Nyob rau hauv qhov tseeb, luam tawm hauv Circuit Court board (PCB) yog ua los ntawm cov khoom siv hluav taws xob tawm, piv txwv li lawv qhov kev tiv thaiv yuav tsum tsis tu ncua. Yog li vim li cas PCB thiaj qhia qhov tsis ncaj ncees rau hauv lub teeb liab? Cov lus teb yog tias PCB txheej txheem yog “tsis yog-linear” txheeb ze rau qhov twg tam sim no ntws.

Txawm hais tias lub suab nrov tau txais tam sim no los ntawm ib qhov chaw lossis lwm qhov yog nyob ntawm qhov tam sim ntawd polarity ntawm lub teeb liab ntawm lub nra. Tam sim no ntws los ntawm cov khoom siv hluav taws xob, dhau los ntawm kev hla lub tshuab hluav taws xob, dhau los ntawm lub tshuab hluav taws xob rau hauv lub nra. The current then travels from the load ground terminal (or shielding of the PCB output connector) back to the ground plane, through the bypass capacitor, and back to the source that originally supplied the current.

The concept of minimum path of current through impedance is incorrect. The amount of current in all different impedance paths is proportional to its conductivity. Hauv lub dav hlau hauv av, feem ntau muaj ntau dua ib txoj hauv kev uas tsis muaj kev cuam tshuam los ntawm qhov uas feem coob ntawm cov av ntws tam sim no: ib txoj hauv kev yog txuas ncaj qha mus rau bypass capacitor; Lwm qhov ua rau lub zog tawm tswv yim kom txog thaum hla dhau lub capacitor. Daim duab 1 qhia txog ob txoj hauv kev no. The backflow current is what’s really causing the problem.

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Thaum hla cov capacitors tau muab tso rau ntawm txoj haujlwm sib txawv ntawm PCB, cov av hauv qab no ntws los ntawm txoj hauv kev sib txawv mus rau qhov tshwj xeeb hla dhau cov khoom siv, uas yog lub ntsiab lus ntawm “tsis sib xws tsis sib xws”. If a significant portion of a polar component of the ground current flows through the ground of the input circuit, only that polar component of the signal is disturbed. Yog tias lwm qhov kev sib txawv ntawm cov av tam sim no tsis cuam tshuam, lub teeb liab nkag mus hloov pauv yam tsis yog kab. Thaum ib qho kev hloov pauv tau hloov pauv tab sis lwm qhov tsis sib xws tsis yog, qhov tsis sib xws tshwm sim thiab pom tau tias yog qhov kev cuam tshuam thib ob sib txawv ntawm cov teeb liab tso tawm. Daim duab 2 qhia qhov cuam tshuam qhov txawv txav hauv daim ntawv exaggerated.

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Thaum tsuas yog ib qho kev tiv thaiv sab qaum teb ntawm lub sine yoj tau cuam tshuam, qhov ua rau lub ntsej muag tsis yog sine yoj lawm. Simulating lub tshuab ua kom zoo tshaj plaws nrog 100-ω thauj khoom thiab txuas cov khoom thauj tam sim no los ntawm 1-ω resistor rau hauv av qhov hluav taws xob ntawm tsuas yog ib qho polarity ntawm lub teeb liab, ua rau hauv daim duab 3. Fourier transform shows that the distortion waveform is almost all the second harmonics at -68 DBC. Ntawm qhov ntau zaus, qib kev sib txuas no tau yooj yim tsim ntawm PCB, uas tuaj yeem rhuav tshem cov yam ntxwv tiv thaiv kev cuam tshuam zoo ntawm lub tshuab hluav taws xob yam tsis tau siv ntau qhov tshwj xeeb tsis cuam tshuam los ntawm PCB. When the output of a single operational amplifier is distorted due to the ground current path, the ground current flow can be adjusted by rearranging the bypass loop and maintaining distance from the input device, as shown in Figure 4.

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Multiamplifier chip

Qhov teeb meem ntawm ntau lub tshuab hluav taws xob (ob, peb, lossis plaub lub tshuab hluav taws xob) tau sib xyaw los ntawm qhov tsis muaj peev xwm ua kom muaj kev sib txuas hauv av ntawm kev hla lub tshuab hluav taws xob nyob deb ntawm tag nrho cov tswv yim. Qhov no tshwj xeeb tshaj yog rau plaub lub suab nrov. Quad-amplifier chips muaj cov tswv yim txuas rau ntawm txhua sab, yog li tsis muaj chav rau hla hla cov kab hluav taws xob uas txo qis kev cuam tshuam rau cov tswv yim nkag.

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Daim duab 5 qhia txog txoj hauv kev yooj yim rau plaub-amplifier teeb tsa. Cov cuab yeej siv feem ntau txuas ncaj qha rau tus lej plaub tus lej. Cov av tam sim no ntawm ib lub zog siv tuaj yeem cuam tshuam cov khoom siv hluav taws xob hauv av thiab cov av tam sim no ntawm lwm cov khoom siv hluav taws xob, ua rau muaj qhov txawv txav. Piv txwv li, (+Vs) bypass capacitor ntawm channel 1 ntawm plaub lub tshuab hluav taws xob tuaj yeem tso ncaj qha ib sab rau nws cov tswv yim; Lub (-Vs) bypass capacitor tuaj yeem tso rau lwm sab ntawm pob. Qhov (+Vs) av tam sim no tuaj yeem cuam tshuam channel 1, thaum (-vs) av tam sim no yuav tsis.

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Txhawm rau zam qhov teeb meem no, cia rau hauv av tam sim no cuam tshuam cov tswv yim, tab sis cia PCB tam sim no ntws hauv qhov dav dav. Txhawm rau ua tiav qhov no, hla dhau lub peev xwm tuaj yeem teeb tsa ntawm PCB hauv txoj hauv kev uas (+Vs) thiab ( – Vs) cov av hauv av ntws los ntawm tib txoj kev. Yog tias cov teeb liab nkag tau zoo ib yam cuam tshuam los ntawm qhov zoo thiab tsis zoo, yuav tsis muaj qhov txawv txav. Yog li ntawd, teeb tsa ob txoj kab hluav taws xob hla dhau ib sab kom lawv sib koom ua ke. Vim tias ob lub ncov qaum teb ntawm lub ntiaj teb tam sim no los ntawm tib lub ntsiab lus (qhov txuas txuas tiv thaiv lossis lub hauv av thauj khoom) thiab ob qho tib si rov qab mus rau tib lub ntsiab lus (kev sib txuas hauv av ib txwm ntawm kev hla lub tshuab hluav taws xob), qhov zoo/tsis zoo tam sim no ntws los ntawm tib txoj kev Yog tias cov tswv yim tsis kam ntawm cov channel cuam tshuam los ntawm (+Vs) tam sim no, ( – Vs) tam sim no muaj qhov cuam tshuam zoo ib yam rau nws. Because the resulting disturbance is the same regardless of the polarity, there is no distortion, but a small change in the gain of the channel will occur, as shown in Figure 6.

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Txhawm rau txheeb xyuas qhov kev txiav txim siab saum toj no, ob txheej txheej PCB sib txawv tau siv: txheej txheem yooj yim (Daim duab 5) thiab kev teeb tsa tsis sib xws (Daim duab 6). Qhov sib txawv uas tsim los ntawm FHP3450 plaub-lub suab ua haujlwm siv fairchild semiconductor tau qhia hauv cov lus 1. Qhov raug bandwidth ntawm FHP3450 yog 210MHz, txoj kab nqes yog 1100V/peb, qhov tsis ncaj ncees tam sim no yog 100nA, thiab kev ua haujlwm tam sim no ib channel yog 3.6 mA. As can be seen from Table 1, the more distorted the channel, the better the improvement, so that the four channels are nearly equal in performance.

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Without an ideal quad amplifier on a PCB, measuring the effects of a single amplifier channel can be difficult. Pom tseeb, qhov muab lub tshuab ua kom nrov nrov cuam tshuam tsis tsuas yog nws tus kheej cov tswv yim, tab sis cov tswv yim ntawm lwm cov channel ib yam nkaus. The earth current flows through all the different channel inputs and produces different effects, but is influenced by each output, which is measurable.

Table 2 qhia txog kev sib haum xeeb ntsuas ntawm lwm txoj hauv kev uas tsis muaj kev cuam tshuam thaum tsuas yog ib txoj kev tsav. Cov channel tsis tau tso tawm qhia lub teeb liab me me (crosstalk) ntawm qhov pib yooj yim, tab sis kuj tseem ua rau muaj qhov txawv txav ncaj qha los ntawm cov av tam sim no thaum tsis muaj ib qho tseem ceeb tseem ceeb teeb liab. Qhov teeb tsa qis-sib txawv hauv daim duab 6 qhia tau tias qhov kev sib haum xeeb thib ob thiab tag nrho kev sib haum xeeb sib txawv (THD) yam ntxwv tau zoo heev vim tias ze ze ntawm kev tshem tawm hauv av tam sim no.

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Kab lus no xaus

Yooj yim muab tso rau ntawm PCB, qhov rov qab tam sim no ntws los ntawm kev hla dhau qhov sib txawv (rau cov khoom siv hluav taws xob sib txawv) thiab lub zog siv hluav taws xob nws tus kheej, uas yog sib piv rau nws qhov kev coj ua. Lub siab-teeb liab tam sim no ntws rov qab mus rau lub me bypass capacitor. Tsawg-zaus tam sim no, xws li cov cim ntawm lub suab, tuaj yeem ntws feem ntau los ntawm kev hla dhau lub peev xwm loj. Txawm tias qhov qis dua tam sim no tuaj yeem “tsis quav ntsej” tag nrho cov kev hla dhau lub peev xwm thiab ntws ncaj qha rov qab mus rau lub hwj chim coj. Daim ntawv thov tshwj xeeb yuav txiav txim siab tias txoj hauv kev twg yog qhov tseem ceeb tshaj plaws. Fortunately, it is easy to protect the entire ground current path by using a common ground point and a ground bypass capacitor on the output side.

Txoj cai golden rau HF PCB txheej txheem yog ua kom HF bypass capacitor nyob ze rau qhov ntim lub zog pin li sai tau, tab sis kev sib piv ntawm Daim Duab 5 thiab Daim Duab 6 qhia tias hloov kho txoj cai no txhawm rau txhim kho cov yam ntxwv sib txawv tsis ua rau muaj qhov sib txawv ntau. Kev txhim kho yam ntxwv tsis zoo tuaj ntawm tus nqi ntawm kev ntxiv txog 0.15 ntiv tes ntawm cov khoom siv hluav taws xob hla dhau qhov hluav taws xob, tab sis qhov no muaj kev cuam tshuam me ntsis rau AC qhov ua tau zoo ntawm FHP3450. PCB txheej txheem yog ib qho tseem ceeb kom ua tiav qhov ua tau zoo ntawm lub tshuab ua kom muaj suab nrov, thiab cov teeb meem tau tham ntawm no tsis txwv rau hf amplifiers. Cov cim qis dua xws li lub suab muaj qhov yuam kev ntau dua qhov xav tau. Cov av tam sim no cuam tshuam me dua ntawm qhov tsawg zaus, tab sis nws tseem tuaj yeem yog qhov teeb meem tseem ceeb yog tias qhov xav tau qhov ntsuas tsis raug yog qhov zoo dua qub.