Example of Noise Calculation for Amplifier OPA855

                 噪声计算

 一．Amplifier noise calculation       

 OPA855noise spectral density

 The amplified noise calculation is mainly based on the noise spectral density,转换成RMS噪声,The calculation of noise gain is calculated with the same direction amplification,Our noise noise gain here is 7,噪声带宽：

BWn =1.57*（ 8G / 7）= 1.57 * 1143M = 1795M

 1.Voltage noise calculation

1>Broadband NoiseCalculation of broadband noise

Enbb = Ebb *√BWn = (0.98nV /√Hz) * √1795 MHz = 41.52uV

It is calculated aboveOPA855的电压RMS噪声.

2>1 / f Voltage noise calculation

Enormal = Enf √f0 = (2.8 nV /√Hz) * ( √1kHz ) = 88.5 nV

Eflicker = Enormal * [√ln( fh / fl) ] = 88.5 *  [√ln( 1795 MHz / 1Khz) ] = 1.27 uV

3>Total voltage noise calculation

En_v =√（Enbb^2 + Eflicker^2）

          =√(41.52uV^2 + 1.27uV^2) = 41.53 uV

2.Calculation of current noise

等效电阻Req= 453R // 75R = 64R

1>Broadband NoiseCalculation of broadband noise

En_r= (2.5pA √Hz ) * 64R = 0.16 nV √Hz

En_i = En_r * √BWn = (0.16nV /√Hz) * √1795 MHz =6.78 uV

3.电阻的热噪声

En_rt = √4KTRf = √[4*(1.38*10^-23)*298k*64R*1795 MHz]

          = 0.44uV

4.total input and outputRMS噪声为：

En_in=√（41.53uV^2 + 6.78uV^2 + 0.44uV^2 ）= 42.08uV

En_out= 42.08 * 7 = 294.6uV

It can be seen that the total input noise is still dominated by voltage noise,Current noise and resistor thermal noise account for a small fraction

The total noise peak-to-peak is ：294.6uV * 6 = 1.77mV

 A quick way to simplify noise calculations：

1.Look at the input type of the amplifier.

对于CMOS和JFETfor the amplifier,Current noise is not noticeable,一般都是fA√Hz级别的,BipolarThe op amp is inpA√Hz,On the contrary, in terms of voltage noiseBipolar是比CMOS和JFET小的.

2.The effect of equivalent resistance.

The general equivalent resistance is less than 1k时,The current noise of the amplifier is very small,regardless of the amplifier input typeBipolar或者CMOS,In many cases, it can be approximately ignored.

3.When can be ignored1/f噪声.

when the system bandwidth ratio1/fThe corner frequency of noise is large10倍以上时,可以忽略1/f噪声.The noise knee frequency for most precision amplifiers1Hz到1kHz,So when your system bandwidth is greater than 10k的时候,1/fNoise is negligible.

4.When there are multiple amplifier circuits.

Generally in multi-stage amplification system,Usually the highest gain is used in the first stage,这样的话,The error sources of the first stage will account for the major part,Errors caused by later stages are negligible.