- able to test low frequency noise of voltage references
- 0.1Hz to 10Hz bandwidth
- portable, easy to use
- low floor noise, 0.16uVpp level(same as Linear described in AN124f by Linear)General considerations
- self-contained, not necessary depending on oscilloscope or DMM for reading
- one 9V rechargeable lithium battery powered
- charge port, oscilloscope output port provided
- case: aluminium, 103mm by 76mm by 35mm, my usual type
- LED display: 4.5 digitsSchematics
There are several modifications to the Linear one
1. Input capacitor C1
I bought those wet tantalum several years ago but tested not good recently. After applied 10V for 10 hrs, there is still leakage of several uA. May be the voltage I applied is much larger than Linear(2.5V) or simply the caps are bad. Either way, I give up. I have several 80uF and 22uF film caps, tested very good, but requires too many to built up to 1000uF, and the volume is huge. Now the only option left seems to be MLCC. I bought some of those 47uF/50V before and there are still about 50 left. However, there are problems when I installed those capacitors, will be described later.
Add: later test showed that the current noise of ADA4528 is much smaller than specified. Therefore, a smaller value for C1 can be used such as 1000uF or 470uF.
2. Input resistor R1
It provides DC bypass for C1 to charge/discharge, and also function as the 0.1Hz HPF together with C1. The attenuation is -1.5dB at 0.1Hz because there is another HPF at later stage with similar property, summing up to -3.0dB at 0.1Hz.
3. Input protection
I first implement it without Rp2, D3 and D4, an opamp was fried.
Because the noise current pass thru those Rp1 and Rp2, keep them as low as possible, better not exceed R1 combined.
I use bc junction of low power transistors(such as 2SC1815, 2SA9012) for D1-D4, leakage current is around 1pA at -4V.
R2 and the switch provide slow charge for the capacitor when connect to a voltage source.
4. The amplifier
The magnification is 10000, same as Linear.
I don't want to use FET front end, and I'm not prepare to measure >10Hz.
My target is a dual opamp, voltage noise <=100nVpp 0.1Hz to 10Hz. There are many so called ultra low noise opamps that not satisfied this because they suffered from severe 1/f noise effect. For instance ADA4898 with 500nVpp noise, even if 20 paralleled, still >100nVpp.
Also, the current noise should be <=50pApp in 0.1Hz to 10Hz range, which generates <=50nVpp voltage noise at 1k impedance. Be noted also that there are many so called ultra low noise opamps that not satisfied this.
It seems that there are not many opamps left to satisfy these criteria except ADA4522-2, but I cannot find the source of purchase. I choose ADA4528-2 instead with very similar performance except the supply voltage is a bit low.
5. Post opamp part
I use two amplifiers in parallel to further reduce the noise, the outputs are connected together by two 620 ohm resistors, and add an 33uF capacitor(C3) for 10Hz LPF. Now the LPF is second order.
Becasue C4 and R5 is another 0.1Hz HPF, there is no need for separate filter stage as in AN124f circuit.
6. Meter part
This part can be omitted if one decide to use an oscilloscope only for output.
Unlike Liner that use paralleled bc junction and be junction of transistors for the low leakage diodes, I use only bc junction for the peak detection diode. The reverse break down voltage of a be junction may be very low, therefore Linear has to use added resistors and diodes for clamp.
R6a and R6b are just jumpers, value not important.
U4a and U4b are photMos AQY212GS, leakage only around 1pA at -4V and turn on current of only 1.5mA with sub-ohm turn on resistance.
(There is no photoMos symbol in the software I use, so that I use phototransistor symbol insdead)
U2B is the instrument amp because my LED meter is earth referenced.
I use manual reset switch only, although an automatic reset can be added if required.
The LED meter has 1.9999V range, operable from 3.4V to 20V and draw 18mA current. I modified the decimal point so that it display 199.99(uV)
7. Power supply
There are two lithium cells inside a 9V rechargeable battery, supply 7.2V to 8.4V for small current, nominal 8V.
U2A split this 8V to +4V and -4V. I choose RS3 a bit small so that it share the current necessary for LED meter which only draw current from positive rail.
D5 and D6 provide the negative supply for U1(max 5.5V for this ADA4528-2). This can be omitted if U1 is an ADA4522-2.
D7 and D9 provide charge route by input socket when power off. Caution should be taken to disconnect anything from the input socket when power off.
Rp3 provide the trickle charge current for C1 when power is off.
Here is the photo of the finished meter: