convert noise figure V/sqrt Hz into dB

[jlim]

Dear all,

Could anyone teach me how to convert noise value in V/sqrt Hz into dB?
For example, if i have input noise= 9.758mV/sqrt Hz and output noise= 156.2pV/sqrt Hz, the bandwidth (cutoff freq)= 1.4354MHz, Then how i can convert the noise value into noise figure in dB?

 

[erikl]

Your above given figures are point values valid for the noise voltage per √Hz at a certain frequency (or in your frequency band, if the noise were real white noise). For the calculation of the noise factor F or the noise figure NF, however, you need the signal-to-noise-ratios (SNR) at the input and output, see e.g. the Wiki explanation on Noise Figure.

If your above given figures were SNR values (which they are not, because SNR values don't have a unit), your system would have a noise suppression of -78dB .

 

[jlim]

Thanks to reply~~~i' m not really don't understand what you said~~is it V/sqrt Hz cant be convert into noise figure in dB..
Because actually i m doing filter and carried out noise simulation using cadence..then i get input noise= 9.758mV/sqrt Hz and output noise= 156.2pV/sqrt Hz..But my specification is about 40dB...Then i don't know how to convert it into noise figure in dB. So that i compare with my target specification.

 

[erikl]

You need to measure or calculate the input & output SNRs of your system, then calculate the NF from them.

 

[jlim]

but i don't know how to get the input and output SNR from the cadence simulation..so V/sqrt Hz can't to be convert into dB by using calculation??

 

[godfreyl]

so V/sqrt Hz can't to be convert into dB by using calculation??

No, you can't convert a single value to dB. dB is a measure of the ratio between two signals.

Examples:
Gain = 20dB means the output voltage is 10 times greater than the input voltage.
SNR = 60dB means the signal voltage is 1000 times greater than the noise voltage.

 

[jlim]

Thanks to reply..Then input noise= 9.758mV/sqrt Hz and output noise= 156.2pV/sqrt Hz, the bandwidth (cutoff freq)= 1.4354MHz, can make those noise values into dB?

 

[sarge]

but i don't know how to get the input and output SNR from the cadence simulation..so V/sqrt Hz can't to be convert into dB by using calculation??

Why don't get NF into db direct from cadence simulation?
What type analysis did you use?

 

[jlim]

i use the noise analysis~~i know that at Direct Plot Form there can choose Noise Figure Function, but my input source is not a port, so can't be executed...Then i just execute output noise function and input noise function..and get input noise= 9.758mV/sqrt Hz and output noise= 156.2pV/sqrt Hz

 

[godfreyl]

input noise= 9.758mV/sqrt Hz and output noise= 156.2pV/sqrt Hz

There's something wrong with those numbers. Output noise is 62500000 times smaller than input noise, i.e. -156dB. Also, 9.758mV/sqrt Hz seems almost impossibly high for any kind of circuit.

 

[jlim]

sorry~~can i know how to get the correct noise value for my designed filter?? my designed filter get the cut-off frequency at 1.4354 MHz..
is it the noise value need to get at cut-off frequency??
Here i attached the input noise and output noise result~~

 

[godfreyl]

cut-off frequency at 1.4354 MHz

I assume this is a low-pass filter. If so, why are you checking the noise at 1GHz? You should be interested in the noise in the pass band. e.g. The output noise is 60nV/sqrt(Hz) over most of the pass band. For the input noise, you need to change the scale. On the graph you showed, it's too close to zero to tell the difference.

It would help if you showed your circuit. So far we know nothing about it, not even whether it's active or passive, or how you measured the noise.

 

[jlim]

I assume this is a low-pass filter. If so, why are you checking the noise at 1GHz? You should be interested in the noise in the pass band. e.g. The output noise is 60nV/sqrt(Hz) over most of the pass band. For the input noise, you need to change the scale. On the graph you showed, it's too close to zero to tell the difference.

It would help if you showed your circuit. So far we know nothing about it, not even whether it's active or passive, or how you measured the noise.

Here is the schematic of OTA-C 3rd-order filter~~