majority of the transient energy away from the adm3485e . bourns tbu - ca065 - 200- wh transient blocking unit ( tb u ) is a nonlinear overcurrent protection device between the primary and secondary protection devices that ensur es coordination occurs. t he tbu is a n overcurrent blocking device that becomes an open circuit at a predefined current. in blocking mode, tbu has very high impedance to block transient energy. this protection scheme offers up to 8 kv contact and 15 kv air discharge esd, 2 kv eft , and 4 kv surge protection. the third protection scheme , shown in figure 1 as tvs/tbu/ gdt , operates in a similar fashion to protection schem e 2 . in this scheme , a gas discharge tube (gdt) is used instead of the tisp. the gdt protect s to higher overvoltage s and overcurrent s than the tisp described in the previous protection scheme. a gdt is a gas discharge plasma device that provides a low impedance path to ground to protect against over voltage transients. the selected gdt is the bourns 2038 - 15- sm - rplf . this third protectio n scheme offers up to 8 kv contact and 15 kv air discharge esd, 2 kv eft , and 6 kv surge protection. the adm3485e has a 120 termination attached to the bus pins .
CN-0313 circuit note rev. 0 | page 4 of 5 circuit evaluation a nd test apply 3.3 v to v cc t o power the e va l - cn0313 - sdpz board. the voltage can be checked on the v cc test points near each adm3485e . the transmit and receive path s can be tested by connecting one of the adm3485e circuits as shown in figure 3 . a signal or pattern generator can be connected to di . the outputs of the driver can be monitored on the a and b test points , and the output of the receiver can be monitored on the ro test po int . jumper configurations are also shown in figure 3 . this test setup can apply to any of the three circuits. figure 3 . tr ansmit and receive test setup according to iec 61000 -4- 2, esd testing implies using two coupling methods, contact discharge and air gap discharge. contact discharge implies the discharge gun is placed in direct connection with the port being tested. with air discharge, the charged electrode of the discharge gun is moved toward the port under test until a discharge occurs developing an arc across the air gap. apply d ischarges to the screw terminal conne ctor of each bus line. for iec 61000 -4- 4 eft testing, a capacitive coupling clamp is used to couple the eft bursts onto the cable connected to the bus lines. the coupling capacitance of the clamp depends on the cable diameter, material of the cables, and cable shielding. iec 61000 -4-5 s urge testing implies the use of a coupling / decoupling network (cdn) to couple the s urge transient into the bus pins. according to the specification, this must be done using two 80 resistors for a two port test. figure 4 shows the test setup for s urge testing. connect t he cdn to the a and b terminals , and the common of the surge generator t o the ground connection of the four terminal screw connector. figure 4 . iec 61000 -4- 5 surge cdn to adm3485e setup all protection circuits described were characterized in the lab with the adm3485e and verified by an external independent emc compliance test house. for further information on the ezlinx i coupler? isolated interface development environment, go to http://wiki.analog.com/resources/eval/ezlinx . information on other important emc issues can be found at http://www.analog.com/rs485emc . oscilloscope 3.3v power supply b a signa l generat or 1 1250-003 de di ro a b c a b c re protection components 80 cdn 80 di de ro re v cc adm3485e b a 120? 11250-004
circuit note cn- 0313 rev. 0 | page 5 of 5 learn more c n - 0313 design support package: http://www.analog.com/cn0313 - designsupport ezlinx? icoupler? isolated interface development environment, ezlinx board quick start guide certified emc compliant rs - 485 interface design tool marais, hein. application note an - 960. rs - 485/rs - 422 circuit implementation guide. analog devices, inc. electromagnetic compatibility (emc) part 4 - 2: testing and measurement techniques C electrostatic discharge immunity te st (ie c 61000- 4 - 2) electromagnetic compatibility (emc) part 4 - 4: testing and measurement techniques C electrical fast transient/burst immunit y te st (ie c 61000- 4 - 4) electromagnetic compatibility (emc) part 4 - 5: testing and measurement techniques C surge immunity test (iec 61000 - 4 - 5) data sheets and evaluation boards ad m3485e data s heet system development platform (eval - sdp - cb1z) ezlinx? icoupler? isolated interface development enviro nment (ezlinx - iiide - ebz) ug - 400 user guide, ezlinx? icoupler? isolated interface development environment , analog devices. cdsot23 - sm712 surface mount tvs diode data sheet , bourns tisp4xxxm3bj overvoltage protector series ( tisp4240m3bjr - s ) data sheet, bourns tbu - ca series tbu? high speed protectors ( tbu - ca065 - 200- wh ) data sheet, bourns 2038 series miniature symmetrical three electrode surface mount gas discharge tube ( 2038 - 15- sm - rplf) data sheet, bourns rev ision history 2 / 13 revision 0: initial version (continued from first page) circuits from the lab circuits are intended only for use with analog devices products and are the intellectual property of an alog devices or its licensors. while you may use the circuits from the lab circuits in the design of yo ur product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the circuits from the lab circuits. information furnished by analog devices is believed to be accurate and reliabl e. however, circuits from the lab circuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warrant y of merchantability, noninfringement or fitness for a particular purpose a nd no responsibility is assumed by analog devices for their use, nor for any infringements of patents or other rights of thir d parties that may result from their use. analog devices reserves the right to change any circuits from the lab circuits at any tim e without notice but is under no obligation to do so. ? 2013 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn11250 - 0 - 2/13(0)
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