Direct Waveform Recording  

aaaaThe heart of our “Direct Waveform Recording” technology is a novel low-cost multi-channel digitizer, based on a unique analog-to-digital converter (ADC), has the required sampling rate (1 GS/s), sampling depth (128 ns), resolution (10 bits), and event rate (>10 kHz) to accurately capture over 10,000 fluorescence waveforms per second per channel. For most applications we use 5X interleaving to increase the effective sampling rate to 5 GS/s. The traditional means for capturing waveforms are digital storage oscilloscopes or transient waveform digitizers. The high costs of the laser and digitizer have limited the commercial opportunities for this desirable technique. The laser cost problem was recently relieved by the invention of the microlaser. This small, relatively low cost solid-state laser uses passive Q-switching to produce intense sub-nanosecond pulses at rates of 1-10 kHz. With the advent of the microlaser, the cost-reduction focus shifted to the digitizer. To substantially lower the total system cost, the need has been for an inexpensive means for rapidly capturing fluorescence waveforms.

aaaaThe key to low-cost high-performance digitization is a fast-in slow-out (FISO) approach to A/D conversion. Also called “flash capture” (not to be confused with “flash A/D conversion”), the idea is to de-couple the sampling and conversion rates with an analog memory. A fast sampling mechanism records the input waveform in the analog memory as a series of analog samples. Later, the stored analog samples are converted into digital form. This approach permits high-speed sampling without requiring equally high-speed conversion. The lower A/D conversion rate admits the use of low-power high-resolution techniques that are not feasible or possible at the higher sampling rate. A further innovation is to increase the event rate by converting the analog samples in parallel.

Figure 1 . In the FISO approach, an analog memory de-couples the sampling
and quantization processes, allowing high-speed sampling to be combined with
low-power high-resolution conversion.

Size, Connections, and Power

aaaaAn external view of the digitizer is pictured in Figure 2. The compact 6.30” x 4.06” x 1.20” box occupies a volume of less than 31 cubic inches. Barely visible on the left are the four analog input channels. Each has a SMA connector. Transient voltage suppressors protect the analog inputs and input signals are allowed to range from –1 to 0 volts and 0 to +1 volts. Each channel supports both ranges. Each input path also includes a relay that is opened when internal calibration signals are being applied to that channel and a fixed gain amplifier that adds an adjustable voltage to the input signal before presenting the sum to the ADC.

Figure 2 . The digitizer.

aaaaOn the right side of the digitizer (see Figure 2) are two trigger inputs, a USB port (not visible), and the power connector (also not visible). There are actually three trigger sources, but one is internal. The external sources are optical (with the red cover) and electrical. The internal source is a general-purpose I/O port of the internal digital signal processor (DSP). USB 2.0 processing is implemented by an internal microcontroller. The input power is expected to be at a nominal potential of 5 volts.

aaaaTwo guiding concepts in the digitizer design were reaction and self-calibration. The reaction concept means that the digitizer reacts to external events rather than polling for them. The triggered sampling strobe generator illustrates this most significantly. All the sampling is done in response to the arrival of a triggering event. No free-running clock is involved. This ensures repeatability. There is no jitter introduced by a need to synchronize external events to a separate clock. Calibration is the means by which a repeatable system can be made to deliver accurate results. So the digitizer was designed to be reactive and self-calibrating.

aaaaCalibrations require measurements and standards upon which to base those measurements. So the digitizer produces three test signals, each of which is based on a particular reference. For relative time, there is a periodic signal based on a crystal oscillator (XO). This is used to calibrate the sampling speed, which is the waveform time dimension. For absolute time, there is a pulse signal that is generated in response to a trigger event. This is used to calibrate trigger delays to achieve interleaved or segmented sampling. For amplitude, there are band-gap voltage references in the digital-to-analog converters (DACs). To calibrate the waveform amplitude dimension, the speed of the ramp (a common input to the Wilkinson-style A/D converters) is adjusted. This involves the XO and a DAC. The ramp speed setting is also how an apparent gain is achieved. Slowing the ramp causes a sub-range to be scanned with the same number of quantization levels. So by slowing the ramp by a factor of 10, the analog signal is effectively amplified by that same factor.

aaaaAll these calibrations are done automatically by the digitizer when commanded to do so by a PC (through the USB port). Other adjustments can also be made from the PC to affect things such as the operation of the trigger, ADC current bias and voltage reference levels, and the routing, shape and timing of the test signals.

Waveform Capture, Conversion, and Processing

aaaaFigure 3 shows the major functional blocks of the digitizer. Information flows from left to right, passing through the ADC, DSP, and mC (microcontroller) before going to a PC via the USB port.

Figure 3 . Digitizer block diagram.

aaaaThe ADC is responsible for capturing and digitizing the input signal, producing the digital waveforms that are read by the DSP. In response to a trigger event, the ADC samples all 4 inputs at 1 GS/s, producing 128 analog samples per channel. The 128 samples of each channel are converted in parallel into 128 10-bit digital values that are read out by the DSP. The DSP selects the channels to be converted and readies the ADC for the next trigger event. In between trigger events, the DSP is fast enough to perform operations such as waveform averaging or computing moments. The results are pushed into stacks within the microcontroller and transferred via USB to a PC.