Understanding A/D terminology

[DG]

Hi everyone,

In this datasheet, http://focus.ti.com/lit/ds/symlink/ads1252.pdf on
page 7 it says,

"Third, to prevent aliasing of the input signal, the bandwidth of the
analog-input signal must be band limited..."

OK, fine, I understand this.

"...the bandwidth is a function of the system clock frequency."

OK, I think this makes sense. According to Nyquist (I think) we can
only sample without aliasing at 2*B, where B is the bandwidth of the
signal we are sampling, so depending on what the system clock frequency
(which is related to the sampling clock) is, the required bandwidth of
the input signal is different.

"With a system clock frequency of 16 MHz, the data-output rate is
41.6667 kHz with a -3dB frequency of 9kHz."

This is where I started to get confused. I know that they get 41.6667
kHz from the fact that the modulator frequency is 16 MHz / 6, and it has
64 times oversampling, so 16 MHz / (6*64) = 41.6667 kHz, according to
page 8. But where did they pull this 9kHz number from?

"... The -3dB frequency scales with the system clock frequency"

OK, this makes sense but they don't say anywhere HOW it scales? Mabye
if I knew where they 9kHz came from I would understand how it scales.

Dave

 

[Spehro Pefhany]

Hi everyone,

In this datasheet, http://focus.ti.com/lit/ds/symlink/ads1252.pdf on
page 7 it says,

"Third, to prevent aliasing of the input signal, the bandwidth of the
analog-input signal must be band limited..."

OK, fine, I understand this.

"...the bandwidth is a function of the system clock frequency."

OK, I think this makes sense. According to Nyquist (I think) we can
only sample without aliasing at 2*B, where B is the bandwidth of the
signal we are sampling, so depending on what the system clock frequency
(which is related to the sampling clock) is, the required bandwidth of
the input signal is different.

"With a system clock frequency of 16 MHz, the data-output rate is
41.6667 kHz with a -3dB frequency of 9kHz."

This is where I started to get confused. I know that they get 41.6667
kHz from the fact that the modulator frequency is 16 MHz / 6, and it has
64 times oversampling, so 16 MHz / (6*64) = 41.6667 kHz, according to
page 8. But where did they pull this 9kHz number from?

"... The -3dB frequency scales with the system clock frequency"

OK, this makes sense but they don't say anywhere HOW it scales?

0.216 times the data output rate- see the digital filter description

Best regards,
Spehro Pefhany