PXI-6255 M series

[jilwan]

Hallo together,

i`m using PXI-6255 for my measurement (Diff mode sample rate 9kHz):

Why do i have an offset (PGIA) from 2mV and a noise from ~+/-1mV on a channel (AI+ AI- bypassed)? The board is 1 year old... do we need to calibrate it or is it normal?

My other question: I`m using it for the following measurements:

Voltage on actuator (with a bleeder, voltage divider from 14V to 7V) with a range from 0 till 10V

Voltage drop on  the cable (0...400mV) range from 0 till 1V.

Regarding my bleeder, i was using 2 *15kOhm serial to divide my voltage from 14V to 7V...With my software i adjust my values!

Till i read in the M series user manual page 4-7: use low impedance sources to ensure fast settling time, < 1kHz. 

I tried 2 * 500 Ohm serial, it works but i noticed that my noise is lesser (about the half). Why?

Is it a settling error or do i have another filter properties (with the 500 Ohm)?

not to forget that i`m using only 2 Channels with a rate from 9kHz (PXI-6255 with 16-Bit, 1.25 MS/s(Max), 750 kS/s(Scanning)).

Thanks

Regards

GJ

Message Edited by jilwan on 02-10-2010 04:12 PM

[Alex_Pi]

Hello GJ,

 I have just the richt piece of literature for Noise troubles. I can only recommend this, its better than  I could ever explain in short:

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

You can Calibrate your device in MAX, rightclicking on it and using the self calibration option.

also the 625x Specs (https://www.ni.com/docs/en-US/bundle/pci-pxi-usb-6255-specs/page/specs.html) state:

Input impedance
Device on
AI+ to AI GND ......................>10 GΩ in parallel
with 100 pF
AI– to AI GND.......................>10 GΩ in parallel
with 100 pF
Device off
AI+ to AI GND ......................820 Ω
AI– to AI GND.......................820 Ω
Input bias current.............................±100 pA 

which will cause a additional voltage drop of 1.5 uV, This isnt very much but a reason why in gereral smaler resistors are recommended(Althoug your Resistors arent anywhere near to this dimension) . Also the 100pF over the Resistor will have a cutoff frequency at about 100 MHz (with 15k Ohm) Lowering the Resistor will result in a higher cutoff frequency while larger paralel capacities will result in a lower Cutoff frequency.

Well your parameters arent anywhere near this, but I guess this is the reason why they tell you to use low resitors in gereral.

(Mayby I got your Question wrong. You wanted a general Explaination why the recommend lower resitors?)