For laser-scanning microscopy, circuity is needed that digitizes the photomultiplier tube output from the PMT preamps and sends the digital values to the PC for each pixel. We have gone through a number of designs for our lab-built systems, with a previous design based on a lab-built digitizing board that sent four channels of image data to the PC through an Advantech high-speed digital IO board (PCI-1755). The PCI-1755 was a digital input/output PCI board that could stream 32 bits of data at rates as fast as 80 MHz. However, Advantech only provides a 32-bit driver for the PCI-1755, and has no plans to offer 64-bit driver as the board is no longer produced.

We are now developing two new designs for digitizing 4 channels of data on our LSMs. One is USB-based and uses a high-speed DIO board from Measurement Computing (USB-DIO32HS) with a lab-built digitizing board attached to it (4 channel ADC module). The USB-DIO32HS is a USB 2.1 device that can stream digital data into the PC through the USB bus at a rate of 8 MHz allowing for pixel data to sent as fast as every 125 ns. Our typical pixel times our 0.5 to a few microseconds, so the ADC counts are averaged 4 or more times per pixel. The USB-DIO32HS board also provides the pixel clock that is gated by a signal from the scan card and serves as the ADC clock for the digitizing board. The ADC board uses four Texas Instruments TLC5540 8-bit 40MHz ADCs.

The second system is PCIe based using a high speed DIO board from Contec, Inc (DIO-32DM3-PE) that can stream 32-bit data to the PC at up to 50 MHz. Data transfer from the ADC board to the DIO-32DM3_PE card is synchronized by a pixel clock generated by a clock on the ADC board and gated by a scan active signal from the scan card. The ADC board uses the also uses four TLC5540 ADCs, but their digital output is level shifted (5v to 3.3V) since the PCI DIO board requires 3.3V logic levels.

The preamplifiers we use have two outputs, a DC-coupled output that is fed into the amplifier on the digitizer board where it is further amplified and a black level adjustment added, and a second AC-coupled output for photon counting and TCSPC.