Author Topic: Low frequency Noise of Zero Drift Amplifiers  (Read 2495 times)

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Online chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #25 on: January 19, 2019, 02:56:15 pm »
Thanks for the work combining those graphs. Making a combined chart like that is on my to-do list.
 
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Online chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #26 on: January 22, 2019, 01:33:39 pm »
Here are the combined results of the Modified Allan Deviation analysis for the OpAmps.
 
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Offline Inverted18650

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #27 on: January 22, 2019, 04:48:43 pm »

Nonetheless: Thanks for this measurement series. Its impressive how much energy you put in this interesting project.  :-+

I'm retired now so I look for interesting projects to stay busy. Another fellow I worked with retired at the same time and he comes by 20-30 hours a week to help. I would have a lot more unfinished projects if it wasn't for Jim.

thanks chuckb and jim, great project.
 

Online chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #28 on: February 17, 2019, 07:10:20 pm »
The third version of the pcb has been built and data collection is happening now. The components were rearranged to minimize trace length between the Op-amps under test and the DIP switch that changes the configuration. This is a 4 layer PCB. Reference the attached photos

Three changes were made, just to make things better.
1. Two of the regulators were moved to a different corner of the pcb to spread out their heat.
2. A low frequency ferrite core was added to the input power line to help with 1 MHz interference.
3. Each power pin to each Op-amp now has a 100 ohm series resistor and a parallel combination of a 100ufd Tant Polymer cap and a 0.1ufd X7R cap for filtering. This filtering starts working (-3dB) at 20Hz.

Jim added heaters to the three metal enclosures so I can control the housing temperature better. I will also be able to test how some parameters change with temperature. I plan to test at 30 and 40 deg C. The heaters have 100k thermistors on the same PCB for quick heater temperature feedback. I monitor this resistance with a K2015 (same as K2000). I also added a TI TMP235 bandgap temperature sensor to the PCB. The ground pin of the TMP235 is soldered directly to a via connected to the three ground planes. The sensor was then covered with hot glue for mechanical strength and isolation from air drafts. This provides 10mV /K to another K2015 DVM. The thermistor on the heater is in the inner temperature control loop. Because of the tight thermal coupling the heater temperature is tightly controlled. I also added a slow outer integration loop with +-0.1 deg C authority that corrects if the PCB temperature is off of target. An insulating cover slows down the rate of temperature change to help the thermal control works better.

So far the TMP235 on the PCB and the control loops hold the PCB indicated temperature to +-0.02 deg C. The TMP235 is powered from 2.5v regulated supply and it draws 9uA so the temperature rise is less than 0.01C. This low current also makes it a noisy temperature sensor so I may change it in the future.

I added more selections for capacitance across the 100k shunt resistor that measures Bias current. Reference the attached schematic.

I also added a selectable 100k shunt resistor in the non-inverting input of the Op-amp. This allows testing similar to how other researches have tested the low frequency current noise.

I am currently checking the ADA4528 (5V), OPA189 (30V), ADA4522(30V), and the LTC2058(30V) Op-amps. So far the noise spectrums are within 1dB of previous testing. The gain is the same at 10,000 (80db). For some tests I am collecting data on one Op-amp for 2 days. These are not quick tests. A 2 channel spectrum analyzer and two K2015 DVMs are used to collect the data.

Future testing -
Low bias current choppers.

I have the OPA2189 (dual OPA189) Op-amps for testing. These are not stocked in Mouser or Digikey yet. I had to get them from the factory web site.

I also have an engineering sample of a very old Maxim MAX426 Super Op-amp from the early 90s. It has not been powered for close to 30 years. This Op-amp was way ahead of it's time but it never made it into production. This device used internal 16 bit DACs to store the offset voltage instead of caps. Caps take up a lot of space on the die. There is a note about this chip in AoE3 but I could not find it. I might be mentioned in the 1st or the 2nd book as the best Op-amp available...

 
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