NI-9260 Getting Started

Connector Types

In this document, the NI-9260 with BNC and the NI-9260 with mini XLR are referred to inclusively as the NI-9260. The information in this document applies to all versions of the NI-9260 unless otherwise specified.

NI-9260 Block Diagram

The Delta-Sigma DAC has a resolution of 24 bits and can update at a maximum rate of 51.2 kS/s. The output stage of the NI-9260 with BNC is pseudo-differential. AO- is terminated using a 50 Ω resistor to GND.

The output stage of the NI-9260 with mini XLR is balanced and fully differential. The AO- pin represents one output of the differential output driver and is capable of driving signals into the minimum load. Connecting this pin to GND will short circuit the output.

The outputs of the DAC are buffered, conditioned, and filtered before reaching the module connectors. The NI-9260 may drive a maximum signal of 3 Vrms on AO+ with respect to AO-. Each channel is DC-coupled and protected against short circuits and ±30 V overvoltages.

Data Rates

The frequency of a master timebase (f_M) controls the data rate (f_s) of the NI-9260. The NI-9260 includes an internal master timebase with a frequency of 13.1072 MHz , but the module also can accept an external master timebase or export its own master timebase. To synchronize the data rate of an NI-9260 with other modules that use master timebases to control sampling, all of the modules must share a single master timebase source.

The following equation provides the available data rates of the NI-9260:

f_{s} = \frac{f_{M} \div 256}{n}

where n is any integer from 1 to 31.

However, the data rate must remain within the appropriate data rate range. When using the internal master timebase of 13.1072 MHz , the result is data rates of 51.2 kS/s, 25.6 kS/s, 17.067 kS/s, and so on down to 1.652 kS/s depending on the value of n. When using an external timebase with a frequency other than 13.1072 MHz , the NI-9260 has a different set of data rates.

Note The NI-9151 R Series Expansion chassis does not support sharing timebases between modules.

Filtering

The NI-9260 uses a combination of analog and digital filtering to provide an accurate representation of in-band signals while rejecting out-of-band signals. The filters discriminate between signals based on the frequency range, or bandwidth, of the signal. The three important bandwidths to consider are the passband, the stopband, and the anti-imaging bandwidth.

Passband

The signals within the passband have frequency-dependent gain or attenuation. The amount of variation in gain with respect to frequency is called the passband flatness. The digital filters and the analog switched capacitor filter of the NI-9260 adjust the frequency range of the passband to match the data rate. Therefore, the amount of gain or attenuation at a given frequency depends on the data rate.

Stopband

The filter significantly attenuates all signals above the stopband frequency. The primary goal of the filter is to prevent images of the fundamental frequency to propagate to the connected measurement device. Therefore, the stopband frequency scales precisely with the data rate. The stopband rejection is the minimum amount of attenuation applied by the filter to all signals with frequencies within the stopband except for interpolation images.

Interpolation and Anti-Imaging

A sample signal repeats itself throughout the frequency spectrum. These repetitions begin above one-half the sample rate, f_s, and, theoretically, continue up through the spectrum to infinity. Because the sample data actually represents only the frequency components below one-half f_s (the passband), it is necessary to filter out the extra images of the signal. The NI-9260 accomplishes this filtering in three stages.

First, the data is digitally interpolated by the digital interpolation filter, which moves the effective sample rate at eight times f_s. A linear-phase digital filter then removes almost all energy above one-half f_s as specified in the Stopband section.

Second, the DAC resamples the data to a new frequency that is eight times higher than the f_s. This filter has a sin x/x response, yielding nulls at multiples of eight times f_s. Images still exist at 8 * f_s - output frequency.

Third, a switched capacitor filter and a four-pole analog filter with fixed cut-off frequency filters the remaining images. The switched capacitor filter scales with the sample rate but the four-pole analog filter does not. Using a higher sample rate pushes the images at a higher frequency where the analog filters are more effective at reducing their amplitude.

NI-9260 Pinout

Table 1. Signal Descriptions
Signal	Description
AO+	Positive analog output signal connection
AO-	Negative analog output signal connection
Chassis GND	Chassis ground connection

NI-9260 with mini XLR Analog Output Connection

Note You can only use the NI-9260 with mini XLR with differential instruments.

Notice Do not connect AO- to chassis ground on the NI-9260 with mini XLR. Connecting this pin to chassis ground will short circuit the output.

Notice Do not connect the NI-9260 with mini XLR to a ground referenced single-ended instrument. Refer to the NI-9260 Block Diagram section for further information.

NI-9260 with BNC Analog Output Connections

NI-9260 Connection Guidelines

Make sure that devices you connect to the NI-9260 are compatible with the module specifications.
To minimize ground noise, make sure you connect the chassis ground to earth ground.