Grounding, Shielding, and Interference in Amptek Processors

Transcription

1 Prducts fr Yur Imaginatin Grunding, Shielding, and Interference in Amptek Prcessrs Electrmagnetic interference (EMI) is frequently a prblem in radiatin detectin. The detectr utput is a very small current pulse, typically a few nanamps with a duratin f tens f nansecnds. The preamp utput is a small vltage pulse, typically a few millivlts in amplitude. Near these sensitive ndes, ne ften finds pwer supplies drawing large current (e.g. the high vltage supply fr an X-ray tube), digital circuits which switch at high speeds, and ther pssible surces f interference. Grunding, shielding, and ther techniques can be used t minimize interference, r when misapplied, can make it wrse. It is quite cmmn fr a user t assemble a system, start taking data, and t discver that the perfrmance des nt match expectatins. The mst cmmn prblems are trivial: cables r pwer supplies nt cnnected, sftware imprperly installed, r incrrectly cnfiguratin f the signal prcessr. Once these are slved, nise due t interference is the mst cmmn prblem. This applicatin nte prvides a few suggestins fr minimizing these prblems with Amptek's X-ray detectrs and signal prcessrs. This is nt an exhaustive treatment: there are t many pssible surces f interference t cver them thrughly. There are many excellent bks and articles, listed in sectin, which prvide a mre general apprach. Amptek prvides many different packaging ptins fr its spectrmeters systems, and the packaging cnfiguratin is very imprtant in determining susceptibility t interference. The X-, shwn n the left belw, puts the whle system (detectr, preamp, signal prcessr, pwer supplies) inside a single, cmpact metal bx with minimal external cnnectins. It is well shielded with simple grunding and is usually least susceptible t EMI. The pht n the right belw shws a bard level slutin. It has all f the same cmpnents, but the user prvides packaging and intercnnectins. The user is respnsible fr shielding, and grunding. The bard level slutin requires mre effrt and expertise n the part f the custmer. AMPTEK, Inc. DeAngel Drive, Bedfrd, MA USA Fax: sales@amptek.cm

2 Prducts fr Yur Imaginatin. GENERAL SUGGESTIONS Grunding Grunding is a subject which is surprisingly cmplicated and is ften prly understand. Imprper grunds are respnsible fr mre interference prblems than any ther cnditin. The key recmmendatins are: Cables Yu shuld always have a cnnectin t earth grund, fr persnnel safety. We recmmend a single pint grund, i.e. ne and nly ne cnnectin between the system grund and earth grund. It is usually best t grund either the preamp r the chassis f the spectrmeter. Laptp cmputers are a cmmn surce f grund nise prblems. Sme laptps have very nisy pwer supplies, which inject large current pulses int their grund pin. We recmmend islating laptp pwer frm earth grund. Pay clse attentin t the pwer return path fr nisy equipment (e.g. high vltage pwer supplies fr X-ray tubes, r stepper mtrs). Make sure that the return currents d nt flw thrugh the spectrmeter's grund cnnectins. If the detectr and preamp are separate frm the signal prcessr (e.g. when using an XR00 r PA0 preamp), cnnect the signal grund f the preamp t the signal grund f the prcessr using the lwest impedance feasible. It is usually best t cnnect the signal and chassis grunds t earth grund at the detectr r preamplifier and islate them elsewhere. Shrt cables are gd. Custmers ften ask "Hw lng can I make the cables befre I get interference?" The answer is "It depends n the electrmagnetic envirnment". Lnger cables are mre susceptible t EMI. Shielding Shielded cables are usually a gd idea thugh it depends n hw they are grunded. Recmmended practice is t send a signal, particularly the small preamp utput, thrugh a shielded cable which is grunded at the preamplifier (tied t preamp signal grund) and flating at the amplifier input. Signal grund shuld be prvided separately. When transmitting nisy signals, i.e. thse with high current fluctuatins, it is best t use a shielded cable with the return n the utside. Twisted pair may als reduce cupling. Fr shielding t be effective, yu must understand the surce f the interference and the receiving prtin f the circuit. In general, we recmmend shielding preamplifiers (put a shield arund the preamplifier and cnnect it t the preamp s signal grund). We als recmmend shielding nisy pwer supplies. A switch mde pwer supply may require shielding. The high vltage pwer supply fr an X-ray will, mst ften, require shielding. Thinking abut interference Any electrmagnetic interference prblem invlves three cmpnents: a surce r transmitter, a cupling mechanism, and a receiver. T slve an interference prblem, yu can (a) reduce what is transmitted, (b) reduce the cupling, r (c) reduce the sensitivity f the receiver. T slve the prblem, yu must first identify the transmitter, and identify hw it is cupling, and identify which prtin f the circuit is sensitive. Only then can yu identify which cmpnent t tackle.

3 Prducts fr Yur Imaginatin. REFERENCES Many papers and textbks have been written n grunding, shielding, and interference. We strngly recmmend that yu refer t these surces. We have a few we particularly suggest (the recmmendatins in this applicatin nte cme frm these references). Web resurces Analg Devices, Inc. has published several applicatin ntes n grunding, shielding, and interference which are available nline. Their fcus is n integrated circuits, rather than systems, but they still prvide a very nice intrductin t the main issues. Understanding interference-type nise: Hw t deal with nise withut black magic. AN-5, Grunding fr lw- and high-frequency circuits, by Paul Brkaw and Jeff Barrw AN-0, An IC amplifier user s guide t decupling, grunding, and making things g right fr a change, by Paul Brkaw AN-7, Shielding and guarding: Hw t exclude interference-type nise, by Alan Rich MT-95, EMI, RFI, and Shielding Cncepts: A Tutrial Radiatin detectin texbk Helmut Spieler has written a textbk n radiatin detectin. Chapter 9 is entitled Why things dn t wrk and mst f the chapter cncerns grunding and shielding fr semicnductr radiatin detectrs. It is an excellent reference. Spieler, Helmut, Semicnductr detectr systems, Oxfrd press, 005 Standard electrical engineering textbks Electrmagnetic interference is a cmmn prblem in electrical engineering. There are several standard reference textbks. Their advice is generally gd fr radiatin detectin systems. Mrrisn, Ralph, Grunding and Shielding Techniques, Jhn Wiley & Sns, New Yrk, 998. Ott, Henry W., Nise Reductin Techniques in Electrnic Systems, nd ed., Jhn Wiley & Sns, New Yrk, 998.

4 Prducts fr Yur Imaginatin. XR-00 with PX5 Amptek s XR00 detectr/preamplifier with the PX5 signal prcessr and pwer supply is cmmnly used in labratry and benchtp settings. A typical setup is shwn t the right. Because the cmpnents are in shielded bxes, with shielded cables, it is immune t much interference. Grunding can be a cncern, due t the cnnectins between the bxes and frm the PX5 t auxiliary equipment. The blck diagram belw summarizes the cnnectins in this system, in a typical cnfiguratin while the pht shws a typical setup. Nte that there are several different detectr cnfiguratins available; this is a generic illustratin. Refer t the PX5 and XR00 manuals fr mre details, including specific pinuts fr the cnnectrs. Detectr Hybrid AXR Preamplifier XR00 PX5 FPGA Feedback Fast Channel Pulse Selectin CF 0.05 pf Drain OUT AMP IN ADC Digital Pulse Shaping MCA Lgic MCA Memry Micrcntrller USB X-RAYS Be Windw Detectr Surce Reset Lgic +8.5V -8.5V Analg Prefilter +8.5V -8.5V Cntrller AUX- PC Stage Thermelectric Cler Bias TEC + TEC - Temperature Lw Pass Filter (several M) t VDC Vmax, 0.Amax 70 A, 70 mv TEC Supply HVPS Digital SCAs LVPS DAC Aux I/O Cntrl Pwer Input AUX- AUX- 5 VDC AC/DC Oscillscpe (Optinal) 0 VAC (islated) N9 Case Shield GND Plarity Plug

5 Prducts fr Yur Imaginatin Ntes and recmmendatins fr XR00/PX5 Detectr and preamp PX5 Cnnectins The signal grund is the surce f the input JFET. This is cnnected t the grund plane f the XR00. The case f the TO-8 can be regarded as the sensr chassis grund. It is cnnected t the XR00 bx (chassis grund). In mst detectrs, the TO-8 case is cnnected t a detectr pin, which cnnects t the XR00 grund plane. The TEC pwer return is tied t the XR00 grund plane. The XR00 grund plane is tied t the XR00 bx (chassis grund) and t the shields f the BNC and the LEMO. It is usually best t cnnect the XR00 bx t grund and t have this be the single pint grund. The BNC fr the amplifier input and the LEMO frm the XR00 bth cnnect t the chassis f the PX5 and t the grund plane f the PX5 bard. On the PX5 back panel, there is a grund plug and grund cnnectins n the tw BNCs, the USB, the AUX cnnectr, and the 5VDC pwer. These are all tied t PX5 signal and chassis grund. The AC/DC pwer supply prvided by Amptek is islated: the pwer return is nt cnnected t grund. The USB grund is tied t PX5 grund. This is frequently tied t grund in a cmputer, s can create a grund lp. If the AUX cnnectrs are tied t scillscpes r ther test equipment, and this is grunded, a grund lp is frmed. Other We have fund that LCD screens, ften used in laptps r scillscpes, t be a surce f nise (the bst regulatr needed t pwer the LCD may emit radiatin). 5

6 Prducts fr Yur Imaginatin. PA0 with DP5 and PC5 Amptek s PA0 detectr/preamplifier with the DP5 signal prcessr and PC5 pwer supply is intended fr use by OEMs, integrating the bards int instruments. Because the cmpnents are in unshielded bards with unshielded cables, this cnfiguratin is mst susceptible t interference and grunding. The blck diagram belw summarizes the cnnectins in this system, in a typical cnfiguratin while the pht shws a typical setup. Nte that there are several different detectr and preamplifier cnfiguratins available. Nte als that the cnnectrs have mre pins than are shwn here. Refer t the PA0, DP5, and PC5 manuals fr mre details, including specific pinuts fr the cnnectrs. Detectr Hybrid AXR Preamplifier PA0 Pulse Prcessr DP5 Analg Prefilter ADC FPGA Fast Channel Digital Pulse Shaping Pulse Selectin MCA Lgic MCA Memry Cntrller Micrcntrller AUX- USB PC CF 0.05 pf Feedback Drain J J5 LVPS Pwer Input 5 VDC GND AC/DC 0 VAC (islated) X-RAYS Signal 6 6 J Be Windw Detectr Surce Reset Lgic +5V -5V +5V -5V LVPS Pwer Cntrl Stage Thermelectric Cler Bias TEC + TEC - Lw Pass Filter (several M) t VDC Vmax, 0.Amax HVPS 0 x crner psts Als n DP5 Temperature A, 70 mv TEC Supply N9 Case 8 8 Pwer Supply PC5 6

7 Prducts fr Yur Imaginatin Ntes and recmmendatins fr PA0/DP5/PC5 Detectr and preamp The signal grund is the surce f the input JFET. It is cnnected t the grund plane n the PA0 preamp bard. The case f the TO-8 detectr package is chassis grund. In mst Amptek detectrs, this is cnnected t a pin and is cnnected t the preamp grund. In the FastSDD TM, the case is nt tied t a pin and bard grund. A separate cnnectin frm the TO-8 case t preamp grund is recmmended fr the FastSDD TM. The TEC return current is nt cnnected t grund in the detectr r preamp. It is grunded in the PC5. We strngly recmmend that the preamplifier be shielded. Amptek prvides an aluminum husing r a custm husing can be made. This husing shuld be tied t the preamplifier grund plane. Fr gd cling and hence lw nise, it is vital that the detectr be attached t a gd heat sink. Gd heat sinks are usually electrical cnductrs; in many cases, this means that the detectr case is electrically tied t the heat sink. We have seen many cases where large grund current flw thrugh here, e.g. due t a nearby X-ray tube. In these cases, it is advantageus t electrically islate the detectr package and the threaded stud, using thermally cnductive but electrically islating materials. PA0 t PC5 cnnectin An 8 inch, ten pin, shielded flex cable is the standard cnnectin between the preamp and the PC5. Lnger cables can be used but are mre susceptible t interference nise. Shrter cables may als be used. The ten pin cnnectr frm the PA0 brings ver signal grund (ften tied t detectr case grund) and separately the TEC return, which is grunded n the PC5. DP5 and PC5 - grunding The DP5 and PC5 signal grunds are cnnected via the signal cnnectr (J n the DP5) and via a large cnnectr (J5 n the DP5). In additin, each bard has fur munting hles thrugh which studs are usually placed. Each hle (and stud) can be cnnected t the bard's grund plane via an adjacent zer hm resistr, r the resistr can be remved t islate the stud. These can studs tie the bard grunds tgether and can grund the stack. One apprach is t grund the stack using ne stud and t remve the ther six resistrs. If the DP5/PC5 stack and the preamp are separately grunded, a grund lp is frmed. A secnd apprach is t cmpletely islate the DP5/PC5 stack and grund the preamp (r vice-versa). The 5 VDC grund pin is tied t the grund plane n bth DP5 and PC5. If a nn-islated supply is used, a grund lp is frmed. Amptek's AC/DC supply is islated s des nt grund the system. The USB cnnectin is tied t the DP5 grund plane s can frm a grund lp. The AUX cnnectin are referenced t the DP5 grund plane s can frm grund lps. DP5 and PC5 ther issues We recmmend shielding the DP5/PC5 stack. 7

8 Prducts fr Yur Imaginatin 5. X- Amptek s X- is a cmplete spectrmeter. It is essentially a packaged versin f the detectr/preamplifier with DP5 and PC5. Because it is in a single metal bx, it is ften less susceptible t interference and grunding that Ammptek s ther prducts. The blck diagram belw summarizes the cnnectins in this system, in a typical cnfiguratin while the pht shws a typical setup. Nte that there are several different detectr cnfiguratins available; this is a generic illustratin. The blck diagram is nearly identical t that already shwn. The preamp has a different part number and the DP5/PC5 are in a metal husing but therwise it is essentially unchanged. Detectr Hybrid AXR Preamplifier PA Pulse Prcessr DP5 Analg Prefilter ADC FPGA Fast Channel Digital Pulse Shaping Pulse Selectin MCA Lgic MCA Memry Cntrller Micrcntrller AUX- USB Signal 6 6 PC CF 0.05 pf Feedback Drain J5 LVPS Pwer Input 5 VDC GND AC/DC 0 VAC (islated) X-RAYS J Be Windw Detectr Surce Reset Lgic +5V -5V +5V -5V LVPS Pwer Cntrl Stage Thermelectric Cler Bias TEC + TEC - Lw Pass Filter (several M) t VDC Vmax, 0.Amax HVPS 0 x crner psts Als n DP5 Temperature A, 70 mv TEC Supply N9 Case 8 8 Pwer Supply PC5 X Husing 8

9 Prducts fr Yur Imaginatin Ntes and recmmendatins fr X Detectr and preamp The signal grund is the surce f the input JFET. This is cnnected t the grund plane f the XR00. The case f the TO-8 can be regarded as the sensr chassis grund. It is cnnected t the X husing (chassis grund). In mst detectrs, the TO-8 case is cnnected t a detectr pin, which cnnects t the XR00 grund plane. DP5/PC5 in its husing Cnnectins Internally, the DP5 grund plane is tied t a single metal stud and thus t the husing. The PC5 grund plane is als tied t a single metal stud and t the husing. The stud and screw nearest the extender prvides this single grund cnnectin. We recmmend a single pint grund. If ne lcatin is t be used, this munting screw nearest the extender is best. The AC/DC pwer supply prvided by Amptek is islated: the pwer return is nt cnnected t grund. The USB grund is tied t PX5 grund. This is frequently tied t grund in a cmputer, s can create a grund lp. If the AUX cnnectrs are tied t scillscpes r ther test equipment, and this is grunded, a grund lp is frmed. Other We have fund that LCD screens, ften used in laptps r scillscpes, t be a surce f nise (the bst regulatr needed t pwer the LCD may emit radiatin). 9

10 Prducts fr Yur Imaginatin 6. Other Infrmatin The pulse shaping netwrk functins as a bandpass nise filter. It is ptimized fr filtering the bradband intrinsic nise at the preamplifier input, primarily white current nise frm the detectr s leakage current and white vltage nise frm the preamplifier. This filter will als affect the respnse f the circuit t interference, whether cnducted via the grund r thrugh ther mechanisms. The plt belw shws the transfer functin f the digital prcessr s nise filter, fr representative settings. There are tw pulse shaping channels; the slw r "shaped" channel is used fr pulse height spectrscpy while the "fast" channel is used fr timing and pile-up rejectin. The curves here are fr a. s peaking time in the slw channel, with a respnse peaking near 70 khz, and fr a 0. s peaking time in the fast channel, with a respnse peaking near MHz. The curves scale inversely with the peaking time. The circuit will respnd t ripple r interference (grund lps, radiated, micrphnic) as shwn here. 0