shunt voltage references can sink or source just fine right?

Question

i need a 5V reference that can sink or source current for a DAC. a simple shunt reference like the LM4040 can do both just fine right? my thinking:

1- when the DAC is SINKING current, that means that current is flowing through the shunt resistor and into the DAC reference input. as long as the current into the DAC does not drop the voltage across the shunt resistor below the voltage at which enough current will pass through the shunt to maintain its accuracy, im fine, right?

2- and when the DAC is SOURCING current, all that source current flows direcly through the shunt, and as long as it doesnt exceed the shunts maximum current, the reference voltage is accurate.

thought #2 seems the easiest to analyze and Im pretty sure im 100% right.

thought #1 im not so sure about...am I completely right or just close? i feel like im close to the right concept but i dont know what the exact numbers are.

Asa

Phil Hobbs · Accepted Answer

Basically, yes, though you're better off thinking about #1 as a current balance problem--you don't want the reference current to go to 0. There are transient issues to consider as well, depending on how fast the load current changes on the MDAC--you'll want a nice bypass across the shunt regulator, or else any load capacitance will make nasty spikes on the reference.

Cheers

Phil Hobbs

Tony · Answer

What you said was a bit mixed up. Current into the DAC doesn't drop the voltage across the shunt resistor at all. It robs current from the shunt regulator, and when there's not enough for regulation, the regulated voltage drops and the voltage across the shunt resistor INCREASES.

Jon Elson · Answer

The LM4040 is a TWO TERMINAL device. No way can it source current, all by itself. if given a higher voltage supply and a series resistor, then that setup can source current. But, it is NOT the regulator, itself, that is sourcing the current, but the resistor.

So, just kind of a semantic quibble, here.

Jon

shunt voltage references can sink or source just fine right?

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