DBS - Digital Baseline Stabilizer

Background

Acquisition of time-domain pulses is a common measurement in many applications, and often the pulse characteristics is determined relative to a fixed baseline (DC level). It is crucial that this baseline is measured with high accuracy in order to avoid false readings and/or performance degradation.

Factors such as component aging, temperature variations, and pattern noise all adversely affect the accuracy of the baseline measurement and often induce a baseline fluctuation/drift. Tracking and removing baseline variations is crucial as it otherwise can lead to missed pulses, erroneous detection of false pulses, and incorrect analysis of pulse characteristics.

How to address baseline variation

The baseline variation consists of a slowly drifting DC-level, often in combination with a zigzag pattern – so-called pattern noise - caused by time-interleaved analog-to-digital converters (ADCs). Time-interleaving is a common technique used in many of today’s high-performance ADCs in order to achieve high sampling rates.

Figure 1. The baseline variation consists of a slowly drifting DC-level due to temperature variation (left) in combination with a zigzag pattern originating from the difference in DC-offset between individual time-interleaved ADC cores (right).

Both these contributing sources need to be considered for removing the baseline fluctuations and thereby increase the sensitivity of the system. Tracking and correction should be done in real-time so that an already corrected data stream is transferred to the host PC. It is also important that the correction is done with high accuracy, especially in systems where many records will subsequently be averaged in order to improve the signal-to-noise ratio (SNR).

Digital Baseline Stabilizer (DBS)

The Digital Baseline Stabilizer (DBS) technology from Teledyne SP Devices is built-in to all our firmware packages for time-domain applications. It removes baseline variations in real-time with high accuracy and operates in the background without any need for calibration signals or measurement disruptions. DBS stabilizes the baseline by removing unwanted variations and adjust its level to a user-defined target value. This helps capture even the smallest pulses, increase the dynamic range and improve the accuracy of the pulse analysis.

Figure 2. Since DBS is always active, it will automatically and continuously monitor baseline variations and correct for any time-invariant behavior.

In addition to correcting for baseline drift DBS also corrects offset errors between individual ADC cores in time-interleaved ADCs. The errors are visible in the digitized waveform and typically show up as a zigzag pattern, hence the name pattern noise. These errors can be a significant noise source and can therefore severely distort the baseline if left uncorrected.

Figure 3. Left image: the noise level (black) caused by pattern noise is significantly reduced when using DBS (blue). Right image: when zooming in, the zigzag pattern due to the four interleaved ADC cores becomes apparent.

Common use of DBS

The combination of high sampling rate and high resolution have led to advances in many different application areas. However, with increased sensitivity, the negative effect of analog imperfections becomes more evident and can limit the overall system performance. Baseline variation is no exception and DBS is therefore used by many of our customers in a wide range of applications. Here are some examples:

• In LiDAR it is crucial to use high-performance waveform digitizers to capture and analyze the return pulse. Target characteristics influence the shape of the return pulse, and multiple targets separated by small distances produce complex waveforms. Furthermore, the amplitude of the return signal is often very low – especially in bathymetric systems where the signal strength attenuates exponentially through the water column. Baseline variation in LiDAR can result in distorted waveforms which leads to incorrect analysis and performance degradation. Our LiDAR customers typically use the built-in DBS function in either the ADQ14 or ADQ7DC digitizer.
• In time-of-flight mass spectrometry (TOFMS) high sampling rate is important for achieving good mass resolution and high dynamic range enables accurate measurement of mass concentration. Baseline variation in TOFMS has a negative effect on both of these parameters and therefore needs to be removed. Customers in this segment normally use DBS in ADQ7DC or ADQ14 in combination with optional firmware packages for pulse detection (FWPD) and/or real-time waveform averaging (FWATD).

Further reading / next steps

• For further information about digitizers with built-in digital baseline stabilization please take a look at the product overview here or visit the product pages for ADQ7DC or ADQ14.
• If you want to know more about pulse detection in general then please read our application note “The art of pulse detection” here.
• The optional pulse detection firmware, FWPD, is described more in detail here and there is also a short video introduction here.
• More information about the real-time averaging firmware option, FWATD, is available here and in the short video here.
• For frequency-domain applications, we recommend using ADX rather than DBS.