NI-9252 Getting Started
- Updated2023-02-23
- 9 minute(s) read
NI-9252 Getting Started
Connector Types
The NI-9252 has more than one connector type: NI-9252 with screw terminal and NI-9252 with DSUB. Unless the connector type is specified, NI-9252 refers to all connector types.
NI-9252 Pinout
| Signal | Description |
|---|---|
| AI+ | Positive analog input signal connection |
| AI- | Negative analog input signal connection |
| COM | Common reference connection to isolated ground |
| NC | No connection |
NI-9252 Grounded Differential Connections
Floating Differential Connections
Connect the negative lead to COM through a 1 MΩ resistor to keep the signal source within the common-mode voltage range. The NI-9252 does not read data accurately if the signal source is outside of the common-mode voltage range.
Single-Ended Connections
Connect the ground signal to COM to keep the signal source within the common-mode voltage range.
NI-9252 Differential Connections with Common Mode Voltage
NI-9252 Connection Guidelines
- Make sure that devices you connect to the NI-9252 are compatible with the module specifications.
- You must use 2-wire ferrules to create a secure connection when connecting more than one wire to a single terminal on the NI-9252 with screw terminal.
Wiring for High-Vibration Applications
- Use ferrules to terminate wires to the detachable connector.
- Use the NI 9928 backshell kit.
NI-9252 Block Diagram
- Input signals on each channel are buffered, conditioned, and then sampled by an ADC.
- Each AI channel provides an independent signal path and ADC, enabling you to sample all channels simultaneously.
- The module protects each channel from overvoltages.
Data Rates
The frequency of a master timebase (fM) controls the data rate (fs) of the NI-9252. The NI-9252 includes an internal master timebase with a frequency of 12.8 MHz. Using the internal master timebase of 12.8 MHz results in data rates of 50 kS/s, 33.3333 kS/s, 25 kS/s, 20 kS/s, and so on down to 10 S/s, depending on the decimation rate. However, the data rate must remain within the appropriate data rate range.
The following equation provides the available data rates of the NI-9252:
where a is the decimation rate.
| f s (S/s) | Decimation Rate | f s (S/s) | Decimation Rate | f s (S/s) | Decimation Rate |
|---|---|---|---|---|---|
| 50000.0 | 2 | 2272. 7 | 44 | 347.2 | 288 |
| 33333.3 | 3 | 2083.3 | 48 | 312.5 | 320 |
| 25000.0 | 4 | 2000.0 | 50 | 284.1 | 352 |
| 20000.0 | 5 | 1785.7 | 56 | 260.4 | 384 |
| 16666.7 | 6 | 1562.5 | 64 | 250.0 | 400 |
| 14285.7 | 7 | 1388.9 | 72 | 223.2 | 448 |
| 12500.0 | 8 | 1250.0 | 80 | 200.0 | 500 |
| 11111.1 | 9 | 1136.4 | 88 | 195.3 | 512 |
| 10000.0 | 10 | 1041.7 | 96 | 142.1 | 704 |
| 8333.3 | 12 | 1000.0 | 100 | 125.0 | 800 |
| 7142.9 | 14 | 892.9 | 112 | 100.0 | 1000 |
| 6250.0 | 16 | 781.3 | 128 | 97.7 | 1024 |
| 5555.6 | 18 | 694.4 | 144 | 60.0[1] | 1666 or 1706[2] |
| 5000.0 | 20 | 625.0 | 160 | 50.0[1] | 2000 or 2048[2] |
| 4545.5 | 22 | 568.2 | 176 | 10.0[1] | 10000 or 10240[2] |
| 4166.7 | 24 | 520.8 | 192 | ||
| 3571.4 | 28 | 500.0 | 200 | ||
| 3125.0 | 32 | 446.4 | 224 | ||
| 2777.8 | 36 | 400.0 | 250 | ||
| 2500.0 | 40 | 390.6 | 256 |
The NI-9252 can also accept an external master timebase or export its own master timebase. To synchronize the data rate of an NI-9252 with other modules that use master timebases to control sampling, all of the modules must share a single master timebase source. When using an external timebase with a frequency other than 12.8 MHz, the available data rates of the NI-9252 shift by the ratio of the external timebase frequency to the internal timebase frequency. The programmable filter specifications, expressed in Hz, will also scale with the external timebase. Refer to the software help for information about configuring the master timebase source for the NI-9252.
Filtering
The NI-9252 uses programmable hardware filtering to provide an accurate representation of in-band signals and reject out-of-band signals. The filters discriminate between signals based on the frequency range, or bandwidth, of the signal.
The NI-9252 programmable hardware filter supports both Butterworth and comb filter responses.
Butterworth Filter
The NI-9252 has a programmable hardware Butterworth low-pass filter. The Butterworth filter provides two selectable filter orders, each with six selectable cut-off frequencies that are configurable per module. The cut-off frequency (fc) of the filter is independent of the data rate (fs). However, using an external master timebase (fM) will influence both the cut-off frequency (fc) and data rate (fs). The following figures show the overall filter response with different filter settings.
Comb Filter
The NI-9252 comb filter frequency response is characterized by deep, evenly spaced notches and an overall roll-off towards higher frequencies. The NI-9252 provides five per module-configurable comb filter settings. The different options provide a trade-off of noise rejection (refer to Idle Channel Noise table) for filter settling time (refer to Settling Time equation) and latency (refer to Input Delay equation). To control the response of the programmable comb filter, you can select to have the first notch at 1, 1/2, 1/4, 1/8 or 1/16 of the data rate. The following figure shows the overall filter response with different filter settings.
Choosing the Right Filter for your Application
The NI-9252 Butterworth filter response is a low pass filter that allows signals with frequencies below the filter cutoff frequency to pass through while attenuating signals with frequencies higher than the filter cutoff frequency. This is useful to filter out unwanted high frequency noise in a signal. The Butterworth filter has a better flatness in the passband compared to the comb filter.
The NI-9252 Butterworth filter is a programmable-order filter. The different filter orders are characterized by the steepness of the filter response roll-off. The higher the filter order, the steeper the roll-off is. However, the trade-off of using higher order response is the higher input delay. The NI-9252 Butterworth filter allows user to trade-off between filter roll-off and input delay.
The NI-9252 comb filter frequency response is characterized by deep, evenly spaced notches and an overall roll-off towards higher frequencies. This is useful in rejecting specific frequencies and all its harmonics at a specific data rate. For example, the NI-9252 comb filter rejects powerline frequency of 50 Hz and all its harmonics when running at 50 S/s. The comb filter has lower settling time compared to the Butterworth filter.
For more information about filters, refer to the NI-9252 Getting Started Guide.
Appendix
NI-9252 Filtering
The NI-9252 supports two types of lowpass filtering:
- Butterworth
- Comb
| Attribute | Butterworth | Comb |
|---|---|---|
| Passband | Configurable independent of sample rate | Tracks sample rate |
| Latency | Medium to high (configuration-dependent) | Low |
| Phase Delay Variation versus Frequency | Variable input delay | Constant input delay |
| Flatness | Best | Good |
| Step Response (Time Domain) | Mid-level delay, overshoot | Short delay, no overshoot/undershoot |
| Typical Applications |
Filtering out high frequency noise sources Reducing measurement noise |
Filtering out specific noise sources Control applications |
Refer to the specifications for details on the amount of variation in the response you can expect for different input frequency ranges.
Frequency Response of NI-9252 Filters
The NI-9252 uses programmable hardware filtering to provide an accurate representation of in-band signals and reject out-of-band signals. The filters discriminate between signals based on the frequency range, or bandwidth, of the signal. How the filter discriminates signals based on their frequency is known as frequency response. In general, the frequency response of a filter is described by a signal attenuation (magnitude response) and a input delay (phase response) for every input frequency.
- Magnitude
Response—The three important frequency ranges, or bandwidths, to
consider for magnitude response are passband, transition band, and stopband:
- Passband—The range of frequencies at which the filter attempts to pass a signal without modifying it. The small amount of variation in magnitude at these frequencies is called passband flatness. This is the frequency range of signals that you want to measure.
- Transition band—The range of frequencies in which the filter magnitude response has started to roll-off such that it attenuates signals by some amount, but has not reached the full attenuation amount. The shape of the transition band has an impact on the alias rejection and how signals are represented in the time domain (for example, step response).
- Stopband—The range of frequencies at which the filter attenuates input signals to its maximum attenuation level. Ideally, you want to choose a filter with a stopband that covers frequencies of noise sources that you do not want in your measurements.
- Input
Delay—Filters delay the input signal by some amount when processing
data. In some cases, the delay is a function of the input signal frequency; when
this is the case, the input delay plot is useful for knowing the exact delay at
different input frequencies and the maximum variation between signals of
different frequencies within the passband.
Figure 5. Comparing Typical Input Delay for NI-9252 Filters 
Each NI-9252 filter has a different frequency response to serve different applications:
- Butterworth—Has a passband independent of the sampling rate (as opposed to the comb filter), which offers more flexibility when filtering out noise that is below one-half of the sample rate. However, depending on your settings, you may see alias components of higher frequency signals in your measurement that extend beyond one-half of the sample rate due to the larger transition band.
- Comb—Has a smaller passband because its transition band starts early in the frequency range. The comb filter has shorter group delay than other filters and better representation of signals in the time domain (step response). The comb filter's transition band features equally-spaced notches at different frequencies. It is common to use the comb filter with a specific sample rate to align the notches of the transition band thereby removing a specific noise-source frequency from measurements.
The NI-9252 filter delay across signals in the passband varies between filters:
- Butterworth—Delays signals by a variable amount depending on their frequency.
- Comb—All input frequencies have the same amount of delay when going through the filter. Choose this filter for applications where linear phase, short delay, or data correlation of different devices and configurations is required.
Refer to the specifications for details on the amount of variation in the passband gain and input delay you can expect for different input frequency ranges.
Step Response of NI-9252 Filters
The shape of the magnitude and phase responses of a filter impacts how signals look in the time domain. The step response of a filter is typically used to identify the behavior of a filter in the time domain.
Three important factors of the filter step response are group delay, rise time, and overshoot/undershoot. The three filters differ in step response across signals in the transition band:
- Butterworth—Has a short group delay and the longest rise time. The output signal shows overshoot.
- Comb—Has the shortest group delay and the shortest rise time. The output signal does not show overshoot or undershoot.
Figure 1 compares the settling time and the latency with the step response between the three filters at the their fastest setting. The results shown are 50 kS/s for the comb filter and a 4 kHz bandwidth for the 2nd and 4th order Butterworth filter.
