Hi Volt-Nuts, :-)
I got several emails from people on this forum why I dont start my own topic over "Voltage References"
Your Wish Is My Command But, please bear with me, I'm a dyslexic monkey, it takes a lot of time, to make reasonable sentences in Englels.
And yes, Google is my Friend.
OK enough blah blah...
The first design that I want to show here, I already posted in another topic.
It is designed approximately 3 to 4 years ago by me.
I've been playing for over 30 years with voltage references and the last 5 years more intensive.
But only in the last view years, I was able to buy the instruments, to test voltage references.
I still have no Fluke 732a reference, therefore I myself am trying to build a voltage reference with good specifications.
i have not the resources like Fluke to test my reference design.
But i have a fresh calibrated HP3458A and two new KEYSIGHT 34461A DMM's.
This is an older design of mine, but there are good decisions,
such as multiple safeguards against the misuse.
Let us begin the upper left, where the power supply is made for the 4x LT1021.
The power supply is built around a LM10 IC, which has a 200mV reference, I scale to approximately 8V. R8 and C3 filter the noise of this voltage, this is a low pass filter of about 5Hz.
The alert viewer falls on, that R 1 and R 8 has the same value, in order to avoid bias errors.
At the output of the opamp, which is pin 6, there is a 47 Ohm resistor (R2) which, together with R1, C1 and C2 also create a low pass filter.
The effect of this circuit is that there are only about 5uV RMS noise is present on the 8V power supply for the LT1021. The bandwidth of the measurement was 20Hz to 22Khz.
The 8V power supply is clean, there is no visible popcorn or schutgun noise.
The circuit might be a bit overdesigned.
It is necessary to have a good quality capacitor for C3, as it is not DC offset corrected, like C1.
Next part, mixing the references...To make the noise and drift lower of the already good LT1021, I use four of them in parallel mode.
R4, R5, R6 and R7, together with the capacitors C4 and C5 create a low pass filter, having a frequentie of about 3.9 Hz.
This filter is a "bootstrapt", R10 of 10K, ensures that the upper 330uF only sees a very low DC voltage, the leakage current is than very low.
Also, use good quality capacitors.
Gain of 2 amplifier.I used a selected LT12ACN opamp, i want to test with a LTC2057 or a Max44241.
The problem is, there is not much info, about about the low frequentie noise, especially the current noise is not properly specified...
I'm always fighting with the values of R12 and R17 (10K, down 0.01% Vishay S102) and the current noise.
The lowest noise and the greatest stability, you need low value for these resistors.
Low value for these resistors also means, that the dissipation in these resistors will go up.
D1 and D2 is for protection, normaly there is no voltage over the inputs, so no leakage.
To trim the Reference i use small 0.1% film resistors in parallel with a 10-turn potentiometer.
Perhaps it is better to inject current through a resistor a high value in the -input of the opamp?
Less connection @ the -input.
BufferThe output of the opamp goes to a 1x buffer.
As a result, the opamp has its maximum open loop gain.
At the output of the buffer are 4 resistors of 100 ohms, C11 and 330UF again make of a low pass filter.
C9 and R17 are also involved in this filter.
ProtectionThe reason for the 4x100 Ohm is this, it is an inexpensive way to ensure that the reference remains intact, when power is put on the output. Bad Boy!!!
L1 keeps HF away from de output of de buffer.
F1, fast blow 150mA, u can choose a lower value, if you want.
D5 PGKE18CA protects IC3 a buffer IC like EL2001, BUF634, LT1010 etc.
And this is a high resolution picture
http://www.bramcam.nl/Diversen/Quad-LT1021-10Vref-01.pngPlease shoot @ it, make it better ect. ect.
Make it a community project if you want
My version is housed in a 42C heathed BimBox, but about ovens, i wil show you later my designs.
Kind regarts,
Blackdog (Bram)