High Noise on Analog Output PCI 6289

Question

Hi guys
I am using  a PCI 6289 M series device for A/D characterization , I am using the device both for the analog output generation and also for data acquistion of the digital lines , The dynamic tests for the A/D were showing a very poor SNR ratio of the ADC, it is a  12 bit adc so technically in the ideal case i should atleast see  90 db but the resulting figure was way lower around 40 db .
I connected the A0 from the PCI 6289 brought out to a scb-68 block to a RHODE and SCHWARZ spectrum analyzer , the above spectrum analyzer is a decent one with good analysis capability and I was surprised to see a SNR of 50 db , the data sheet says that PCI 628X devices have  a internal 16 bit DAC so technically the SNR should be atleast 90 db even if 6 db were removed for the harsh environment of the computer .
please advise , have anybody else faced this problem , the data sheet does not give a figure for the SNR ratio of the card , since i need to test a 12 bit adc , thought that 6289 would be up to the task since it has a 16 bit DAC.I am using the Noise shielding cables and also SCB-68 block , the block is far removed from other power supplies .
any ideas why the noise is occuring , one of my colleagues says maybe the harsh environment inside the pc enclosure causes it , is it necessary to use a separate PCI box for the card and then connect the card to the computer , is this standard procedure ?????.
and also does the PCI -6289 have a filtering option for the analog output , all i could find was the filtering for input.

Answer 1

Hi Tom
Thanks a lot for responding to the thread , did not expect a response in the holiday season . The SNR for the 12 bit one was a typo , was referring to the 16 bit onboard dac in the M series card , which technically should ensure a clean input signal having atleast 90 db .
The info you have given helps a lot , i suspect the conditions inside the pc might be causing it , also i am driving the analog output from the card at almost its limits of 2.5 MS/s , the sample rate specification for the card on the data sheet says 2.8 MS/s , also i notice the settling time requirement for the outputs is 3 us which i suspect will put the onboard DAC under quite a load.
If the conditions inside the PC cannot be rectified , where can i find a external PCI box which can be connected to the computer or does such a box exist only for the PXI cards ? will that necessarily rectify the problem and also since filtering is not possible on the card itself , I remember reading the scb -68 connector block has  single RC output filter option , i would have to use discrete R,C components to make it but notice on the right hand corner some pcb holes are provided , would this help.
Would appreciate your Response
Thank you
Chip learner