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Why do we need an external ADC when there are microcontrollers available with internal ADC?

I think it makes it economically expensive to add to 2 chips separately on the PCB, adding peripherals, using board space.

Are there any other advantages of using an external ADC?

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  • \$\begingroup\$ Higher resolution, higher sample rate, noise suppression, etc. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 17 '15 at 4:11
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Usually the kind of ADC integrated on-chip within a microcontroller, is typically a capacitive charge-balancing successive-approximation type (SAR), and usually no more than 12 bits resolution. Sometimes a bandgap reference is included on-chip, but often they just measure ratiometric to the digital power supply -- so they are low accuracy, no point in going with higher resolution.

The typical 12-bit microcontroller-integrated ADC is adequate for lots of general-purpose applications (like most Arduino/hobbyist stuff and even some industrial applications), but higher-end applications require more specialized ADCs.

Some motor control schemes require simultaneous sampling on multiple channels; can't do that with on-chip ADC.

Weigh scale applications typically use a Delta-Sigma ADC. Despite the low output data rate, they often use oversampling to reject 50Hz/60Hz powerline-induced noise.

Capturing sound at decent quality requires a moderately high-speed ADC (CD audio sample rate is 44.1kHz, DVD audio sample rate is 48kHz). A typical microcontroller's on-chip ADC doesn't go that fast, and its resolution is too limited -- some may be able to get old-timey "telephone quality" sound adequate for voice, but the distortion is somewhat objectionable.

Video frame buffer capture requires a very high-speed ADC. So does a digital sampling oscilloscope. And RADAR and specialized imaging applications require very, very high-speed ADC.

Medical equipment requires high precision ADC, with noise rejection. For example an ECG (electrocardiogram) must sense very fine details of the heart rhythm, on a set of electrical signals that are significantly less than the 50Hz/60Hz powerline-induced noise from electrical wiring in the room with the patient.

So yes, there are a lot of applications that can work just fine with the cheap 12-bit on-chip ADC using VDD as the reference -- for those applications, you're absolutely right, it makes sense to just use the on-chip ADC if it's good enough. That's why it's there. But there are times when more performance is needed.

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  • \$\begingroup\$ Very helpful! I got the point. I am considering to develop an ADC module (like a breakout board) with a J type thermocouple connected to it. The plan is to start up a small business of electronic subsystems. Please let me know your views about it. \$\endgroup\$ – Shaun Sep 17 '15 at 8:45

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