sensitivity calculation?

[david753]

I have a AD convert AD9057BSR-80.
http://www.analog.com/UploadedFiles/Dat ... 9057_d.pdf
I design a decoder circuit, that 40MHZ modulation signal input(BPSK) -> AD9057 -> FPGA.<p>Does anybody know what the value of minimum sensitivity is?
And how do you calculate it?

[Engineer1138]

Do you mean the minimum voltage change the ADC will resolve? If so it's Vref/(2^bit_resolution-1)
In the case of this device it's 2.5/255 = 10mV

[Dean Huster]

That's the basic resolution, but for all practical purposes, you have to add in a ±1 digit bobble just because the output of an ADC is digital. That's why the display of a digital scope looks so crappy compared to an analog scope.<p>Dean

[david753]

In my practical circuit, the minimum sensitivity is -22dbm.
It means the noise floor is -22dbm.
Is it reasonable?
How to calculate the reasonable noise floor?
Which item of specification is refering in terms of noise issue?

[david753]

I calculate the power of resolution.
dynamic range = 1 Vpp
8 Bits resolutions = 255 status
resolution of Vpp = 1/255 = 4mVpp
4Vpp = -43.9dbm
BUT, In the practical circuit experiment.
The resolutoin is -22 dbm.
So, there are about 20dbm that have gone.
Does it noise issue?
Is it reasonable?
What can I do??
Appreciated for your reply.

[Engineer1138]

If I understand you, your A/D can resolve signal levels that are below the existing noise floor. This means that the useful resolution of the ADC is now the level of the noise floor. There are ways to measure signals in the presence of noise, but they're specific to the signal type and the amount of noise rejection required. <p>I'd suggest looking at the Analog Devices website and reading through some of the back copies of Analog Dialogue (I'm pretty sure it's available online). They have lots of articles on just this kind of thing.