R3 / R4 do what?

In these circuits...

http://www.adafruit.com/products/2200#technical-details-anchor

...what is the purpose of R3 / R4.

All the LM4040 reference circuits I can find are sans the second resistor. For example... http://www.ti.com/general/docs/datasheetdiagram.tsp?genericPartNumber=LM4040-N&diagramId=25110

I found one other circuit with the second resistor but have not found any explanation for it.

You don't really need them. My guess is they only provide a small 10K load, perhaps for stability for an analog input such as A0 on the Arduino.

Typically these devices would be powered by a regulated supply.

I believe the second resistor is added to create a voltage divider, allowing for a wider range on the supply (5V-12V) as specified by adafruit. Both resistors would be sized so that IQ meets the min/max requirements as specified in the datasheet.

Thank you both for replying.

You don't really need them.

From what I can tell the Adafruit boards don't need them either.

My guess is they only provide a small 10K load, perhaps for stability for an analog input such as A0 on the Arduino.

While I like the idea I cannot find anything in the datahsheet to indicate a small load is necessary.

I believe the second resistor is added to create a voltage divider, allowing for a wider range on the supply (5V-12V) as specified by adafruit.

Unless Kirchhoff and I are on bad terms, that does not play out. Under no load, the 10K, as far as I can tell, has no affect. I believe it reduces the maximum current that can be supplied.

Any other theories?

Several more guesses that I hope won't break any laws:

The 10K resistor:

  • discharges any stray, parasitic or self-capacitance when handling the board or making connections, thereby helping to protect the LM4040 IC.
  • ensures proper power-on conditions by having the reference voltage pulled down when the power source is off or disconnected.

You could always ask on their forum.

After doing the math on the Adafruit version, doing the math on what I need, studying the posts above, and studying other circuits, I've come to the conclusion that the top resistor can be 1k to 2.2k and the bottom resistor is pointless. So, the momentum to understand the bottom resistor is quickly declining.

But, I tend to obsess about mysteries so later this afternoon I'll post at Adafruit.