I am using NI PCI-6281, for precise voltage acquisition.
When I connected the 6281 Analog-Input to an external voltage reference the acquired signal noise span is 3 mV. As I was surprised, to cross verify, I connected this voltage reference to an Agilent 6.5 digit DMM. The noise span observed on the DMM was < 10 μV.
Then again, I looped-back a voltage output of 50 mV from the 6281 Analog-Output to the Analog-Input, this time I observed a noise span of 100 μV (at +/- 100 mV range, 1 kHz DAQ rate, low pass filter enabled).
Why am I experiencing such large spans of noise when connecting to an external voltage reference and comparatively very little with a loopback? Any help will be appreciated. Thanks.