MODEL SR560

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MODEL SR560LOW-NOISE PREAMPLIFIER1290-D Reamwood AvenueSunnyvale, CA 94089 U.S.A.(408) 744-9040Copyright 1989, 1990, 1997, 1999, 2011, 2013Stanford Research SystemsAll Rights Reserved.Revision 3.0(8/2013)

TABLE OF CONTENTSIntroduction and SetupInstrument OverviewPreparation for UseLine VoltageLine FuseLine CordVentilationPower-UpRepackaging for ShipmentUse in Biomedical ApplicationsWarning Regarding Use with PhotomultipliersAccessories FurnishedEnvironmental ConditionsSymbols1222222333334SpecificationsSR560 Low Noise Preamplifier Specifications Chart5Operation and ControlsFront Panel Operating SummaryPowerSourceFiltersGain ModeGainOutputResetStatusRear Panel Operating SummaryAC Power InputAmplifier Power OutputBattery ChargerBlanking InputRS-232 InterfaceBattery Care and UsageRechargingBattery Care777899999101010101111111111Circuit DescriptionDifferential Low-Noise Front EndConfigurable Filters and GainOutput StagesOverload DetectionMicroprocessorBattery Charger and Pre-regulatorsPower RegulatorsRear Panel InterfacesBatteries and P.E.M.Front Panel13131314141415151616i

TABLE OF CONTENTSCalibration and RepairOffset AdjustmentCalibrationFront End ReplacementSR560 Offset Adjustment ProcedureBattery ReplacementFuse ReplacementNoise ContoursInput Voltage NoiseDynamic Reserve171717181818192020Appendix ARemote ProgrammingIntroductionCommandsA-1A-1A-1Appendix BNoise Sources and CuresIntrinsic Noise SourcesJohnson Noise'1/f' NoiseOthersNon-Essential Noise SourcesCapacitive CouplingInductive CouplingResistive Coupling ('Ground Loops')MicrophonicsThermocouple EffectB-1B-1B-1B-1B-1B-1B-2B-2B-3B-3B-3SR560 - Component ListFront Panel BoardMain BoardMiscellaneous PartsC-1C-2C-13D-1PCB - SR560 Revision F (9/89)E-1 to E-10Schematicsii

INTRODUCTION AND SETUPFigure 1: SR560 Block DiagramINSTRUMENT OVERVIEWBNC amplifier" with the amplifier groundisolated from the chassis and the AC powersupply. Opto-isolated input blanking controland listen-only RS-232 interface lines areprovided for instrument control. Digitalnoise is eliminated by shutting down themicroprocessor's oscillator except duringthe short time required to alter theinstrument's configuration, either through afront-panel pushbutton or through an RS232 command. Internal sealed lead-acidbatteries provide 15 hours of line-The SR560 architecture is diagrammedabove. The instrument provides DCcoupled low-noise amplification of singleended and true differential input signals atgains of 1 to 50,000. Two configurable R-Cfilters are provided to selectively conditionsignals in the frequency range from DC to 1MHz. The user can choose high dynamicreserve or low noise settings, and can invertthe output relative to the input. The SR560normally operates with a fully floatingground and can be viewed as an "in-line-1

INTRODUCTION AND SETUPindependent operation. Rear panel bananajacks provide access to the internalregulated power supplies (or batteries) foruse as a bias source.Line FuseVerify that the correct line fuse is installedbefore connecting the line cord to the unit.For 100 V and 120 V, use a 1 Amp fuse andfor 220 V and 240 V, use a 1/2 Amp fuse.PREPARATION FOR USE**********CAUTION**********Line CordThis instrument may be damaged ifoperated with the LINE VOLTAGESELECTOR card set for the wrong appliedAC input source voltage or if the wrong fuseis installed.The SR560 has a detachable, three-wirepower cord with a three-contact plug forconnection to both the power source andprotective ground.The protective groundconnects to the accessible metal parts ofthe instrument except for BNC shields.To prevent electrical shock, alwaysuse a power source outlet that has aproperly grounded protective-groundcontact.Line VoltageWhen the AC power cord is connected tothe unit and plugged into an AC outlet, theunit automatically switches the amplifierpower source from internal battery operationto line operation. The internal batteries arecharged as long as AC power is connected.VentilationAlways ensure adequate ventilation whenoperating the SR560. The unit will generateheat while charging dead batteries.The SR560 can operate from a 100 V, 120V, 220 V or 240 V nominal AC power sourcehaving a line frequency of 50 or 60 Hz.Before connecting the power cord to apower source, verify that the LINEVOLTAGE SELECTOR card, located in therear-panel fuse holder of the unit, is set sothat the correct AC input voltage value isvisible.Power-UpAll instrument settings are stored innonvolatile memory (RAM backed-up) andare retained when the power is turned off.They are not affected by the removal of theline cord. If the power-on self test passes,the unit will return to the settings in effectwhen the power was last turned off. If anerror is detected or if the backup battery isexhausted, the default settings will be used.Additionally, if the RESET key is held downwhen the power is turned on, the instrumentsettings will be set to the defaults shownbelow:Conversion from one AC input voltage toanother requires a change in the fuseholder's LINE VOLTAGE SELECTOR cardposition and a new fuse. Disconnect thepower cord, slide the fuse holder cover tothe left and rotate the fuse-pull lever toremove the fuse. Remove the small printedcircuit board. Select the operating voltageby orienting the printed circuit board. Pressthe circuit board firmly into its slot, so thedesired voltage is visible. Rotate the fusepull lever back into its normal position andinsert the correct fuse into the fuse l ADCOFF

INTRODUCTION AND SETUPROLLOFFHIGH-PASSLOW-PASSGAIN MODEGAINLISTENDEVICE ADDRESSbypassed0.03 Hz, 6 dB/oct1 MHz, -6 dB/octHigh DynamicReserve20, calibratedONAs per SW601Accessories Furnished- Power cable- Operating ManualEnvironmental ConditionsOPERATINGTemperature: 10 C to 40 CRelative Humidity: 90% Non-condensingRepackaging for ShipmentThe original packing materials should besaved for reshipment of the SR560. If theoriginal packing materials are not available,wrap the instrument in polyethylenesheeting or equivalent and place in a strongbox, cushioning it on all sides by at leastthree inches of high-density foam or otherfiller material.NON-OPERATINGTemperature: -25 C to 65 CRelative Humidity: 95% Non-condensingWarning regarding batterymaintenanceBatteries used in this instrument are seallead acid batteries. With usage and timethese batteries can leak. Always use andstore this instrument in the feet-downposition. To prevent possible damage to thecircuitboard, it is recommended that thebatteries be periodically inspected for anysigns of leakage.Use in Biomedical ApplicationsUnder certain conditions, the SR560 mayprove to be unsafe for applications involvinghuman subjects. Incorrect grounding,component failure, and excessive commonmode input voltages are examples ofconditions in which the instrument mayexpose the subject to large input currents.Therefore, Stanford Research Systemsdoes not recommend the SR560 for suchapplications.Warning Regarding Use withPhotomultipliersThe front-end amplifier of this instrument iseasily damaged if a photomultiplier is usedimproperly with the amplifier. When leftcompletely unterminated, a cable connectedto a PMT can charge to several hundredvolts in a relatively short time. If this cableis connected to the inputs of the SR560, thestored charge may damage the front-endFETs. To avoid this problem, provide aleakage path of about 100 kΩ to groundinside the base of the PMT to preventcharge accumulation.3

4

SPECIFICATIONSSR560 LOW-NOISE PREAMPLIFIER SPECIFICATIONS CHARTInputsSingle-ended or true differentialImpedance100 M 25 pF, DC-coupledMaximum Inputs1 VDC before overload; 3 V peak to peak max AC coupled;protected to 100 VDCMaximum Output10 VppNoise 4 nV/ Hz at 1 kHzCMRR 90 dB to 1 kHz, decreasing by 6 dB / octave (20 dB / decade)above 1 kHzGain1 to 50,000 in 1-2-5 sequencevernier gain in 0.5% stepsFlatness 0.3dB to 300kHz(gains up to 1000)-3 dB at 1 MHz, 1 Vpp outputGain Stability200 ppm / CDC Drift5 V/ C referred to input (DC coupled)Filters0.03 Hz to 1 MHz, 10% typical accuracyDistortion0.01% typicalPower100, 120, 220, 240 VAC (50/60 Hz), 60 Watts MaxInternal Batteries: 3 x 12 V, 1.9 Ah sealedlead-acid (rechargeable) 12 VDC in / out through rear panel banana jacks.Battery Life15 hours nominal250-1000 charge / discharge cyclesCharge Time4 hours to 80% of capacityMechanical1/2 Rack-Mount width, 3 1/2" height, weight 15 lbs.Dimensions14-7/8" x 8-1/8" x 3-1/2"Warranty1 year parts and labor on materials and workmanship5

SPECIFICATIONS6

OPERATION AND CONTROLSFigure 2: SR560 Front PanelFRONT PANEL OPERATING SUMMARYlighted. As the batteries near exhaustion,this indicator will change from green to red,indicating that the unit should be connectedto AC power to charge the batteries.The operation of the SR560 Low-NoisePreamplifier has been designed to be assimple as possible. The effect of eachkeypress on the front panel is reflected inthe change of a nearby LED. The frontpanel LED’s will remain lighted at all timesunless dip switch SW601 (accessiblethrough the bottom cover of the unit)positions 3 and 4 are placed in the "off"position. All front panel functions can becontrolled through the rear-panel RS-232interface.When connected to an AC power source,amplifier power is derived from regulatedline power, and the internal batteries areautomatically charged. When operating onAC power, the front panel "LINE" indicator ison to indicate the source of amplifier power.Charging status is indicated on the rearpanel by the "CHARGE" and "MAINTAIN"LED indicators.PowerSourceThe SR560 is turned on by depressing thePOWER switch. When disconnected fromAC power, the unit will operate forapproximately 15 hours on internal sealedlead-acid batteries. Up to 200 mA ofunregulated battery power is available at therear panel banana jacks as long as thepower switch is in the ON position. Batterylife will be reduced when the unit isproviding external power through the rearpanel jacks. When operating on batteries,the front panel "BATT" indicator will beThere are two input connectors located inthe SOURCE section of the front panel.The pushbutton located between themselects either single-ended (A or B) ordifferential (A-B) inputs.The A and B inputs are voltage inputs with100 MΩ, 25 pF input impedance.Theirconnector shields are completely isolatedfrom chassis ground, but can be made7

OPERATION AND CONTROLSMHz. The filters in the FILTER CUTOFFSsection can be configured in the followingsix ways:common with chassis ground by connectingthe "AMP GROUND" and "CHASSISGROUND" banana jacks on the rear panelof the SR560. When connected to ACpower, the chassis of the unit is alwaysconnected to the grounding conductor of theAC power cord. The inputs are protected to100 VDC but the DC input should neverexceed 10 Vp. The maximum DC inputbefore overload is 1 V peak.i. high-pass filter at 12 dB / octaveii. high-pass filter at 6 dB / octaveiii. high-pass filter at 6 dB / octave,and low-pass filter at -6 dB /octave (band-pass)iv. low-pass filter at -6 dB / octavev. low-pass filter at -12 dB / octavevi. no filters in the signal pathThe COUPLING pushbutton selects themethod of connecting the A and B inputs tothe amplifier. The inputs can be AC (0.03Hz - 3 dB) or DC-coupled, or the inputs tothe amplifier can be internally grounded withthe A and B input BNC’s left floating. Thisfeature makes for simple offset nulling,particularly useful when operating theamplifier DC-coupled at high gains. Pleaserefer to CALIBRATION AND REPAIR -OFFSET ADJUSTMENT for information onthe offset nulling procedure.The filter settings are controlled by theROLLOFF, HIGH-PASS and LOW-PASSpushbuttons. Each time the ROLLOFFpushbutton is pressed the instrumentconfigures the two R-C filters to conform tothe progression shown above. The fourROLLOFF LED’s give a visual indication ofthe current filter configuration. For theHIGH-PASS filter the left pushbutton servesto decrease its cutoff frequency. The twopushbuttons for the LOW-PASS filterfunction in an analogous manner.NOTE: When the coupling is set to AC, a0.03 Hz cutoff high-pass filter is alwaysengaged. All high-pass filter modes can stillbe selected while AC-coupled, but the 0.03Hz filter will always be in, even if the filtersare set to DC. Because one of the two filtersections is always used as a high passwhen AC coupling is selected, low-passfilters are only available with a 6 dB / octaverolloff.When the FILTER CUTOFFS section isconfigured solely as high-pass or low-pass(i, ii, iv and v ), the cutoff frequency isilluminated by one of sixteen LED’s in therange from 0.03 Hz to 1 MHz, and the slopeof the rolloff is shown by one of the fourROLLOFF LED’s. When the filter section isconfigured as band-pass (iii), the cutofffrequencies are illuminated by two LED’s.The frequency setting on the left marks thecutoff for the high-pass filter, and the settingon the right is the cutoff for the low-passfilter. The two 6 dB / oct ROLLOFF LED’sare also illuminated. In this case the twocutoffs can be set to the same frequency toprovide a narrow bandpass. When bothfilters are removed from the signal path (vi)all rolloff and cutoff frequency LED’s areextinguished from the FILTER CUTOFFSsection and the DC LED is on.The INVERT pushbutton allows the user toinvert the output of the instrument withrespect to the input when operating withsingle-ended or differential inputs. TheINVERT LED displays the output senserelative to the input for all SOURCEsettings.FiltersThe SR560 contains two identical 1st-orderR-C filters whose cutoff frequencies andtopology (high-pass or low-pass) arecontrolled from the front panel. Themaximum bandwidth of the instrument is 1NOTE: High pass filters are not availablefor the four highest frequency settings. Seethe note under Source: Coupling for8

OPERATION AND CONTROLSinformation on using filters with the amplifierin AC coupled mode.OutputThe outputs of the instrument are locatedwithin the OUTPUT section of the frontpanel. Two insulated BNCs are provided: a600Ω output and a 50Ω output. Theamplifier normally drives high impedanceloads and the instrument's gain is calibratedfor high impedance loads. When driving a600Ω load via the 600Ω output (or a50Ω load via the 50Ω output) the gain of theamplifier is reduced by two. The shields ofall the front-panel BNC’s are connectedtogether and form the amplifier's floatingground.Gain ModeThe allocation of gain throughout theinstrument is set using the GAIN MODEpushbutton The Gain Mode is displayed bytwo indicator LED’s: HIGH DYNAMICRESERVE and LOW NOISE. For a givengain setting, a HIGH DYNAMIC RESERVEallocates the SR560's gain toward theoutput stages after the filters. This preventssignals, which are attenuated by the filtersfrom overloading the amplifier. The LOWNOISE setting allocates gain toward thefront-end in order to quickly "lift" low-level(nV range) signals above the instrument'snoise floor.ResetThe OVLD LED indicates a signal overload.This condition can occur when a signal istoo large or the dynamic reserve is too low.Reducing the gain, reducing the input signaland/or switching to the HIGH DYNAMICRESERVE setting should remedy thiscondition. If an overload occurs with filtersettings of long time constants, the RESETpushbutton will speed the SR560's recoveryfrom overload.GainThe instrument's gain is increased ordecreased using the GAIN pushbuttons.Gain settings from 1 to 50,000 are availableand are displayed as the product of a factor1, 2 or 5 and a multiplier (none (i.e. 1), 10,100, 1,000 or 10,000). In addition to thesefifteen fixed gain settings, the user mayspecify arbitrary gains through the UNCALfeature. To set an uncalibrated or arbitrarygain the user must press both Gain buttonssimultaneously, lighting the UNCAL LED. Inthis mode by pressing the Gain Up or GainDown pushbuttons, the user may reduce thecalibrated gain in roughly 1% incrementsfrom 100% down to 0% of the selected gain.In contrast to other front-panel functions,when in UNCAL the instrument's key-repeatrate will start slowly and increase to a limitas long as either Gain button is depressed.Simultaneously pressing both Gain buttonsonce again will restore the unit to thepreviously calibrated gain setting, and turnoff the UNCAL LED.StatusThe ACT LED indicates communicationsactivity over the SR560's optoisolated RS232 port. Please refer to Appendix A:Remote Programming for further details onprogramming the instrument via RS-232.The BLANK LED indicates the optoisolatedBLANKING input (on the rear panel of theSR560) is active. The SR560 responds to ablanking input by internally grounding theamplifier signal path after the front end andbefore the first filter stage.9

OPERATION AND CONTROLSFigure 3: SR560 Rear PanelREAR PANEL OPERATING SUMMARYground is connected to the AC line groundconductor.The SR560 rear panel is pictured in Figure3. Various interface and power connectorsare provided, along with fuses and chargerstatus LEDs.Battery ChargerAC Power InputThe two 3 A slow-blow fuses protect thebattery supply and charging circuitry. Ifthese fuses are blown, battery power will beunavailable, and charging of the batterieswill not be possible.The power entry module contains thereceptacle for the AC line cord and fuse.The line fuse should be a 1 A slow-blow for100/120 VAC operation, or a 1/2 A slowblow for 220/240 VAC operation.When both the positive and negative supplybatteries are dead, the red "CHARGE" LEDwill be on brightly, and the batteries will becharging at a fast rate. When the batteriesapproach a fully charged condition, thecharging current will be reduced to completethe charge and maintain the batteries.Because the batteries charge at differentrates, the indicators on the rear panel canreflect the charge status of the positive andnegative batteries independently. Whenone set of batteries switches to the"MAINTAIN" mode, the red "CHARGE" LEDwill be reduced to half brightness, and theyellow "MAINTAIN" LED will turn on at halfbrightness. When both batteries switch to"MAINTAIN", the red "CHARGE" LED willAmplifier Power OutputThe -12 V, 12 V, and AMP GROUNDbanana jacks provide external DC power upto 200 mA for use as a bias sourcereferenced to the amplifier's floating powersupplies.The CHASSIS GROUND banana jack isprovided to allow the amplifier's ground tobe referenced to the chassis. If the unit isconnected to an AC power source via athree prong grounding plug, the chassis10

OPERATION AND CONTROLSthe batteries from the amplifier if the unit isoperated for too long in the low batterycondition. This protects the batteries frompermanent damage, which could occur ifthey were to remain connected to a loadwhile dead.turn off and the yellow "MAINTAIN" LED willbe on full brightness.Blanking InputThe blanking input accepts a TTL-levelsignal and grounds the amplifier signal pathafter the front end for as long as the input isheld high. The response time of theblanking input is typically "on" 5 µs after therising edge and "off" 10 µs after the fallingedge.The internal battery charging circuitry of theSR560 will automatically charge deadbatteries at a quick rate until they areapproximately 80% charged. The chargerate is then lowered to a level that is safe formaintaining the batteries. During ACoperation, the batteries will be in this"maintain" charge condition indefinitely, andwill suffer no degradation from prolongedcharging. The sealed lead-acid batteriesused in the SR560 differ in this respect fromnickel-cadmium batteries, which do suffershortened lifetimes due to overcharging.The sealed lead-acid batteries will providethe longest service life if they are notallowed to discharge too deeply and if theyare charged immediately after use.RS-232 InterfaceThe RS-232 interface connector allowslisten-only communication with the SR560at 9600 baud, DCE. Communicationparameters should be set to 8 bits, noparity, 2 stop bits. Data sent must bedelimited by CR LF . All front panelfunctions excluding power and blanking, areavailable over the RS-232 interface. Formore information on programming andcommands, see Appendix A: RemoteProgramming.Battery CareWARNING: As with all rechargeablebatteries, for safety reasons the chemicalrecombination processes within the cellsrequire that the batteries be allowed to ventnon-corrosive gases to the atmosphere.Always use the batteries in an area withadequate ventilation.BATTERY CARE AND USAGEThe SR560 can be powered from either anAC power source or from three 12 V, 1.9Amp-hour maintenance-free sealed leadacid rechargeable batteries. Integral to theSR560 is an automatic battery charger,along with battery protection and chargeindication circuitry.As with all instruments powered byrechargeable batteries, the user must takesome precautions to ensure long batterylife. Understanding and following theprecautions outlined below will result in along operating life for the batteries in theSR560.RechargingDuring battery operation, the front panelBATT LED will change from green to red toindicate that the batteries are low andrequire charging. For the longest batterylife, the batteries should be immediatelycharged by plugging the unit into AC powerwhenever the BATT indicator lights red.Internal protection circuitry will disconnectThe SR560's internal lead-acid batteries willhave a variable service life directly affectedby THE NUMBER OF DISCHARGECYCLES, DEPTH OF DISCHARGE ANDAMBIENT TEMPERATURE. The usershould follow these simple guidelines belowto ensure longest battery life.11

OPERATION AND CONTROLS AVOID DEEP DISCHARGE KEEP THE BATTERIES COOLRecharge the batteries after each use. Thetwo-step fast-charge / trickle-chargeoperation of the SR560 allows the chargerto be left on indefinitely. ALWAYS rechargethe batteries immediately after the BATTindicator LED on the SR560 turns red.Built-in protection circuitry in the unitremoves the batteries from the load once adead-battery condition is detected.Avoiding deep discharge will provide thelongest battery life - upwards of 1,000charge / discharge cycles.When not in use, the SR560 should bestored in a cool, dry place with the batteriesfully charged. This reduces the selfdischarge of the batteries and ensures thatthe unit will be ready for use when calledupon. A SR560 in storage should be"topped off" every three months with anovernight charge to maintain its batteries inpeak condition. AVOID TEMPERATURE EXTREMESBatteries used in this instrument are seallead acid batteries. With usage and timethese batteries can leak. Always use andstore this instrument in the feet-downposition. To prevent possible damage to thecircuitboard, it is recommended that thebatteries be periodically inspected for anysigns of leakage.Warning regarding battery maintenanceWhen using battery power, operate theSR560 at or near room temperature.Operating at lower temperatures will reducethe capacity of the batteries. As well, moretime will be required to recharge thebatteries to their rated capacity. Highertemperatures accelerate the rate ofreactions within the cell, reducing cell life.12

CIRCUIT DESCRIPTIONselect either the output of the second stageamplifier or ground as the input to the nextstage, the first filter section.DIFFERENTIAL LOW-NOISEFRONT ENDTwo high-impedance inputs A and B allowthe instrument to operate in either singleended or true differential modes. RelaysK103 and K104 allow the inputs A and B tobe individually grounded, while K101 selectsAC or DC coupling. Inversion of the inputsis provided by relay K105. The inputcapacitances and R101 and R102 establishthe front end's input impedance at 25 pFand 100 MΩ.CONFIGURABLE FILTERS ANDGAINThe two filter stages in the SR560 eachconsist of 16 R-C filters which can beconfigured as either high pass or low passby a relay. In the following description, partreferences in parentheses refer to filter two.Relay K201, (K301) selects either the highpass or low-pass configuration for all of thesixteen filters. The output of one R-Csection is selected by multiplexer U202 orU203, (U301 or U302) and passed on tonon-inverting buffer U202, (U303).U106 is an NPD5564 low-noise matchedFET pair, which, along with U102 and U103form the first differential amplifier stage.U102 compares the currents in the drainloads of U106, and U103 maintains the sumof those currents at a fixed level by varyingthe total current in both FETs. C109provides open-loop compensation for U102,and front-end gain is nominally establishedby the sum of R118 and R112 over the sumof R114 and R128. K102 is a gain switchingrelay which selects a front end gain of 2 or10. In the gain of 2 position, gain to thenext stage becomes 1 when R116 divideswith the input attenuator to the next stage.For a gain of 10, relay K102 shorts the topof R115 and R128 together, essentiallyeliminating them from the gain loop. P103allows adjustment of front-end offset, andP104 allows for offset compensation whenin the low gain configuration. P102 allowsadjustment of the front-end common-moderejection ratio, along with P101, whichadjusts the CMRR in the low gainconfiguration.Approximately 80 pF input capacitance ofthe multiplexers is included in thecalculation of the R-C time constants of thefilters. The four highest frequency stagesare not available as high-pass filtersbecause of unacceptable attenuation of thesignal that occurs when the filtercapacitance forms a divider with the inputcapacitance of the multiplexers.DG444 U205D, (U401A) is used to bypassthe filter sections entirely and U101D,(U304D) is used to "reset" the filter stagesby discharging them through R228, (R329).U201, (U305) is the third, (fourth) gain stagewith a fixed gain of 5. The input attenuatorU205, (U304) allows setting the gain ofthese stages to 1, 2, or 5 under computercontrol.In the second gain stage, U105 isconfigured with a fixed gain of 10. Byswitching the input attenuation of this stagewith DG444 U101, the overall gain of thisstage can be computer selected as 2, 5, or10. C111 provides high frequencycompensation for U105. The output of thisstage passes through all three sections ofU104, a CMOS multiplexer that serves asthe blanking control. The three parallelswitches provide a low "on" resistance toOUTPUT STAGESThe fifth gain stage consists of op-ampU402 which is configured as a non-invertingamplifier with a gain of 5. U401 is a DG444that again serves to switch the inputattenuation of this stage for overall gains of1, 2, or 5. Additionally, output offset13

CIRCUIT DESCRIPTIONpanel key is pressed and instrumentsettings are to be changed, or while there isactivity on the RS-232 port.adjustment is provided by this stage.U405B, half of an AD7528 dual 8-bit DAC isused to provide a 5 volt offset voltage atthe non-inverting input of U402. The frontpanel offset control also sums at thisjunction, and provides an offset voltage of 5 V that is buffered by U407D.The SR560 uses a 16 K x 8 CMOSEPROM, (U504) containing systemfirmware and calibration bytes, along with a2 K x 8 CMOS RAM, (U505) which isbattery backed-up at all times to retaininstrument settings.Following amplifier U402 is the other half ofthe 8-bit DAC U405A, which along with opamp U404 forms a digital gain vernier. Thisvernier is used in calibration to compensatefor gain variances that occur withconfiguration changes such as inputcoupling and filter settings. This DAC alsoprovides the front panel "uncal" gain vernierfunction.U507 generates port strobes for system IO,and U510 provides a buffered data bus.The buffered data bus is active only duringIO instructions to keep digital noise in theamplifier to a minimum while the processoris running.U601 through U606 are control latchesproviding the 48 DC control lines thatconfigure all of the instrument's hardware.U607 is an input buffer and takes data fromthe front panel and RS-232, as well asproviding a processor input indicating lineoperation and address from SW601 forganged RS-232 operation. SW601additionally controls power to the front panelLED’s through positions 3 and 4.The sixth and final gain stage consists ofU403 and output buffer U406, configured fora gain of 5 and with input attenuator U409to select overall gains of 1, 2, or 5. TheLM6321, (U406) provides the output drivecapability for both the 600Ω and 50Ωoutputs.OVERLOAD DETECTIONThe overload detector constantly monitorsthe front-end output, filter 1 output, U402(after the second filter) output, and finalstage output for excessive signal levels.Comparator U408 compares both positiveand negative signal excursions against a 5volt reference and lights the front paneloverload indicator if any levels areexcessive.BATTERY CHARGERAND PRE-REGULATORSThe 17 volt AC line transformer providesunregulated power for both amplifieroperation and battery charging. Diodebridge D706 and filter capacitors C706 andC707 generate unregulated DC that is preregulated to 12 VDC by U706 and U707 totake the place of the batteries when theinstrument is operating on AC line power.Relay U705 switches the amplifier frombattery to pre-regulated AC whenever theAC line cord is plugged in.MICROPROCESSORThe system processor U503 is a CMOS Z80processor running at 4 MHz. The systemclock consists of Schmitt trigger U506A andan R-C network. The oscillator is designedso that latch U508A can shut down theclock oscillator completely, therebydisabling all digital circuits in the amplifier sothat no digital noise will be present. Theprocessor and clock only run when a frontDiode bridge D710 and C709 and C710provide unregulated DC to charge thebatteries. U701 and U702 operates as "AC"regulators, limiting peak battery chargingvoltage. As there are two positive batteriesand one negative battery, U701 is a LM35014

CIRCUIT DESCRIPTIONregulator that provides twice the current ofthe LM317 negative battery regulator.POWER REGULATORSThe 5 V and 10 V supplies are producedwith three-terminal regulators U801 andU802, respectivel

Noise Contours 19 Input Voltage Noise 20 Dynamic Reserve 20 Appendix A Remote Programming A-1 Introduction A-1 Commands A-1 Appendix B Noise Sources and Cures B-1 Intrinsic Noise Sources B-1 Johnson Noise B-1 '1/f' Noise B-1 Others B-1 Non-Essential Noise Sources B-1 Capacitive Coupling B-2 Inductive Coupling B-2

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