Hi,
I'm going to explain why the schematic is structured like this.
But first this...
I still had a handful of good parts lying around and another one of my component test ovens, so I thought about building a 10V reference with it.
I just want to do the best I can with the parts I have here.
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The setup shown here will probably only be built once, so I don't take into account the parts costs and labour as this is unimportant to me.
About a month is here in a test box 2x LM399AH to age and from these then two are selected for low noise and absence of popcorn noise.
Andreas, thanks, i forgot the heater capacitor in the next version of the schematic is show it wil be there.
R4 and R5 mix the reference voltage of the two LM399AH IC's and these resistors are also used to create the low pass filter together with C1 to C6.
R6 helps to protect the +input of the opamp a little with the charge of the capacitors when switching off.
The opamp + buffer is besides a 1.4x amplifier also a low pass filter.
This is mainly meant to keep the noise of the amplifier and the buffer low.
The measurement of the noise without already being optimized was about 1uV RMS measured with my Audio Precision measuring set at a 22KHz bandwidth.
Of course this was using the 400Hz high pass filter.
This was necessary because despite the decent build on a breadboard en some shielding the 230V 50Hz field is still present.
This measurement was done in the test setup with 1x LM329 as zener.
But the noise tests were meant to find out which opamp works best with the precision resistors I have at my disposal.
Yesterday I tested three opamps, the LT1007AC, the OPA140 and the ADA4522 which is currently doing the best in my test setup.
Andreas, you're asking for the extra buffer if the ADA4522 can deliver peak currents of up to 80mA?
I don't like that this precision opamp is directly connected to the output.
My starting point was also, that the 10V output can be abused.
D7 and R20 is used for startup, this was not necessary for an ADA4522, but it was sometimes necessary for the OPA140 and LT1007.
When power on this circuit, it pulls the input of the LT1010 buffer positive for a moment.
In normal operation D7and R20 do nothing, a little leakage current if there is one, is controlled by the opamp loopgain, normaly that wil be no problem.
I'm going to do some more tests on these parts, to see if these two components can be connected directly to the +15V output, then they're completely out of the loop.
Q1 limits the output current, and in my test circuit this is a little less than 35mA short circuit current.
The problem I run into at the moment, is that I am against the noise floor of my Audio Precision Analyser...
This is about 0.8uV RMS in 22KHz bandwith.
A simple extra preamplifier is not possible.
This should be a symetrical preamplifier with low noise, high common mode suppression and a not no too low input impedance.
I am thinking about how I am going to build it.
So, this schematic isn't ready yet, but I'm satisfied with the output buffer.
It's solid and relieves the opamp of eny load and overload.
I also tested some of my transistor buffers after the opamp, but that wasn't a good plan as far as the noise behavior of the circuit is concerned.
This needs to be tested further:R10 + C8 and C10 with a small series resistor of say 10-Ohm and a 1uF capacitor directly over the output.
This to keep enough bandwidth to control the noise behavior of the LT1010 buffer.
The 2nd order filtering of the opamp + buffer is only meant to keep the noise of this circuit as low as possible, it is not meant to filter the noise out of the 2x LM399.
R4 +R5 and the 6x 10uF have a much lowerfilter corner.
So, this is not a circuit to become a better than an LTZ1000 reference, that was not the starting point.
But to build as good a circuit as possible with the present parts.
By building the temperature sensitive parts in a oven, I have almost no more problems with the Seebeck effect.
These are de components for R4 and R5, R7, R8 and R13, R14 and one of the 10uF capacitors.
When I've done some more tests, I'll show you the results here.
Thanks for your remarks!
Kind regards,
Bram