I am looking at the voltage reference circuit used in a source measurement unit, which is meant to be stable and precise:
The circuit above is said to be a 10V reference as described in the block diagram. (You can find it here on page 132.)
I have two questions about this.
- I understand that this will amplify the 6.4V Zener voltage to 10V, but the resulting voltage is not exactly 10V as I have calculated. It is about 10.3V instead. Even though the additional R76 is added, it will be even greater than 10.3V. Why is this referred to as 10V?
- The top of the Zener diode is not connected to 15V but the op-amp output instead. I can also see a low-pass filter along with it to filter out any high-frequency noises for stability. Why isn't it connected to +15V? Is there any difference?
I have done the simulation in terms of noise. The result shows no difference between both, see the image below. The
noise signal is a 15V with noise, which is defined as
I just noticed that the 15V might use a different ground reference point compared to the Zener diode's. Not sure whether this is the underlying reason of this.