Low frequency Noise of Zero Drift Amplifiers

[chuckb]

I tested two of the LT1028A Ultralow Noise Bipolar Op Amps. The power supplies were +- 5Vdc and the chips were in a plastic DIP package in a machined pin gold plated socket. The "A" version of the Op Amp has been tested for better DC performance and lower noise. The SMD version of the Op Amp is not available in the "A" version.

The LT1128A has slightly less speed (20 MHz vs 75 MHz) but it is stable for a +1 buffer application. The voltage noise and current noise should be similar to the LT1028A. There is also a version just for audio, the LT1115. It has relaxed DC specs.

I first tested the Voltage noise with the gain at 80 dB and the input grounded. The results are attached.  I was surprised how well the noise curves matched. The offset voltage was less than 10uV for both Op Amps. The temperature coefficient was +0.3uV / deg C between 31 and 41 deg C.
 
Then I tested the Current noise of both Op Amps with a 1000 ohm shunt on just the non-inverting input. The bias current was around 30na. The final test was with a 1000ufd Aluminum Electrolytic cap connected across the 1000 ohm current shunt. This lowered the current noise down to the level of the voltage noise for frequencies higher than 10Hz.

The next results will be for the MAX426. It has several operating modes so it takes longer to gather the data.

[chuckb]

The first three LT1028A Op Amps that I tested were manufactured in 2000-2001. So I picked up another one with 1835 date code (35th week of 2018). The voltage noise was the same as the other three. Looks like their process is not changing over the years.

I wanted to see where the LT1028A flicker noise floor changed to white noise. So I did an extra test at a much higher sampling speed of 20 Hz. As you can see in the second graph if you are averaging the signal for longer than 2 seconds the OPA189 or ADA4528 have a lower noise than the LT1028A.

I tested the OPA2189 (Dual version of the OPA189) at 30V in a SOIC-8 package. The voltage and current noise was the same as the single version.

[maxwell3e10]

It looks like OPA2189 is an all-around winner, with a higher voltage range and a little smaller input current noise than ADA4528.  I was also surprised that it has  a much larger slew rate and gain bandwidth than other zero-drift op-amps. So got some and tested them briefly:
https://www.eevblog.com/forum/projects/what-is-your-favorite-most-versatile-op-amp/msg2457006/#msg2457006
I was only able to find one peculiar behavior at 67.5 kHz that is probably associated with the chopper.

[alex-sh]

Did you test OPA388 or ADA4528?

[chuckb]

Did you test OPA388 or ADA4528?

I have not tested the OPA388 but it looks like a very modern design (2016).
The ADA4528 was one of the first designs tested. The results can be found in the first post in this topic.

[chuckb]

Here are two more low noise Op Amps in this never ending story.
The ADA4523 is a new very low noise chopper from Analog Devices. The datasheet was released April 20, 2020. I had to get samples from the factory because the distributor did not have any stock yet.

https://www.eevblog.com/forum/metrology/new-chopper-opamp-from-ad-ada4523-1/msg3076193/#msg3076193

The LT6018 is a very low noise bipolar Op Amp. It was introduced in 2017 time frame. It has a 33V power supply limit and high bias currents. It has been on my list of Op Amps to test.

Today I just have the voltage noise spectrum for you. I collected current noise data also and will present that when I get some time.
I used +-5V power for both Op Amps. The enclosure was not temperature controlled this time. However the lab had less than +-0.3 deg C variation and the heavy insulated enclosures provided several hours of thermal lag. Each amplifier was configured for a gain of 80dB using 10 ohm and 100k ohm metal film SMD resistors. The amplifier outputs were measured with an HP35665A Low Frequency Spectrum Analyzer.

The new ADA4523 chopper has 20% less flicker noise than the OPA189. The bias current of my sample was 50pa.

The LT6018 is an exceptional bipolar opamp for white and flicker noise. It has roughly 1/3 the flicker noise of the LT1028A. The bias current of my sample was 61na.
More results to follow.

[Bud]

The vertical scale (or Y axis label) is incorrect.

[chuckb]

I corrected the Spectrum Analyzer model number and the label on the Y axis of the graph in my previous post.

[exe]

Wow, those modern auto-zero opamps seems to be quite good. I'm talking about opa189.

[chuckb]

Attached are the bias current noise plots for the ADA4523 and the LT6018.

The non-inverting input has the indicated resistance and capacitance to ground. The amplifier is configured for a gain of 80dB with 10 ohms on the Inverting input.

The ADA4523 bias current noise is much improved compared the the previous ADA4522.

The LT6018 bias current noise is very similar to the LT1028A noise spectrum. For this test I measured unbalanced current noise. The input resistances are not balanced (1 kohm and 10ohm). My results match the datasheet unbalanced data within 10%. The datasheet shows a 6x current noise reduction with balanced input resistances in the flicker noise region, I have no reason to doubt that. I will not be testing that.

[macaba]

Thank you for all your hard work. The ADA4523 is an impressive device.

I have seen references to putting a small capacitor across the inputs of zero drift devices to reduce noise, I would be interested to see an investigation into this for the leaders (ADA4523, OPA189).

[chuckb]

I noticed early on that the bias current appeared to change with different input capacitance.
I decided to plot offset voltage vs input impedance for the ADA4523. The Op Amp has a 330kHz input switching freq so that's what I used to compute input Z vs offset. There are usually input current spikes at 10x this freq also.

The DC input resistance is 100 kohms, that causes the 5uV offset from the 50pa of input bias current. The extra input capacitance is added to reduce the input impedance at the switching frequency. This seems to stabilize the operation of the Op Amp. Reference the attached graph. In this setup the inverting input has 10 ohms to ground so it is not disturbed by the input switching spikes.

The OPA189 needed the full 100nF to stabilize it's operation also.
The OPA187 (Low bias current, low power) only needed 1nF to achieve stable voltage.
 
There is much more to research and experiment on this topic.

[SilverSolder]

I noticed early on that the bias current appeared to change with different input capacitance.
I decided to plot offset voltage vs input impedance for the ADA4523. The Op Amp has a 330kHz input switching freq so that's what I used to compute input Z vs offset. There are usually input current spikes at 10x this freq also.

The DC input resistance is 100 kohms, that causes the 5uV offset from the 50pa of input bias current. The extra input capacitance is added to reduce the input impedance at the switching frequency. This seems to stabilize the operation of the Op Amp. Reference the attached graph. In this setup the inverting input has 10 ohms to ground so it is not disturbed by the input switching spikes.

The OPA189 needed the full 100nF to stabilize it's operation also.
The OPA187 (Low bias current, low power) only needed 1nF to achieve stable voltage.
 
There is much more to research and experiment on this topic.

Are you placing the 100nF between non-inverting and ground, after a 100K resistor?  (like a ~15Hz filter?)

[Kleinstein]

The impedance / capacitance at the input effects the charge injection of the chopper switches. The switching should be really fast (e.g. more like 10 ns range) and thus really high frequencies (e.g. 100 MHz to a few GHz) that matter.
The effect of capacitance on the charge injection at CMOS switches is known.
At the high frequencies it may be more than just the capacitance, but also ESR, ESL that can have an effect.

It is a little surprising the it takes so much capacitance to saturate the effect.
It could be interesting to have the same curve with a smaller resistor (e.g. 1 K), so that one also has more of the range where  R*C is less than some 3 µs.

[SilverSolder]

The LT6018 looks pretty amazing as well,  but it isn't exactly cheap...

[SilverSolder]

The impedance / capacitance at the input effects the charge injection of the chopper switches. The switching should be really fast (e.g. more like 10 ns range) and thus really high frequencies (e.g. 100 MHz to a few GHz) that matter.
The effect of capacitance on the charge injection at CMOS switches is known.
At the high frequencies it may be more than just the capacitance, but also ESR, ESL that can have an effect.

It is a little surprising the it takes so much capacitance to saturate the effect.
It could be interesting to have the same curve with a smaller resistor (e.g. 1 K), so that one also has more of the range where  R*C is less than some 3 µs.

If the impedance seen by the + and - inputs are different, perhaps the charge injection behaves differently between the two inputs, and that difference ends up being amplified by the gain of the op amp?  Perhaps there are slight differences between the internal switches on the two inputs as well, which also end up being amplified?  (Just thinking out loud...)

[chuckb]

I noticed early on that the bias current appeared to change with different input capacitance.
I decided to plot offset voltage vs input impedance for the ADA4523. The Op Amp has a 330kHz input switching freq so that's what I used to compute input Z vs offset. There are usually input current spikes at 10x this freq also.

The DC input resistance is 100 kohms, that causes the 5uV offset from the 50pa of input bias current. The extra input capacitance is added to reduce the input impedance at the switching frequency. This seems to stabilize the operation of the Op Amp. Reference the attached graph. In this setup the inverting input has 10 ohms to ground so it is not disturbed by the input switching spikes.

The OPA189 needed the full 100nF to stabilize it's operation also.
The OPA187 (Low bias current, low power) only needed 1nF to achieve stable voltage.
 
There is much more to research and experiment on this topic.

Are you placing the 100nF between non-inverting and ground, after a 100K resistor?  (like a ~15Hz filter?)

That is correct. Ref this link for the schematic and pictures of the PCB layout with the DIP switches selecting the capacitance.

https://www.eevblog.com/forum/metrology/low-frequency-noise-of-zero-drift-amplifiers/msg2206434/#msg2206434

[souldevelop]

I used ADA4523-1 to make a low noise amplifier which is ULNLFA-100, and its performance is as follows:

Input noise voltage: 40nVp-p in the range of 0.1 Hz to 10 Hz
Spectral density voltage: 2nV/√Hz at 1kHz
Maximum voltage gain: 1000000 or 120dB
Gain control: 4 levels (60dB 80dB 100dB 120dB)
Low-pass filter: 3 gears (<10Hz, <100Hz, Full)
Power supply: 2 pieces of 103450 4.2V 2000mAH lithium battery
Power supply voltage: ±4.2Vdc, maximum power supply voltage ±15Vdc
Charging interface: miniUSB 5V 300mA
External dimensions: (length/width/height) 13CM/7.65CM/3.5CM
Total weight: 308 grams (including battery)

ULNLFA-100 uses 8 pieces of ADA4523-1 in parallel to achieve low noise performance of 40nVpp.
The total cost of making it is about 140$. I still have some remaining machines here but the number is not much. If you need it, plz contact me.

[Andreas]

Input noise voltage: 40nVp-p in the range of 0.1 Hz to 10 Hz

Hello,

how is the input impedance?

Is there any protection of the source against excessive charge current of the input capacitor?

with best regards

Andreas

[souldevelop]

1.input impedance is about 4.7k.
2.Inside are two 4.2v lithium batteries, which are connected in series when charging,
   the maximum charging current protection limit is 500ma, and the working current limit is 100ma.

[TiN]

Andreas was asking about input signal current pulse, when you connect e.g. 10V source to preamp, while input AC capacitor is not charged (at 0V).

[souldevelop]

Sorry, I haven't considered this issue yet. Any good suggestions?

[macaba]

Your board layout looks nice but potentially a few issues:

- 4.7k resistor is an unusually large value that generates noise.
- Input current noise of ADA4523 is high... 8 in parallel? 
- Your Rigol scope traces have AC-coupling enabled (which is a HPF of something like 5Hz/10Hz)

[Kleinstein]

For AC coupling the resistor to ground adds to the noise, but not in the normal sense. It adds to the current noise and thus a smaller resistor adds more noise. So the 4.7 K to ground are more an the low side, but it depends on the amplifier.

The input current noise of the AZ OPs is relatively high and with more units in parallel this adds up. So the amplifier is only good for a low impedance source and a large input cap is needed.  The current noise of the amplifier together with the capacitor set the useful lower frequency limit. For the lowest end, it may have actually be better to have only 1 amplifier active.

Noise wise it would make sense to set the lower frequency limit not by the RC at the input, but have a larger resistor and set the lower frequency limit later (e.g. in software).

[souldevelop]

Thank you for your reply. Indeed, as you mentioned, I experimented with more than 8 4523-1. The noise reduction is not so obvious. I will publish the circuit diagram as follows to see if I can improve it.