INSTRUMENTATION Protecting Instrumentation data acquisition

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INSTRUMENTATION Protecting Instrumentation data acquisition

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INSTRUMENTATION
INSTRUMENTATION
Protecting Instrumentation
data acquisition board designs have contend with ESD, EMI, overvoltages. solution protect circuitry against three hazards?
their nature, instrumentation amplifiers used applications where their inputs often exposed electromagnetic interference (EMI), electrostatic discharge (ESD), overvoltage events. Although these conditions fundamentally different, circuits used protect against each them similar. This implies possibility single application circuit that protect against three conditions. this article, look each interference source propose suitable protection circuitry work toward universal input-protection circuit. often case, there's tradeoff-in this case, between level protection corresponding effect circuit performance. Overvoltage Protection Whenever amplifier's input voltage goes outside supply range, unit damaged, usually high current flow, even when turned off. Typical maximum ratings limit allowable input voltage positive negative supplies, possibly outside supplies. some devices incorporate internal, currentlimiting resistors, which input voltage exceed supplies varying degrees. Furthermore, some instrumentation amplifiers tolerate-and indeed designed operate under-high-input, commonmode voltages. avoid damage, engineers generally clamping limit input current either depending device. This value conservative rule thumb based metal trace widths typical amplifier input stage. Higher currents cause metal migration, which will eventually lead open trace. Migration cumulative effect that might lead failure long time. Failure caused multiple overvoltage events, making failure modes difficult identify. Even though amplifier appear withstand overvoltage currents well above this level short time, limiting
James Bryant, Walt Kester, Chuck Kitchin, Eamon Nash, Analog Devices, Inc.
INSTRUMENTATION current important guarantee longterm reliability. Figure shows equivalent input circuit input stage Analog Devices' AD620 during overvoltage condition. unit internal resistors series, with input transistor junctions their protection diodes. AD620 series designed handle maximum input currents internal diodes protect unit from input voltages greater than either supply voltage supplies, maximum safe input level addition, differential input voltage should also given value that limits maximum input current equivalent circuit shows that input current flows through external RLIMIT resistors, internal resistors, gainsetting resistor diode drops junction Q2). given differential input voltage, input current function amplifier gain. Therefore, gain resistor 49.9 (gain 1000) affects input current more than does 5.49 (gain 10). generalized external protection circuit using Schottky diodes external currentlimiting resistors ensure input protection (see Figure page 64). circuit incorporates protection against both differential- common-mode overvoltages. amplifier internal protection diodes supplies shown), diodes conduct about forward drop above below supply rails. internal diodes-whose primary function protect against ESD-actually serve dual role because they also clamp input voltage beyond supply rails. such cases, external diodes unnecessary. must choose external current-limiting resistor that maximum input current limited This cause large values LIMIT, resulting increase resistor (Johnson) noise might acceptable. Resistors contribute noise according following equation: Noise (nV/Hz) 4KRT
Figure equivalent circuit input stage Analog Devices' AD620 during overvoltage event includes series resistors. safely flow through resistors. Adding external series resistors (RLIMIT) provides additional protection.
Device Input Noise input Rext Rext Current additional noise additional noise AD620 nV/Hz AD623 nV/Hz AD627 nV/Hz 8.08 10.0 2.49 40.2 45.3
Figure External series resistors combine with internal electrostatic discharge diodes create overvoltage, current-limiting clamping circuit. Higher currents carried external Schottky diodes, allowing smaller series resistors. Zener diodes TransZorb diodes provide differential overvoltage protection RLIMIT small omitted.
INSTRUMENTATION where: Boltzmann's constant (1.38 10-23) resistance ohms temperature kelvin (~300 room temperature) example, resistor Johnson noise nV/Hz room temperature. Because protection circuit includes equal resistors, whose noise uncorrelated- that noise sources independent each other-the above result must multiplied square root (the root square noise voltages). have find reasonable balance between protection provided increased resistor noise introduced. Circuits that amplifiers with relatively high noise able tolerate more series protection without experiencing serious deterioration performance. good rule thumb choosing protection resistors select values that contribute more than additional total circuit noise. example, circuit using amplifier with rated noise level nV/Hz tolerate additional nV/Hz Johnson noise. Because lower value root squared with higher value, effect lower value total noise almost negligible. situations where required protection resistor generates much noise, external Schottky protection diodes shown Figure These begin conduct about overvoltage current shunted through them supply rails rather than through internal diodes. Therefore, RLIMIT maximum allowable diode current, which much larger than internal limit instance, RLIMIT resistor would limit diode current Unfortunately, most ordinary diodes (e.g., Schottky, silicon, germanium) have high leakage currents that cause large offset errors amplifier's input, leakage increases directly proportion temperature. This tends rule external diodes applications which amplifier used with high-impedance
Figure 1000-4-2 test pulse amplitude (air-gap discharge) much faster rise time than Human Body Model pulse.
sources. most applications, limiting resistors alone provide adequate protection against overvoltage conditions. series protection resistor also produces voltage drop because amplifier bias current flowing through This drop appears increase circuit offset voltage (and offset drift bias current changes with temperature). because same series protection resistance generally used both amplifier's inputs, effect serious only with amplifiers that have large input offset currents (offset current difference between amplifier's bias currents). Input series resistors also limit current when differential overvoltage condition occurs. low-noise applications, achieve differential overvoltage protection using Zener TransZorb diodes, shown Figure diodes Figure limit maximum differential input voltage less than |VPOS VNEG|, required. Two-op-amp instrumentation amplifiers don't generally need such differential protection because their input current function gain-setting resistor, case with three-op-amp instrumentation amplifier configuration. Electrostatic Discharge high voltages high peak currents
generated partially permanently damage Several specifications related safe handling semiconductors-such MIL-STD-883B, METHOD 3015.7 (also called Human Body Model, HBM), Machine Model (MM), Charged Device Model (CDM)-are well known semiconductor business. only recently have system-level standards susceptibility emerged. Some devices require higher immunity than others. example, board surrounded other circuits generally less likely subject than signal-conditioning devices (e.g., amplifiers that interface with sensors outside world). Since 1996, electronic equipment sold European community must immune ESD, defined specification IEC1000-4-2. This system-level specification applying equipment, individual ICs. practical reality that signal-conditioning components (e.g., amplifiers, which "first line fire") must protected from specified must exhibit immunity However, precision analog circuits, which often feature bias currents, actually tend more susceptible damage than common digital circuits. This mainly because they cannot traditional protec-
INSTRUMENTATION tion structures that tend increase input leakage currents. Traditional test methods don't fully test product's susceptibility type discharge specified IEC1000-4-2. Figure (page shows profile test pulses used types testing. There some important differences between method specification. specification more stringent terms discharge energy. peak current injected more than four times greater. current rise time significantly faster test, test conducted with device under power. IEC1000-4-2 specifies compliance testing using coupling methods: contact discharge air-gap discharge. Contact discharge, which maximum discharge voltage calls direct connection unit being tested. Air-gap discharge uses higher test voltage does make direct contact with test unit. With air-gap discharge, moves toward test unit, developing across gap. This method influenced humidity, temperature, barometric pressure, distance, discharge gun's closure rate. contact-discharge method, although less realistic, more repeatable gaining acceptance preference air-gap method. Figure (page shows simple technique protecting amplifiers against highvoltage ESD. technique quite simple relies again input-series-resistor protection limit current flowing into device during ESD. Carbon resistors, which noninductive, should used protection resistors PROT Figure instead devices made metal film carbon film. There however, tradeoff here, because carbon film resistors imprecise noise. Reducing Radio frequency interference (also called EMI) seriously affect performance high-accuracy circuits. Because their relatively bandwidth, amplifiers don't accurately amplify signals
Figure External series resistors augment internal protection resistors limiting fault current flow into instrumentation amplifier's inputs.
range. these out-of-band signals (either differential mode common mode) couple into precision amplifier through input, output, power-supply pins. Then, through various junctions amplifier, unexplained unwanted offset output result. Fortunately, proper filtering minimize prevent these errors. Precision amplifiers particularly sensitive common-mode RFI. Figure (page shows recommended filtering component values, along with equations calculating appropriate components. figure shows both common-mode filtering (R1/C1, R2/C2) differential-mode filtering C3). R1/R2 C1/C2 well matched, some input common-mode signal will converted differential signal amplifier's inputs. this reason, should matched within least each other. should metal film resistors ensure matching. Capacitor helps attenuate differential signal that result from
perfect matching common-mode filters. this type filter, should much larger than that differential signal caused mismatching common-mode signals will sufficiently attenuated. either traditional silver-mica capacitors, miniature micas, Panasonic film capacitors. overall filter bandwidth should least input signal bandwidth. optimum filter performance, filter components should symmetrically mounted board with large ground plane should close amplifier input. Figure (page shows typical layout standard 8-pin amplifier. This type layout easiest achieve surface-mount components. board should have least layers, ideally with continuous ground plane running under component plane shown). Multiple vias should between sections ground plane component layer continuous ground plane (either other side circuit board buried layer). tested common-mode rejection
INSTRUMENTATION
Device AD620 AD623 AD627 R1/R2 1000 1000 1000 0047 0022 0022
AD620 circuit shown Figure applying common-mode signal input. AD620 gain 1000. measured offset voltage frequency sine-wave source varied from MHz. maximum input offset voltage shift filter bandwidth ~400 Common-Mode Chokes Common-mode chokes offer simple, onecomponent alternative passive filters. Because their resistance ohms), they attenuate little noise compared with networks. Selecting proper common-mode choke critical, however. choke used circuit shown Figure (page Pulse Engineering B4001 designed Digital Subscriber Line data receivers (through-hole mount). B4003 equivalent surface-mount choke. maximum offset shift measured from Unlike filter Figure choke-based filter offers differential-mode filtration. differential-mode filter after choke. advantage this configuration that there penalty potential common-mode-to-differentialmode conversion, there with combination networks discussed earlier. addition filtering inputs power pins, have protect amplifier outputs from RFI, especially they must drive long lengths cable. output couple into amplifier, where rectified appears again output offset shift. resistor ferrite bead series with output simplest output filter. Adding capacitor resistor shown Figure page improves filter. general, however, shouldn't connect such capacitor resistor's amplifier side because could cause amplifier become unstable. Multipurpose Protection Circuit clear from this discussion from examples that simple circuit protect against more than type interference. example, circuit Figure
Figure Protection against electromagnetic interference requires attention both differential-mode common-mode interference. R1/C1 R2/C2 time constants should closely matched preserve highfrequency common-mode rejection.
Figure Symmetrical layout filter components increases effectiveness filter. continuous ground plane should under component layer (either other side circuit board buried layer) with plenty vias connecting sections ground plane component layer.
INSTRUMENTATION provides some protection against three types interference-the input series resistors limiting current during overvoltage, well constituting part low-pass differential common-mode filter. this example, don't need protect against large differential voltages because large series resistors limit worst-case current (for differential overvoltage) around Further Reading Amplifier Applications Guide. 1992. Analog Devices, Inc., Norwood, Section XI:1-10. EIAJ ED-4701 Test Method C-111, Electrostatic Discharges (available from Japan Electronics Bureau, York, NY). Association Draft Standard DS5.3 Electrostatic Discharge (ESD) Sensitivity Testing-Charged Device Model (CDM) Component Testing (available from Association Inc., Rome, NY). Association Standard S5.2 Electrostatic Discharge (ESD) Sensitivity Testing-Machine Model (MM)-Component Level (available from Association Inc., Rome, NY). Prevention Manual. 1986. Analog Devices, Inc., Norwood, Reliably Protect CMOS Circuits Against Power Supply Overvoltaging,
Figure Common-mode chokes offer simple, one-component alternative filters. simple filter protect amplifier's output from EMI.
Application Note AN-311, Analog Devices, Inc., Norwood, Linear Design Seminar. 1994. Analog Devices, Inc., Norwood, Section 1:19-22. Lyne, 1995. Electrical Overstress Damage CMOS Converters, Application Note AN-397, Analog Devices, Inc., Norwood, MIL-STD-883 Method 3015, Electrostatic Discharge Sensitivity Classification (available from Standardization Document Order Desk, Philadelphia, PA). Pulse Engineering, Inc., Diego,
Systems Applications Guide. 1992. Analog Devices, Inc., Norwood, Section 1:1.37-1.55 56-72. TransZorbs (available from General Semiconductor, Inc., Melville, NY).
James Bryant European Applications Manager, Walt Kester Corporate Staff Applications Engineer, Chuck Kitchin Technical Support Engineer, Eamon Nash Applications Engineer, Standard Linear Products Div., Analog Devices, Inc., Woburn St., MS-125, Wilmington, 018873462; 781-937-1292, 781-937-1024.
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INSTRUMENTATION Protecting Instrumentation data acquisition

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