# Connecting a high-speed comparator into a computer

I need to store the output of a stand alone comparator (Hittite HMC875LC3C) in computer RAM drive. Currently, I'm using an 8bit 2GHz ADC (an Agilent PCIe - Acquiris U1071A) to bridge between the comparator and the RAM drive. This solution is redundant, slow, and very expensive (the Agilent ADC is almost $10k). I would enjoy a better solution where no ADC is present and the comparator connects directly to the PC. Perhaps through a USB port? Ideally, I would like the comparator to operate at around 1GHz (it supports operations up to 20 Gbps). I would suspect the maximal throughput essentially depends on the computer's processor and interface i/o characteristics. The comparator outputs pulses are about 60ps long and the Output Voltage High Level is -10mV while the Output Voltage Low Level is about -420 mV. The comparator is emmbeded on a PCB with SMA connectors. Question: Is it possible to connect a high speed (1GHz) comparator into a PC without having to use an ADC? If so would some form of synchronization be required? Thanks Louis - How much data do you need to capture at a time or is this constant, real-time? – Nick T Mar 14 '11 at 12:13 In the end, I would like to send these bits to remote clients via the PCs network card. The RAM is used to store the bits while they await a request. If client demand is high, RAM storing is not really necessary (I think). If there is no client demand, the RAM is full and new incoming bits from the comparator are simply discarded. – Louis Mar 14 '11 at 15:31 ## 2 Answers If we are talking about custom solutions, one might convert output of comparator to LVDS for example and feed to FPGA, where data is deserialized and pumped into PCI/PCI-E bus. USB is not fast enough. Also, if you have long sequences of 0 or 1, FPGA might have RLE-like compressio built in. FPGA would cost <500$, which is defenetly less than your ADC, but development costs are not that low. If you need it for just 1 instance, you might want to go with what you have now.

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Assuming you can get it in a state suitable for an FPGA, the opal kelly PCIe FPGA board might be of use opalkelly.com/products/xem6110 –  ralight Mar 14 '11 at 13:52
BarsMonster, ralight, thanks for the input. FPGA is def something I am looking at. I would need to outsource since I have no experience with FPGAs. –  Louis Mar 14 '11 at 15:43

If you only need a single board I think the current solution is probably the cheapest given the cost of any form of electronics development.

However, if you need more boards I think the FPGA way is probably the best way to go. Your requirements are kinda vague. You will definitely need an ADC to sample your signal, but you don't need that particular ADC. If you want to sample 8 bit at a data rate of 1GHz (8Gbits per second) you will need something like infiniband to give you the required throughput (or at least many ethernet ports). To get those data rates I would probably drop the computer entirely and look for an FPGA board with an Ethernet connector (they're quite common) and either a good enough ADC or buy an external ADC and connect it to the FPGA.

For a base architecture I would suggest using a synthesizable microcontroller which basically parses the ethernet packets and move bytes from the ADC buffer to ethernet.

Possible boards:

Possible microcontrollers: