Author Topic: Low frequency Noise of Zero Drift Amplifiers  (Read 6904 times)

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Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #25 on: January 19, 2019, 03:56:15 am »
Thanks for the work combining those graphs. Making a combined chart like that is on my to-do list.
 
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Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #26 on: January 22, 2019, 02:33:39 am »
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, 05:48:43 am »

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.

Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #28 on: February 17, 2019, 08:10:20 am »
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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Offline serg-el

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #29 on: February 25, 2019, 11:27:53 pm »
Max426 datasheet  ;)
« Last Edit: February 26, 2019, 06:47:02 am by serg-el »
 
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Offline SilverSolder

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #30 on: February 26, 2019, 04:32:03 am »
That's pretty impressive specs for the 1990s!
 

Offline serg-el

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Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #32 on: February 27, 2019, 05:35:06 am »
This data collection takes a while when you have two temperatures and two different current measurement techniques to check. Along with different capacitance values for filtering. I'm about halfway through the testing with these 4 Op-amps. I'll generate some plots tomorrow for the data I have collected so far.

The MAX426 is an interesting chip for the electronic historians. Attached is some more info about it.
 

Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #33 on: February 27, 2019, 05:39:01 am »
The preliminary Datasheet for the MAX426.
 
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Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #34 on: February 27, 2019, 05:54:08 am »
Here is how Cirrus built a Zero Drift Op-amp with 300dB gain at 1 Hz, the CS3001. Also attached is some chopper history.
« Last Edit: March 08, 2019, 09:24:03 am by chuckb »
 
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Offline serg-el

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #35 on: February 27, 2019, 08:50:28 am »
First mention MAX425/426.
And datashit in better quality  ;)
 
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Offline Echo88

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #36 on: February 27, 2019, 11:20:52 am »
Interesting datasheet and autozero-concept, thanks! Its a bit like the OP mentioned in AoE 3 (dont remember the name) which eliminates its own Offsetvoltage once when its powered up.
 

Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #37 on: March 05, 2019, 09:51:09 am »
When I went to the Rev C PCB I added temperature control to the three metal housings.

I started with the TMP235 sensor from TI because it has a very low operating current of 9ua. However it has a surprising amount of noise. I measured 0.030 K peak to peak. This sensor was better than no temperature control but I thought I could find a sensor with lower noise.

After a week of testing Jim changed the Chopper PCB temperature sensor to a 300ua LM335. He kept the sensor ground lead as short as possible for best heat conduction to the three ground planes. I should have also glued the TO-92 sensor down to the PCB. That will be the next revision.

The LM335 sensor is much quieter than the TMP235; it had less than 0.001 K peak to peak noise. The indicated temperature was within 0.5 K of the TMP235 sensor. Note: I used a K2015 DVM with a moving average of 20 samples for both sensors. The TMP235 used the 1V range and the LM335 uses the 10V range.

The normal AC coupling of the HP35665A Digital Spectrum Analyzer works down to 1 Hz. I needed AC coupling down to 0.001 Hz so I built a custom coupling capacitor. I used eight of the KEMET Polyester 47 ufd 63 V, R60DR54705050K film caps. The 400ufd was 0.5dB down at 0.001 Hz. For faster settling I added a switch for selectable low frequency limits of 0.1 and 0.01 Hz. See photo and schematic. I use the 0.1 Hz position for quick charging and settling when a configuration changes.

I completed current noise testing for the ADA4528 and the OPA189. The first test (labeled Single) was with a 100 kohm sense resistor on just the non-inverting input. A 111 nF C0G was across the resistor to average current spikes. The second test (labeled DIFF) kept the first resistor and cap but added a second resistor and cap in series with the inverting input and the feedback network. The two current curves are on the attached plots.
The 111 nf causes the rolloff of the blue trace at 10 Hz. When a 10ufd is placed across the 100k the current noise roll off happens at 0.1 Hz.

I plotted the Voltage noise of the ADA4522-1 and the OPA189 at different power supply voltages. All power supplies were symetrical. For example the
30 V curve is configured as +-15V power. The    ADA4522-1   ADA4522-2 picks up a little low frequency noise at 30 V. The chip has    6   12 times the power dissipation at 30 V compared to 5 V. It's just a sample of one also.
   
« Last Edit: April 16, 2019, 02:13:14 pm by chuckb »
 
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Offline SilverSolder

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #38 on: March 05, 2019, 02:09:33 pm »

Can we be sure that the HP35665A  is as quiet at those low frequencies as the very high quality parts under test?
 

Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #39 on: March 05, 2019, 04:15:53 pm »

Can we be sure that the HP35665A  is as quiet at those low frequencies as the very high quality parts under test?

That's a great question. See the attached file.

I first started evaluating the amplifiers with a gain of 60 dB. Then I saw that all three amplifiers had the same low frequency noise. So I increased the gain to 80dB. I only use metal thin film resistors for this low level work. The 10 and 100,000 ohm resistors in the feedback network are 5  10 ppm/ deg C, 0.1% accuracy parts. So the gain is stable. The temperature is also stable to minimize thermocouple and potential issues with op amp offset voltage drift.

The downside of using that much gain is that it limits the high frequency performance of the op amps because of GBWP limitations. I collect data out to 1KHz but because of two different high frequency roll off issues I only report performance out to 100 Hz. The OPA189 with a 14MHz GBWP has much better performance at 1 kHz than other op amps with a 1 or 3 MHz GBWP.

Part of this research is to decide which chopper to use as a preamp for my DSA. Just for fun, I attached is a photo of the 1.5F coupling cap for that preamp. There are a lot of 22,000 ufd caps in that box! It uses a triax connector to connect with the preamp. The switch adds a 10k resistor in series to limit charging current.
« Last Edit: March 26, 2019, 02:49:05 pm by chuckb »
 
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Offline SilverSolder

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #40 on: March 05, 2019, 04:44:55 pm »
Interesting, so it seems the HP35665A is more or less out of steam at 0.001Hz for this kind of testing? (Which is impressive, still!)

Impressive capacitor box...  I'm beginning to see why filtered references might not be completely practical, if done right!
« Last Edit: March 05, 2019, 05:17:33 pm by SilverSolder »
 

Offline alanambrose

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #41 on: March 07, 2019, 02:18:14 pm »
Super interesting, many thanks for this work :)
“A foolish consistency is the hobgoblin of little minds"
 

Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #42 on: March 08, 2019, 04:11:08 am »
...
I plotted the Voltage noise of the ADA4522-1 and the OPA189 at different power supply voltages. All power supplies were symetrical. For example the
30 V curve is configured as +-15V power. The ADA4522-1 picks up a little low frequency noise at 30 V. The chip has 6 times the power dissipation at 30 V compared to 5 V. It's just a sample of one also.
 

Correction, the noise plot at 30V is using the dual version of the chip, the ADA4522-2. So the 30V curve has 12x the power dissipation of the 5V curve (single opamp).
« Last Edit: October 04, 2019, 02:49:48 pm by chuckb »
 

Offline Inverted18650

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #43 on: March 09, 2019, 07:09:01 am »
I’ve downloaded all the files. Thanks again, this is great work!

Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #44 on: March 25, 2019, 02:53:52 am »
I got busy with other projects and I just let the system collect data for 12 days. This is over 900,000 data points for the ADA4528 and the OPA189. At 1 day the Allan Deviation stability of some of the op amps is better than 1nV. That pretty amazing! See attached.

The zero input stability of the ADA4528 and the OPA189 are similar to the HP34420A Nanovoltmeter. All three have a flicker noise floor around 0.3nV at 1000 seconds.
Link to DVM stability results plot - https://www.eevblog.com/forum/testgear/nanovoltmeters-performance/?action=dlattach;attach=501782

I tested the ADA4522-2 for current noise with a single and dual 100k input resistors. See Attached. With a single 100k resistor on the non inverting input I recorded the current noise with a 0.1 uF C0G and a 10 uF Film capacitor across the resistor. During differential current noise testing both 100k resistors were bypassed with 0.1 uF C0G capacitors. The 100k resistors have 0.1% tolerance.

The testing continues...


 
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Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #45 on: March 25, 2019, 03:17:51 am »
Just wanted to note that I corrected this old post. I updated the name on the file also.

I tested three normal amplifiers for low frequency noise. This was to provide a baseline for comparison to the Zero Drift amplifiers.
All three amplifiers were tested at +-5Vdc and a Gain of 10,000. I used the same 10 ohm and 100k ohm feedback resistors to set the gain as I used on previous tests.

The OPA227 (CORRECTED, WAS OPA277) bipolar amplifier was tested. It's low frequency flicker voltage almost reached the performance of the ADA4522 chopper!

...blah, blah, blah...
 
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Offline 3roomlab

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #46 on: March 26, 2019, 01:38:49 am »
this 2017 cern article says the new adc design uses 4522
I wonder why not 4528  :-//
https://indico.cern.ch/event/725164/contributions/2989640/attachments/1642142/2623032/new_ADC.pdf
spheres of influence, example linustechtips. can you feel the brainwashing? showing off equipment, etc. were you swayed and baited? with immense popularity (and social "titles"), can you afford to disagree?
 
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Offline RandallMcRee

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #47 on: March 26, 2019, 02:21:48 am »
this 2017 cern article says the new adc design uses 4522
I wonder why not 4528  :-//
https://indico.cern.ch/event/725164/contributions/2989640/attachments/1642142/2623032/new_ADC.pdf

The 4528 is a +-2.5v opamp. The 4522 is +-18. Their adc is +-13volts or so.
 
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Offline chuckb

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #48 on: March 28, 2019, 08:38:12 am »
Thermal performance summary.
Reference attached pdf.

I did not want the ambient temperature reading influenced by the heated enclosure so the ambient temperature was measured 12" away from the enclosures and 6" above. An HP2804A Quartz Thermometer monitored the ambient temperature and reported it with 0.001C resolution.

Three heaters with integral 0402 sized 100k ohm thermistors monitored the heater temperature. The thermistors were used to form an inner control loop for the heater temperature. The heater temperature was controlled by a PI controller with local, fast thermistor temperature feedback.

The PCB temperature monitoring used an LM335 sensor and a Keithley 2015 6.5 digit DVM. The LM335 provides 10 mV per deg K. At the 30 deg C target temp the sensor provided 3.03V to the K2015 DVM. The DVM was on the 10V range and it used a 40 sample moving average filter (about 40 seconds) to minimize DVM induced noise. Reference the lowest trace on the attached pdf for the DVM noise level with a shorted input on the 10V range. The DVM noise is mostly below 10u deg C.

The three heavy, sealed, metal enclosures (1.5kg total) added several minutes of time delay between external temp changes and surface of the pcb. The LM335 sensor was used to bias the target temp of the external heater controller. The LM335 PI controller was very slow and it had a very limited +- 0.2 C range. There is an offset between the sensor temp and the pcb temp but I'm considering that a constant.

Overall, the temperature control system worked well because the indicated pcb temp stabilty was better than 0.2 mC.

NOTE: The graph indicates a long term (3 days) temperature stability of 20 uC. That may not be reality because the control LM335 is also the monitor LM335. If the sensor drifted I would not be able to tell.

Does anyone have experience with LM335 temp sensor drift at 30 deg C?
 
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Offline Andreas

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Re: Low frequency Noise of Zero Drift Amplifiers
« Reply #49 on: March 28, 2019, 07:53:17 pm »

Does anyone have experience with LM335 temp sensor drift at 30 deg C?

Not directly drift but the self heating with 1mA current is quite high (around 0.8 deg C in still air)
I mentioned this when using as outdoor sensor.
Each time when the wind was blowing I had a jump in temperature.

I reduced this jumps by switching on the LM334 current source only during measurement.

with best regards

Andreas

 
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