Low frequency Noise of Zero Drift Amplifiers

[souldevelop]

thk! There will be a small offset when using DC coupling. I did this to facilitate observation.

[Electrole]

The test suite by chuckb of the various Zero-Drift Amplifiers is highly interesting.
Any ideas how a chopper amplifier like the ICL7650S from Renesas performs in comparison??
The data sheets states a typical "Change in Input Offset with Time" of 100 nV/sqrt(month), a typical input noise voltage of 2 µVpp from DC to 10 Hz, and a typical input current noise density of 10 fA/sqrt(Hz) at 10 Hz, but I have not found any noise density specifications as a function of the frequency.
BR

[chuckb]

Check the data sheet for the Texas Instrument ICL7652. It looks like a similar architecture with external capacitors. It has 90nV / rt Hz Voltage noise density and 4fa / rt Hz Current noise density at low frequencies.

The newest chopper stabilized amplifiers have roughly 20x less voltage noise with a higher bias current. I have not researched low bias current choppers.

[Kleinstein]

The ICL7650 is an old classic auto zero OP.  AFAIR I saw some noise spectrums and it follows the usual way for an AZ OP:  relative high noise for low frequenices and lower noise (like 1/2 - 1/3)  from some 5 kHz on.
The noise current looks good, but maybe too good - some of the very low numbres for the current noise are not real - more like an lower limit derived from the bias.

I just tested the microchip  MCP6V51 , which has good voltage noise specs at some 11 nV/sqrt(Hz).  I had not expected to get current noise as low as the data-seet number, but I had some hope i might be better than the otherwise comparable LTC2057. However the test showed way more current noise - more like 700-800fA/sqrt(Hz)  - so something like 5 x the LTC2057.

[Electrole]

The Texas Instruments ICL7652 looks quite much like the TLC2652A / TLC2652, and the noise density at low frequencies is certainly not impressive. I suspect the ICL7650s would show similar performance, given the age and topology of the device, but I do not know. For this reason, I was thinking about measuring the ICL7650S, but I'm finding the OPA189ID increasingly interesting. The wide supply voltage span is also a bonus, which makes it easier to use in some designs. One thing I have in mind is a buffer for a 10 V reference, for which the OPA189ID seems quite appropriate...
BR

[RandallMcRee]

Have you looked at the ADA4523-1?
https://www.analog.com/en/products/ada4523-1.html

For my 7v-10v transfer a few years ago I used the OPA189, today I would use the above.

[Electrole]

I made a parametric search at the AD web site the other day, but I somehow did not spot the ADA4523-1. Thanks!
Looking at the specifications the ADA4523-1 is definitely a device that targets some of the same applications as the OPA189, such as a voltage reference buffer. The two devices also seem to have a comparable price tag.
May I ask why you came to prefer the ADA4523-1 over the OPA189?
BR

[RandallMcRee]

I made a parametric search at the AD web site the other day, but I somehow did not spot the ADA4523-1. Thanks!
Looking at the specifications the ADA4523-1 is definitely a device that targets some of the same applications as the OPA189, such as a voltage reference buffer. The two devices also seem to have a comparable price tag.
May I ask why you came to prefer the ADA4523-1 over the OPA189?
BR

I wanted the lowest possible flicker noise. The input is several (eight) averaged LTZ1000 circuits so *their* noise is approx. 0.05 uV. So that opamp, in that position, is the major noise contributor.

[macaba]

May I ask why you came to prefer the ADA4523-1 over the OPA189?

I'm not Randall, but as someone who picked the ADA4523-1 for use in ultra precision metrology circuits, I have some input to make.

Have a look at Figure 7-11 of OPA189 datasheet, and Figure 14 of ADA4523-1 datasheet.

Essentially; the ADA4523-1 has input bias compensation that makes it extremely well behaved over temp range. This makes up for the fact that it has 1x - 2x the average input bias current of other devices. In practice, this means that if you have a 10k resistor on the ADA4523 input, the input bias current will only cause a 0.75uV drop over that resistor, and only vary by 0.25uV over the temp range from 25 to 100 degrees.

[Kleinstein]

I would not count too much on the figues give an the input current. Most curves are for typical units. The input current of chopper OPs in general does not change much with temperature, but it can be different a lot between individal units.

The AD4523 is very low current noise, but relatively high bias and high current noise.  There are very few referene that really need voltage noise that low.