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The following description from GBATEK explains the implementation of a low-cost ADC that was used in a video game from the early 2000s. It contained a solar sensor in the cartridge:

A/D Conversion

The cartridge uses a self-made A/D converter, which is (eventually) better than measuring a capacitor charge-up time, and/or less expensive than a real ADC-chip:

It contains a 74LV4040 12bit binary counter (clocked by CPU via the I/O port), of which only the lower 8bit are used, which are passed to a resistor ladder-type D/A converter, which is generating a linear increasing voltage, which is passed to a TLV272 voltage comparator, which is passing a signal to the I/O port when the counter voltage becomes greater than the sensor voltage.

Feeding a ladder DAC with a binary counter and comparing the (increasing and then resetting) output voltage to the voltage from some analog source and recording the binary counter's state when they match sounds like it would be a very simple, low-cost ADC. It looks like it is a good fit for situations that require fairly low resolution (this one is 8-bit), low sample rate (this one has a 666Hz clock), and where the source is expected to be relatively steady (like sunlight). Does this design have a name?

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  • \$\begingroup\$ I think this is what a sigma-delta ADC is but I'm not 100% sure I don't have it mixed up with a different type. \$\endgroup\$
    – Hearth
    Commented Feb 21, 2021 at 4:06
  • \$\begingroup\$ @Hearth Not a sigma-delta because no stream of ones and zeroes. it also uses a multi-bit DAC rather than a one-bit DAC. I first mistook it as a non-charge-redistributing SARs ADC which also has a multi-bit DAC and single comparator, but the described logic was too simple to implement one. \$\endgroup\$
    – DKNguyen
    Commented Feb 21, 2021 at 4:42
  • \$\begingroup\$ @DKNguyen Thanks, I knew something sounded wrong with that. \$\endgroup\$
    – Hearth
    Commented Feb 21, 2021 at 5:08
  • \$\begingroup\$ Tracking ADC... \$\endgroup\$
    – user16324
    Commented Feb 21, 2021 at 13:19
  • \$\begingroup\$ Tracking ADC has an up/down counter and is much faster, when "locked" ... except when changes of analog input are too fast (slew-rate greater then acceptable), then there is some "delay" in tracking ... or nothing ... \$\endgroup\$
    – Antonio51
    Commented Nov 3, 2021 at 19:38

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It is called a digital ramp ADC.

The specific ADC described by the OP does not have a sample-and-hold and relies on the input signal being steady but there is no reason it couldn't have one. It just compares the input signal to the output of a DAC with a comparator, and it ramps up the DAC value and whenever the DAC output just exceeds the ADC input, it latches the DAC input to the ADC output. Then it restarts the ramp. As a result, it has latency that varies based on the input value which is not a desirable trait.

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    \$\begingroup\$ which is not a desirable trait – For something that requires a very low sample rate (like telling if a solar sensor is in direct sunlight), it seems like it's sufficient. \$\endgroup\$
    – forest
    Commented Feb 21, 2021 at 6:45
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    \$\begingroup\$ @forest If you had something like a datalogger it would matter, or an equally slow control loop. \$\endgroup\$
    – DKNguyen
    Commented Feb 21, 2021 at 6:57

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