Fixed-Frequency Analog Filter

[KingWeasel11]

Hello,

The user manual for the PXIe-4302/3 states the following:

"To minimize the error from the ADC alias holes, the NI PXIe-4302/4303 modules feature a fixed-frequency analog filter. This analog filter removes high-frequency components in the analog signal path before they reach the ADC. This filtering addresses the possibility of high-frequency aliasing from the narrow bands that are not covered by the digital filter."

What is the cutoff frequency of this analog filter? Can anyone give me other details about the filter (order, active/passive)? I imagine it's a simple 2 pole passive filter, but I would really like to know the cutoff frequency.

Thanks,

Jacob

[iradovanovic]

Jacob, 

Have you looked at the datasheet for the 4302, on page 6? Is this what you are referring to? 

Best, 

Ivana 

[KingWeasel11]

Yes, I think so. Okay, so I was under the impression that there was an RC circuit at the input with fixed component values. So my answer is actually:

Frequency (for 25 S/s ≤ f_s ≤ 25.6 kS/s)....≤0.57 × f_s

Is that correct? It's still an analog filter?

[iradovanovic · Accepted solution]

Jason, 

It is still an analog filter, but based on your original question, I think the cutoff frequency you were referring to was the Alias-free bandwidth which is listed in the data sheet as: 

Alias-free bandwidth
(for 1 S/s ≤ fs < 25 S/s).....................................0.1 × fs
Alias-free bandwidth
(for 25 S/s ≤ fs ≤ 25.6 kS/s)..............................0.45 × fs

[janaf]

Hopping in here as I have one of these boards.

They have analogue filters, plus various optional digital filtering and the spec datasheet is not very clear on the analogue filter, at least for non-specialist me.

If anyone could clarify the analogue filter part, I'd appreciate it.

[KingWeasel11]

Yeah, it would be great to know the other characteristics of the filter, or at least the part number for the programmable filter IC to look up.

[janaf]

I was guessing it's a fixed frequency only, as delta-sigma A/D with digital filtering should be efficient far above the sampling frequency. There's a mention in the specifications of a (digital filter) window at something like 3MHz, with 100dB attenuation (from the analoge plus digital filter?) if I understand it right.