Looking for a low noise reference circuit for an audio regulator, at 2.5V? But you’ve found bandgap circuits too noisy? Read on!
Consider the simple circuit to the right, which I call the GLED431. Just 3 low cost parts, all easy to get. It acts like an extremely low noise 2.5V zener. On my setup, noise measures around 2nV/√Hz, so if you take out the measuring system noise, the actual noise is likely below 1nV/√Hz. Really quite good. We’ll have more on this later on, in 2016.
While the GLED431 performance is very high for noise, you will need to apply about 5mA (or more) to make it work. Yes indeed, this current threshold is much higher than that of the TL431. But, it also has around 1/100 the noise! Caveat(1): The voltage won’t be as tight as typical bandgap ICs, nor as low for temperature drift. Those are conscious tradeoffs.
Here are some Vout measurements on a sample set of 5 LTL-4231Ns, in the lab prototype shown, after 1 minute warmup:
#1: 2.5094V, #2: 2.5093V, #3: 2.5069V, #4: 2.5019V, #5: 2.5062V
Not too shabby! In the schematic, the leftmost R values are just as shown from lab tests, as trimmed for the 2.500V target Vout. Obviously, just use a single 150Ω RN60D unit for this R. Note that the forward voltage of the LTL-4231N green LED (LiteOn) and the Vbe of the ZTX951 (Diodes Inc.) conveniently add, producing the desired Vout of 2.5V. Caveat(2): These two parts should not be changed if you expect to get close to 2.500V!
In use, if you are building say, a 5V regulator, select a series resistor so that 5mA is supplied to the GLED431 cell (499Ω). With this, also be sure to select a very low noise op amp, and reduce all the surrounding resistances, so as to minimize their noise contributions. Finally, be careful to minimize capacitive loading.
I am now releasing this simple version, as a Christmas present to the readers. Stay tuned for more, have fun with the GLED431, and have a great holiday!
December 24, 2015