NI-9207/9208 Analog Input Module Bandwidth Specification and External Signal Conditioning Requirements

[Umbagogger]

My LabVIEW application is the control of a large hi-power chemistry experiment with LabVIEW 2011 (x86) and a 9178 USB CompactDAQ chassis. I am trying to decide if I should migrate from using 9205/9203 (Voltage and Current, respectively) 250KSPS modules to 9207/9208 500SPS modules for measuring low-bandwidth "industrial" process signals in a noisy (motors, motor drives, large switch-mode power supplies, etc.) machine-control environment.  Most of my process signals can be treated as 1Hz (10Hz at most) bandwidth signals. We have about 16 signals to monitor. The 9205 is overkill as my required sample rate (per chan) should be more like 10-100Hz (and the voltages can all be 0-10V range).

My current conditioning/sampling solution is:

1. Externally filter each input with a 2.5Hz 1P RC filter at the 9205 terminal block.

2. Sample each channel at 1KHz.

3. Average each block of 100 samples.

This provides 10 data pts per second (per chan). This solution is not perfect, but it has been good enough.

I have experimented with substituting digital filters and higher sampling rates in place of the block averaging in hopes of eliminating the mess associated with external filters, but eventually came to the following conclusion:

External conditioning (filters) are _always_ required becasue the bandwith of each input on the 9205 is 370kHz (per the spec), so even for a single channel sampled at the max sample rate of 250kHz, there is NO digital filter than can correct for the failure to sample at a rate that is at least 2X the max frequency content of 370kHz.

So, I am stuck with RC filters. Incidentally, why does no one make a external filter board (conditioning board) that is designed to be plug compatible with the BD37 expansion cable on a 9205 moduel (or similar)? If I am correct, one should ALWAYS require an external conditioning when measuring ANYTHING with a 9205.  I can find one for most other mfg's data acq HW.

I am hoping that the 9207/9208 modules, where they have max sample rates of 500 SPS in high-speed mode and 20 SPS ( and 60Hz filter) in high-res mode, will allow me to directly measure my signals without external conditioning. If not, I am strongly considering replacing the control system with a PLC that has inputs appropriately configured for direct measuremrnt of low-bandwith  signals.

My Question:

What is the bandwith of each input on a 9207 or 9208?

The data sheet does not specify bandwidth. It only specifies conversion time (high speed: 2ms=500SPS, hi-res: 50ms=20SPS) and the 60Hz/-68dB attenuation in hi-res mode.

My guess is that I will still have the same problem.  If I sample 16 channels at the max (all channels) sample rate, then:

In high-speed mode, each channel will be sampled at 31Hz (500/16), and each input will require an external filter <=15Hz.

In high-res mode, each channel will be sampled at 1.5Hz (20/16), and each input will require an external filter <=0.75Hz.

If right, I might as well leave my 2.5Hz external filters where they are and run the 9207/8s in high-speed mode, since I cant get rid of them. SO, why bother changing at all.

UNLESS... the 9207/8 already has each channel (or the ADC converter) filtered as specified above.

Any guidance or explanation would be greatly appreciated, even if it is simply to point out that I completely misunderstand how these things work 😉

Thanks,

Peter

[Wayne_T]

Umbagogger,

The sample rate is not directly correlated to the bandwidth of the signal, but rather the sample rate is identified as suitable for a representation of the signal.  The NI 9207 and 9208 are intended for slow measurements, not waveform acquisition.  Maybe it depends on how you're defining direct measurement of low bandwidth signals.  The channel is filtered through one of the three settings, and then enters the ADC, and you would see the rates you calculated.