Noise measurement conversion

[2KW]

Can someone please tell me what 86nV/sqrt Hz (10Khz bandwidth) is in dB??

Thanks.

 

[jan.didden]

The noise number is an absolute value, in V. dB is a ratio, a dimension-less number.
So at the very least you need to specify: dB relative to what?
For instance, you may want to know the noise level relative to, say, a 1V signal. So you would want to know the noise in dB re: 1V.

I don't know how to convert the noise in your expression, but suppose that the 86nV was an RMS number, then you would have 20*log(86nV/1V) = - 141dB S/N

Edit: the sqr of 10kHz is 100 of course, so your RMS noise level in that 10kHz band would be 8.6uV which gives a 20*log(8.6uV/1V) = -101dB S/N re: 1V. I think.

jan didden

 

[2KW]

Thanks Jan. I'm just trying to form some sort of correlation between simulation results and values I'm familiar with. Simulation figures being the noise density in nV, and the dB figures being a normal S/N ratio number ( which I think is relative to 1V ). It's for a power amp circuit I'm working on.

Cheers,
Paul.

 

[Samuel Groner]

Can someone please tell me what 86 nV/sqrt Hz (10 kHz bandwidth) is in dB?

A figure with dimension V/rtHz (= V/sqrt Hz) is by definition for a measurement bandwidth of 1 Hz. To convert this figure to 10 kHz bandwidth, you multiply by the square root of 10 kHz, which gives 8.6 µV (this is RMS, of course). You can feed this figure to any of the online dB calculators. As Jan noted, what you actually want is dBu or dBV.

Samuel

 

[cbdb]

If thats the noise level at the output, you divide the signal voltage by it to get S/N so for a power amp your signal would be higher than a volt, more like 30v (for a 100 watt amp). This would increase your S/N by about 30 db (I think).

 

[2KW]

So for an amp that has 56.56VRMS out and 8.6uV that comes out to -136dB according to the calculator I just used. I plugged in 8.6uV with reference to 1V and got -101dB, so I guess it all adds up. Seems pretty quiet though. I guess it's only a simulation that doen't take power supply noise into account. I only really want to compare different input configurations anyway, just wanted to get an idea of where I was at to start with.

Thanks everyone for your inputs.

Paul.