| Data Acquisition Toolbox | |
Specify the measurement bandwidth (in Hz)
Description
For an input channel, span specifies the maximum frequency at which valid alias-protected data is received. Frequencies above this value are filtered out. For a source channel, Span specifies the maximum frequency at which the output signal will correctly track the signal that the source is attempting to generate.
The valid values for Span depend of the current clock frequency. You should set the clock frequency before setting Span. Normally, the maximum valid span is the clock frequency divided by 2.56. Valid spans are given by the maximum span divided by powers of two, and the maximum span divided by five and by powers of two. The ratio between the span and the maximum span is called the decimation factor.
For the E1432 module, the maximum number of decimate-by-two passes allowed is nine. Therefore, the maximum decimation factor is 5.29, and the minimum valid span is (clock frequency)/(2.56.5.29). If the clock frequency is larger than 51.2 kHz, then the module is unable to do a decimation factor of one. In this case, the minimum decimation factor is two and the maximum valid span is (clock frequency/5.12.
For the E1433 module, the maximum number of decimate-by-two passes allowed is 12, so the maximum decimation factor is 5.212. Because of limits in the module's DSP processor, when the clock frequency is set higher than 102,400 Hz, it is unable to do any decimation. In this case, the only valid span is (clock frequency)/2.56. If you attempt to use decimation when the clock frequency is above 102,400 Hz, then an error might occur when the measurement starts.
For the Option 1D4 source board, the maximum number of decimate-by-two passes allowed is 16, and the maximum decimation factor is 5.216.
The effective sample rate is defined as the rate at which data is received from an input or used by a source, and is normally equal to 2.56 times the span. If the data is oversampled, then the effective sample rate is 5.12 times the span.
If the digital filters in a module have a cutoff that is sharper than 1/2.56, then some of the frequencies above the maximum span might contain valid alias-protected data. This is the case with the E1432 and E1433 modules, which have a top span filter cutoff of (clock frequency)/2.226, which is 23 kHz when the clock frequency is 51.2 kHz, 88.3 kHz when the clock frequency is 196.608 kHz. However, Span ignores the extra bandwidth so that the maximum span is always 1/2.56 times the effective sample rate.
Span applies to an entire E1432 module rather than to one of its channels. After a hardware reset, each module is automatically set to the maximum legal span.
Characteristics
| Vendor |
Agilent Technologies |
| Usage |
AI, AO, Common |
| Access |
Read/write |
| Data Type |
Double |
| Read-only when running |
Yes |
Values
Normally, the maximum valid span is given by the clock frequency divided by 2.56. Valid spans are given by the maximum span divided by powers of two, and the maximum span divided by five and by powers of two. The value set for Span automatically updates the SampleRate value.
Properties
| | SourceOutput | StandardSampleRates | |