Tepelny sum odporu?

[balu@home]

ono sa to da aspon nahrubo odhadnut este pred tym nez zacnete nieco 
lepit. Neviem ci ste sum uz pocitali, ked nie skusim to urobit pre vas, 
teoria je dole. Predpokladam zapojenie s tromi stupnami v serii ako ste 
posielali pred casom.

AD8021 ma uvedeny sum 2.5nV/sqrt(Hz). Pri zosilneni 10 ma sirku pasma 
ca. 100MHz. Sum 50 Ohmoveho rezistora je 0.92 nV/sqrt(Hz).

To nam vychadza ze len zosilnovac samotny prispeje 2.7 krat viac ako 
vstupny rezistor.

Konkretne pre prvy stupen 2.5nV/sqrt(Hz) * sqrt(100e6) * 10 = 250 uVrms

Dalsi stupen prispeje tiez 250uVrms, ale na vstupe uz ma 250uVrms, ktore 
zosilni 10x, t.j. dostanete 2.5mVrms plus zanedbatelny prispevok sumu 
druheho stupna.

Treti stupen ma na vstupe 2.5mVrms, ktory zosilni 10x na 25mVrms a k 
tomu prispeje svojimi 250nVrms (sum nie je korelovany, takze sa 
nepripocitava linearne ale to nie je teraz dolezite).

Otazka je ci toto pozorujete na osciloskope. Myslim ze ste spominali ze 
hej. Prevod medzi rms a peak to peak je niekde medzi nasobenim 4 az 
nasobenim 7, podla toho kolko sigma chcete zaratat.

25mV je stale maly signal pre operak takze filtrovat podla mna staci az 
na uplnom konci kde sumove parametre zosilnovaca neprispievaju do 
celkoveho noise performance tak velmi ako prvy stupen. Ak zuzite sirku 
pasma zo 100MHz na 1MHz tak by mal sum klesnut o odmocninu zo 100, t.j. 
zhruba 10x

Dajte vediet ako ste dopadli a ci vypocet sedi s pozorovanim, celkom ma 
to zaujima.


http://www.ti.com/lit/ml/sloa082/sloa082.pdf

10.2.5 Noise Units
Noise is normally specified as a spectral density in rms volts or amps 
per root Hertz, V/sqrt(Hz) or A/sqrt(Hz). These are not very 
“user-friendly” units. A frequency range is needed to relate these units 
to actual noise levels that will be observed.
For example:
D A TLE2027 op amp with a noise specification of 2.5 nV/sqrt(Hz)Hz is 
used over an audio frequency range of 20 Hz to 20 kHz, with a gain of 40 
dB. The output voltage is 0 dBV (1 V).
D To begin with, calculate the root Hz part: 􏰀20000 * 20 + 141.35.
D Multiplying this by the noise spec: 2.5 141.35 + 353.38 nV, which is 
the equivalent input noise (EIN). The output noise equals the input 
noise multiplied by the gain, which is 100 (40 dB).
The signal-to-noise ratio can be now be calculated: D 353.38 nV 100 
+ 35.3 mV
D Signal-to-noise (dB) =
20 log(1V B 35.3 mV) + 20 log(28329) + 89 dB
(10–5)
The TLE2027 op amp is an excellent choice for this application. 
Remember, though, that passive components and external noise sources can 
degrade performance. There is also a slight increase in noise at low 
frequencies, due to the 1/f effect (see below).


On May/17/2012 10:28, hutta.j na seznam.cz wrote:
> Vlastne ano, dokonce jsem cekal i odpoved jakou jsem dostal od pana
> Andela. Jen mi to castece prislo divne.
>
> Na problematiku sirky pasma zesilovace me jiz upozrnil pan Kucera.
> Jako reseni jsme jednotlive stupne v zesilovaci predelal vlastne na
> aktivni filtry typu dolni propust s zesilenim 10x, Filtr dolni proust
> 3radu pro 1MHz mi vychazi takto, respektive kazdy stupen by pak mel
> vypadat takto.
> http://files.uloziste.com/2c3df4460e30ff2d/OZLPF.jpg
> Charakteristika filtru by mela byt nasledujici 1.0MHz +19,6dB
> 3.3MHz + 0,0dB
> 7.4MHz - 20.0dB
>
> Je to tak spravne, respektive resi se dany problem takto?
>
> Dekuji
>
> Hutta
>
>> ------------ Původní zpráva ------------
>> Od: balu na home <daniel.valuch na orange.fr>
>> Předmět: Re: Tepelny sum odporu?
>> Datum: 17.5.2012 08:51:00
>> ----------------------------------------
>> pokial sa pytate v suvislosti s predchadzajucim vlaknom o
>> zosilnovacoch tak v pripade ze tam nemate ziadne obmedzenie
>> frekvencnej charakteristiky treba pocitat sum az do sirky pasma
>> zosilnovaca. Co je vo vasom pripade ovela viacej ako 100kHz.
>> b.
>>
>>
>> On May/17/2012 0:48, hutta.j na seznam.cz wrote:
>> > Tepelny sum odporu = Sqrt(4 * kB * T * f * R) pro R 1k, T25C a f
>> 0-100kHz
>> vychazi na 1,28uV RMS, ale jaky bude tepelny sum dvou paralelne
>> razenych 1k
>> odporu?
>> >
>> > Dekuju
>> > Hutta
>> _______________________________________________
>> HW-list mailing list - sponsored by www.HW.cz
>> Hw-list na list.hw.cz
>> http://list.hw.cz/mailman/listinfo/hw-list
>>
>>
>>
> _______________________________________________
> HW-list mailing list - sponsored by www.HW.cz
> Hw-list na list.hw.cz
> http://list.hw.cz/mailman/listinfo/hw-list
>