Understanding ADC Operation and Noise Performance in PXIe-4309 Based on Channel Allocation

Issue Details

When reviewing the noise level specifications for the PXIe-4309, it was noted that noise levels were expected to increase with the addition of more measurement channels. However, actual measurements showed that when increasing the number of channels from 1 to 8, the standard deviation of noise either remained the same or decreased. Why does this behavior differ from the specifications?

Solution

The PXIe-4309 is equipped with 8 independent ADCs, each capable of supporting up to 4 channels in scan mode.

For measurements involving up to 8 channels, one ADC is allocated for each channel. However, for measurements involving 9 or more channels, a single ADC is used to handle two or more channels.When only one channel per ADC is used, multiplexing is not necessary. For example, when sampling is requested at 100 kS/s, the ADC operates internally at a rate of 2 MS/s, which allows for approximately 20 times oversampling, improving RMS noise performance through averaging and digital filtering.

However, when two channels share the same ADC, the ADC must multiplex between them. This means that each channel effectively receives 1 MS/s from the internal ADC, and when accounting for settling time and overhead, the effective oversampling per channel drops to less than 10 times. As a result, RMS noise increases compared to when there is one channel per ADC, even if the same sample rate is requested.