What is the proper baseline bit value?

The AP240 Settings Tab and BitWise include a rarely touched value called "Baseline Bits." Before explaining how to set it, we need to review the basics of the ADC. In particular, the terms Full Scale, Offset, and Baseline.

Paraphrasing the manual (Section: Menu Window->AP240 Setting Tab) :

Full Scale
The AP240 digitizes signals with 255 bits of resolution. The full scale voltage range is user-adjustable and is set based on the amplitude of the largest signal of interest ...

Offset
By default, the board centers its full scale voltage range on 0V, in order to record positive and negative voltages of equal amplitude. The offset setting allows the user to adjust the relative position of the voltage window to better capture specific signals ...

Baseline / Baseline ()
The baseline is the voltage from which signals originate. Ideally this would be 0V in the ToF-AMS, but it typically has a value of a few mV. This value is measured in the BitWise Window ... With thresholding enabled, signals are recorded relative to the defined baseline voltage.

Putting it all together: When configuring the AP240, the user defines the region of voltage space where the signals will be recorded.

• Full scale determines the size of the window
• Offset determines the position of the window
• Baseline (mV) determines the voltage relative to which thresholded signals are recorded

The AP240 can only return integer values. That is, a signal cannot have intensity = 1.302 bits. Critically, a signal with voltage = baseline voltage should return 0 bits. For this to be the case, we must set our offset such that the measured baseline voltage corresponds to an integer bit value.

Rather than put the user through the trouble of calculating this offset, the DAQ takes the desired bit value (Baseline bit) and baseline voltage as inputs and uses them to calculate the appropriate offset. This step is carried out in the menu window when you press "Set" next to the baseline value or in the BitWise window when you press "Set Baseline."

So, what should the Baseline Bit value be? Historically, we set this value to 13 bits, which is 5% of 255. We measure all signals greater than the baseline, so this meant we were using 95% of the ADCs dynamic range. More recently, we have developed a rule of thumb:

"The baseline bit value need not be greater than the threshold bits + 2."

Where does this come from? When acquiring thresholded data, anything less than the base voltage is digitized as a 0. So, if the baseline bit = 13, anything with voltages corresponding to 13 or less will be recorded as a 0. (Actually, anything with voltage equivalent to 13 + threshold or less will be 0 ..) Immediately you can see that there is no sense setting the baseline bit high.

So, if we are not interested in voltages below baseline, why not set the baseline bit to 0? There is one time we look at signals below baseline: Baseline voltage is calculated as the average value of the electronic noise waveform. This signal has positive and negative going components. If we only measure the positive, we will bias the calculation high.

Back to the rule of thumb ... we generally set threshold at a value that is just greater than most of the positive electronic noise. And, the magnitude of the positive and negative noise is approximately equivalent. So, under normal circumstances, if we set the baseline bit at (Threshold +2) we will capture the positive and negative electronic nose, and properly calculate our baseline voltage.

Versions 3.0.17+ of the DAQ will warn you if your baseline bit is set too high or too low.