The SR400 photon counter is a powerful tool for photon counting applications, but its analog voltage output update rate can be a limiting factor in fast-scanning applications like laser scanning microscopy. The output update rate of the SR400 is closely tied to the selected dwell time and the instrument's internal timing circuitry. From your description, it seems that you are constrained by a minimum dwell time of 0.5 ms, which is significantly slower than the 1 µs period required for your standard pixel dwell time of 8 µs. Unfortunately, the SR400 was not specifically designed for high-speed analog output updates, which presents a challenge in your setup.

To address this, the first step is to confirm the minimum dwell time that the SR400 can achieve in its current configuration. The SR400 manual suggests that the dwell time can be as low as 0.5 µs when using the external start input with a properly configured external clock signal. However, achieving this performance requires precise synchronization of the external clock signal and the instrument settings. Ensure that the square wave signal fed into the START input is clean, stable, and has a period matching the desired dwell time (e.g., 1 µs for your application). Verify that the SR400 is set to "EXTERNAL" mode and that its parameters align with the external clock source.

If the SR400 still does not achieve a dwell time below 0.5 ms, this limitation may stem from the design of the analog-to-digital conversion circuitry or internal processing constraints. The D/A update rate might inherently limit the output period, regardless of the external clock or dwell time settings. In this case, the SR400 may not be suitable for direct use in your fast-scanning microscopy application without additional adjustments or supplementary equipment.

One potential workaround is to bypass the SR400’s analog output entirely and instead use the raw photon count data directly via its digital output. Many modern DAQ cards, including the PCI 6110, can handle high-speed digital inputs. By transferring the photon counts digitally to the DAQ, you eliminate the bottleneck of the SR400's analog voltage update rate. With digital input, you can synchronize the photon counts directly with your pixel dwell time and post-process the data to reconstruct the analog equivalent if needed.

Another approach is to consider external hardware to bridge the gap between the SR400 and your DAQ system. For instance, you could use a high-speed microcontroller or FPGA to process the SR400's digital output and generate a corresponding high-speed analog voltage signal. This hardware would essentially act as a real-time converter between the SR400's counting capabilities and your DAQ's analog input requirements, enabling faster update rates compatible with your scanning setup.

Lastly, if high-speed operation is a critical requirement and the SR400's limitations cannot be addressed effectively, you may need to explore alternative photon counting systems designed for fast analog outputs. Modern photon counting electronics with direct compatibility for laser scanning microscopy, such as multichannel scalar modules or high-speed photon counters, might better suit your application. These systems are designed to handle sub-microsecond timing and provide outputs synchronized with high-speed scanning rates.