Basic question of noise

[glias]

Hello all,

I'm using a AD7781 and I don't understand the noise specification in the datasheet :
In the configuration with the lower gain, the noise is higher, why ?

Does it represent the quantization noise of the ADC of the AD7781 divided by the gain ?

here is the extract of the datasheet :

 

[barry]

I suspect that's INPUT referred noise. If you take 44nV and multiply it by 128 you get 5.6 uV, which is in the same range as the gain=1 noise value.

Barry

 

[albbg]

The circuit is composed by a variable gain low noise amplifier followed by a sigma delta modulator. You can imagine this circuit as (simplified):

noise generator (noise N1) + noisless amplifier (gain G) + noisy modulator (noise N2, gain 1)

at the otuput the total noise will be then: totale output Noise = N1*G + N2

referring it to the input we will have:

total input noise = (N1*G + N2)/G = N1 + N2/G

from the data-sheet:

G = 1 ==> N1 + N2 = 2400 nV
G = 128 ==> N1 + N2/128 = 44 nV

Solving the system:

N1 = 25 nV
N2 = 2374 nV

 

[glias]

Hello albbg, Barry

Thanks a lot for your reply, I understand better now, thanks for your demonstration.