Fastest external High Speed ADC?

Question

I am in a situation close to: Hi-speed external ADC

But with 100,000 ~ 500,000 samples per second, I don't need more.

I only want to detect changes in 2 sensors, so each loop is only an "analogread" and a couple of conditionals, so I have the 16MHz almost intact to store data from an external ADC.

What is the fastest thing you can aspire to with an arduino and an external ADC?

• Boggyman: The link you gave explains the problem with an external ADC: No matter how fast the ADC is, the Arduino has to get the data somehow, and put it someplace. You can go faster than analogRead, but 100 kHz is about the limit for a single channel (8 bits, 2MHz clock) in bursts of 1024 samples, or just under 50 kHz with 2 channels. What exactly are you doing? Can you use digital instead of analog? Direct read of 8 digital pins can run at over 400 kHz in 1024-byte bursts.
• user1814720: Thanks @Boggyman, I have 2 analog photodiodes, and I want to detect any change in the fastest possible way. With 8 bits, or either 6, it's enough. But I can not find an external ADC with this characteristics. (that can be bought at a reasonable price)
• StarCat: Just browsing through the Mouser catalog brings up the following: MAX153, 1MSps, 8 bit parallel output with prices from about 8ドル or the TI ADC0820, 667KSps, 8 bit parallel with prices starting at about 5ドル.
• user1814720: Thanks @StarCat, I'm going to buy MAX153 and test it.
• Boggyman: You mention detecting changes. Depending on exactly what you are doing, there are ways of doing that externally without an ADC. For example, if you want to know when the difference between the sensors exceeds a certain threshold, you can use a differential amplifier followed by a threshold comparator. That gives you a pure digital signal that you can detect very quickly, or better yet use to generate an interrupt.