Calculating Input Noise Voltage

[MRW]

I was just checking out various opamp datasheets. I encountered this
opamp:
http://focus.ti.com/lit/ds/symlink/tlv2784.pdf

In the few lines of text, it says that the input noise voltage is 9 nV/
sqrt(Hz) at 10kHz. But in the table at the bottom of the page, it says
that the input noise voltage is 18 nV/sqrt(Hz) at 1kHz.

How did they calculate this?

I'm curious because if I were to use an opamp (not necessarily this
one) then I would like to know how much input noise voltage it
contributes for a certain bandwidth, for instance audio band from 20Hz
to 20kHz. There was another discussion that I saw (don't remember the
link) that mentioned that at one particular design, the amplifier was
designed with a 40dB gain and the input noise got amplified to about
100mV.

 

[Phil Allison]

"MRW" <[email protected]>

** Groper fool alert.

I was just checking out various opamp datasheets. I encountered this
opamp:
http://focus.ti.com/lit/ds/symlink/tlv2784.pdf

In the few lines of text, it says that the input noise voltage is 9 nV/
sqrt(Hz) at 10kHz. But in the table at the bottom of the page, it says
that the input noise voltage is 18 nV/sqrt(Hz) at 1kHz.

How did they calculate this?

** See figure 19 ??

See how the EIN goes up dramatically at frequencies below 3 kHz ?

Go check some other op-amps - discover they are not all like that.

I'm curious because if I were to use an opamp (not necessarily this
one) then I would like to know how much input noise voltage it
contributes for a certain bandwidth, for instance audio band from 20Hz
to 20kHz.

** Normally just multiply the typical or 1 kHz EIN figure by 141 (= sq
rt 20,000)



........ Phil

 

[MRW]

"MRW" <[email protected]>

** Groper fool alert.




** See figure 19 ??

See how the EIN goes up dramatically at frequencies below 3 kHz ?

Go check some other op-amps - discover they are not all like that.


** Normally just multiply the typical or 1 kHz EIN figure by 141 (= sq
rt 20,000)

....... Phil

Thanks, Phil! I'll take a look at other datasheets. Got any opamps you
can recommend for audio?

 

[Phil Allison]

"MRW"

Thanks, Phil! I'll take a look at other datasheets. Got any opamps you
can recommend for audio?

** See:

http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm




....... Phil