A digital signal averager uses a flash ADC (analog-to-digital converter) to sample the analog signal at fixed intervals. This essentially converts the tracing of the analog signal to a digital record of that same trace. If only one scan though the time period of interest is saved, the instrument is normally called a transient recorder. A digital signal averager differs from a transient recorder, in that the former rapidly acquires multiple scans of a repeating analog signal, and sums (or averages) the repeated records in a hardware memory. This same summing can be implemented by software in a computer when a transient recorder is employed. But, the time taken to transfer each record to the computer and perform the sum in software consumes circa 90% of the available time. Only about 10% of the time is available to actually acquire useful data. By accomplishing the summing of records in a dedicated, high-speed hardware memory, the most efficient digital signal averagers can allocate 99% of the time to acquiring useful data.

The ability to allocate a high percentage of available time to acquiring data is determined by the selected time span for a scan, compared to the end-of-scan dead time and the end-of-spectrum dead time. For convenience, a scan is considered to be one pass through the designated time span. A complete record results when all requested sampling points have been populated. A complete record will require only one scan if interleaved sampling is not used. Interleaved sampling requires two or more scans to generate a complete record. To improve the signal-to-noise ratio, multiple records are summed in the hardware memory to form a spectrum. This latter step is the high-speed averaging process.

For TOF-MS the sampling interval needs to be fine enough to adequately define the peak shapes and positions in the time-of-flight spectrum. This requires at least 1.2 samples across the FWHM of the narrowest peak (see Application Note AN61 for an explanation; click here to locate it). Hence peak widths of 1 ns require sampling intervals <833 ps. Finer sampling intervals will provide better peak shape definition at the expense of increasing the file size when the spectrum is saved to hard disk.

Flash ADCs that can sample at 500 ps intervals typically are limited to 8 bits, i.e. 256 levels of voltage encoding. The FASTFLIGHT product line includes a Precision Enhancer that improves the encoding to 12 bits when more than 16 records are summed. The Precision Enhancer also improves the differential non-linearity of the ADC by a factor of 16 when more than 256 records are summed. The differential non-linearity (DNL) is a measure of how much the individual widths of the voltage-encoding intervals deviate from the average width. The ADC DNL affects the accuracy of the vertical coordinate in the TOF-MS spectrum.

A chromatograph/TOF-MS acquisition that extends over ten minutes can easily generate a file size of the order of tens of gigabytes. Thus rapid data compression in the hardware becomes important for significantly reducing the files size. The FASTFLIGHT products offer several techniques for data compression.

The correlated noise specification becomes important when ion rates are extremely low and the number of records summed must be very large (e.g., 65,535 records/spectrum). In this case, the correlated noise of the digital signal averager can become the controlling factor in minimum concentration detection limits.

For a synopsis of the main specifications for digital signal averagers, click here.