f= 1/ (2*π*R*C)
Radio frequenc filter: (AM Fc~=531kHz)
Fc= 234.05kHz= 1/ (2*π*1kΩ*680pF)
Fc= 338.63kHz= 1/ (2*π*1kΩ*470pF)
Fc= 482.29kHz= 1/ (2*π*1kΩ*330pF)
Fc= 530.52kHz= 1/ (2*π*1kΩ*300pF)
Fc= 589.46kHz= 1/ (2*π*1kΩ*270pF)
Fc= 482.29kHz= 1/ (2*π*100Ω*3.3nF)
Fc= 589.46kHz= 1/ (2*π*2.7Ω*0.1uF)
* Johnson Noise of Resistors:
ALL resistors have a voltage noise of VNR =√( 4kTBR)
k = Boltzmann’s Constant (1.38 x 10–23J/K)
T = Absolute Temperature [°K]= T(°C) + 273.15
B = Bandwidth (Hz)
R : Resistance [Ω]
A 1000Ω resistor generatesVrms= 4nV / √Hz @ 25°C > 0,6μV for 20kHz band
R=200kΩ -> Vrms = 58nV/√Hz -> 8μV for 20kHz band
* Op Amp Input Noise
1. Voltage Noise
2. Current Noise
3. Johnson Noise Of R
Example:OP27
Voltage Noise = 3nV / √ Hz
Current Noise = 1pA / √ Hz
T = 25°C
A 3k input R: en+in+Jn = 3nV + 1pA *3kΩ + 7nV = 3+3+7(nV / √ Hz)
A 300k input R: en+in+Jn = 3nV + 1pA *300kΩ + 70nV = 3+300+70(nV / √ Hz)
* Broadcast radio frequencies
Broadcast radio frequencies in the Taiwan are:
• FM: 88.1 to 107.8 MHz
• AM: 531 to 15365 kHz
reference:https://www.ncc.gov.tw/chinese/gradation.aspx?site_content_sn=4172
* Input bias current cancellation resistors:
Rb= Ri//Rf
* Opamp Compensation For Input Capacitance
This unwanted effect can be cancelled using the principle of "matched voltage division". It can be shown that a voltage divider consisting of two R-C parallel combination has no frequency dependence when R1C1=R2C2 (equal time constants) or R1/R2=C2/C1 . (In your case: RGCin=RFCF).
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