What's the point of an internal reference on an ADC/DAC?

Question

I'm designing a circuit using the AD567XX (datasheet) family of Digital to Analog Converters.

I'm confused by the Reference Voltage Pin (Pin 21). It seems that one of the distinctions between different chips in the family (ie: AD5674 vs AD5674R) is whether or not the voltage reference is external or internal.

As best I understand, the reference voltage is the voltage by which DACs and ADCs use against which to compare analog signals coming in and out of the IC, and an external reference means that VREF needs to be provided to that pin whereas an internal reference means that VREF will just appear at that pin due to internal circuitry on a given ADC or DAC.

My question is: why bother having an internal reference? External references make sense to me because you might want to optimize performance by using a voltage regulator and external circuitry to ensure the DAC/ADC has a precise/accurate VREF. But the internal reference doesn't do anything? It just makes VREF appear at the pin, regardless of what's on the other side, so what's the point? Is the circuit designer (me) supposed to put something on the other end of that pin? Can I leave it unconnected and use the digital programmable bit to turn it off? Any explanation would be greatly appreciated!!

Answer 1

Internal reference is nice because you don't have to find space on your board for a separate reference and pay for the separate chip. (And support component for the reference chip, such as power bypassing capacitor)

The internal reference also may automatically power down when the ADC is in an idle/sleep state, while an external reference would continue to draw some power unless you add your own circuit to shut it off.

The datasheet will tell you if you are supposed to connect anything to the VREF pin when using the internal reference. Most ADCs either need a capacitor or don't need anything connected at all.

Answer 2

External references make sense to me because you might want to optimize performance...

You answered your own question here. What if you do not "want to optimize performance"? There are countless embedded applications where the precision of internal reference is good enough.

... using a voltage regulator and external circuitry to ensure the DAC/ADC has a precise/accurate VREF

That is one use of external reference. Another is ratiometric measurement, where the inputs are proportional to variable VREF.

Is the circuit designer (me) supposed to put something on the other end of that pin?

Read the datasheet. It usually requires external bypass capacitor on VREF pin when internal reference is used.

Answer 3

My question is: why bother having an internal reference?

Well, the pin would be unused if they didn't bring it out and, if you relied on a stable reference voltage for other parts of your input circuit it would be really useful because errors would ratiometrically cancel: -

Answer 4

The internal reference is very useful. It allows you to optimize the circuit by using it without any external parts.

You can power your MCU and DAC with any random regulator with large tolerance, or even make a device that operates directly from say a lithium battery that has a range of 2.7V to 4.2V.

Even with that huge range of supply voltage, your DAC output is constant and stable as it has a built-in 2.5V reference that is 0.1% accurate which is far better than any normal voltage regulator and the internal reference has a pretty low tempco of 2 ppm per °C.

If the DAC did not have internal reference, you pay extra for the components to make a stable output.