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I have been doing some work with the TMSC5515 DSP chip and noticed in the process of configuring ADCs for potentiometer control that the chip supports 6 ADC channels (VIN[5:0]), but only has 4 pins that can perform the conversion (GPAIN[3:0]). I've noticed this on other embedded processors as well.

What is the point of doing this? If the chip is only capable of reading 4 values at a time, the extra ADC channels don't seem like they would serve any purpose.

Discription of ADC on TMSC5515: http://www.ti.com/lit/ug/sprufp1c/sprufp1c.pdf

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  • \$\begingroup\$ One kind of IC die, several options of bond out. Want a smaller outline? Use the option without so many pins \$\endgroup\$ – infixed Oct 20 '16 at 16:32
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In the special case of the TMSC5515 you have 3 channels for 3 pins and 3 channels only for the pin GPAIN0.

Pin GPAIN0 allows higher voltages and provides a built-in voltage divider.

You may select the undivided voltage or the divided voltage by selecting one of the three channels. (See page 10 of the data sheet.)

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Often the same ADC IP core is re-used among chip families to save cost (or package options for the same die) but there may also be internal on-die analog values worth measuring, like temperature. The other channels not broken out externally could be connected to an on-die thermal sensor, for example.

For some applications, the same die is packaged say into a QFN and a TSOP. Based on available pins, some ADC pads may get wire bonded to pins while others are left floating.

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An ADC input channel costs very little. A chip needs logic to generate an active-high and active-low select signal for each channel, and it needs a pair of small MOSFETs for each channel to connect it to the analog input circuitry. Where there would be no point including circuitry to select among e.g. 128 inputs if an analog subsystem would be unlikely to ever be used in a chip that size, it's easier to design in extra input channels (which are cheap) and not use them, than to add input channels to a design that didn't include enough.

Note, btw, that analog output channels are often much more expensive. Input channels generally don't need to carry significant current and can thus use small pass transistors. Output channels need to carry enough current to drive external circuitry, and thus require larger transistors to amplify the signal.

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The other answers are valid but frequency and resolution is not mentioned.

If the ADC channels can't be configured to a PIN the other answers are valid. When more ADC can be connected to one PIN they can be multiplexed. There are a few interesting things this allows us to do:

  • Increased sample rate or
  • Decrease noise (average) or
  • Increased resolution
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