Low noise op amp

[Jean-Pierre Coulon]

Is there any opamp better than Linear Technology's LT1028 regarding low
frequency noise? i.e. voltage noise about 1 nV/sqrt(Hz) and current noise
about 1 pA/sqrt(Hz), and 1/f corners below 10 Hz if possible.

It doesn't matter if the GB product is as low as 1 MHz.

Regards,

Jean-Pierre Coulon (here "cacas.pam" is what others call "nospam")

 

[Eeyore]

Is there any opamp better than Linear Technology's LT1028 regarding low
frequency noise? i.e. voltage noise about 1 nV/sqrt(Hz) and current noise
about 1 pA/sqrt(Hz), and 1/f corners below 10 Hz if possible.

It doesn't matter if the GB product is as low as 1 MHz.

AD8599 is damn close and has a GBP of 10 MHz. Might be a useful alternative
candidate.

Graham

 

[Spehro Pefhany]

AD8599 is damn close and has a GBP of 10 MHz. Might be a useful alternative
candidate.

Graham

Unfortunately the current noise 1/f corner is high.. current noise at
10Hz is 6pA/Hz^1/2

I don't know why there are no monolithic JFET-input amplifiers
optimized for low voltage noise (of course they don't have much
current noise). Not many hybrids out there even.

 

[Vladimir Vassilevsky]

Is there any opamp better than Linear Technology's LT1028 regarding low
frequency noise? i.e. voltage noise about 1 nV/sqrt(Hz) and current
noise about 1 pA/sqrt(Hz), and 1/f corners below 10 Hz if possible.
It doesn't matter if the GB product is as low as 1 MHz.

There was application note from Analog Devices about using an opamp with
the external input stage made of four SSM-2210 in parallel. IIRC it had
0.5nV/1pA performance with 1/F corner at 1 Hz.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Fred Bartoli]

JK17PWGBDR a écrit :

Hmmmm. A 32 Ohm resistor generates 1nV/root Hz at room temp.
I know curiosity killed the cat but I have to ask.
What are you trying to amplify that has a source
impedance less than 32 Ohms and a frequency
response where 10 Hz matters?

1nV/rtHz is about 60R, not 32R.
Some other values good to remember are:
50R is 0.9nV/rtHz
1K is 4nV/rtHz
1M is 128nV/rtHz

But 32R isn't that far, at 0.73nV/rtHz.
(obviously about a sqrt(2) factor below 1nV)

 

[David L. Jones]

Is there any opamp better than Linear Technology's LT1028 regarding
low frequency noise? i.e. voltage noise about 1 nV/sqrt(Hz) and
current noise about 1 pA/sqrt(Hz), and 1/f corners below 10 Hz if
possible.
It doesn't matter if the GB product is as low as 1 MHz.

Regards,

Jean-Pierre Coulon (here "cacas.pam" is what others call "nospam")

Once you reach the lower limits like this you can decrease your noise
further by paralleling the best amp you can get.

Interestingly, AD have a patent on this for use at the substrate level:
http://www.google.com/patents/about?id=aZCpAAAAEBAJ&dq=7339428

Dave.

 

[Spehro Pefhany]

Hmmmm. A 32 Ohm resistor generates 1nV/root Hz at room temp.
I know curiosity killed the cat but I have to ask.
What are you trying to amplify that has a source
impedance less than 32 Ohms and a frequency
response where 10 Hz matters?

1K ohm at 4.2K only has ~0.5nV/Hz^1/2 Johnson-Nyquist noise.


Best regards,
Spehro Pefhany

 

[Jean-Pierre Coulon]

Hmmmm. A 32 Ohm resistor generates 1nV/root Hz at room temp.
I know curiosity killed the cat but I have to ask.
What are you trying to amplify that has a source
impedance less than 32 Ohms and a frequency
response where 10 Hz matters?

I am measuring current fluctuations from a photodiode from 1Hz to 10kHz.

I put a 50 Ohms in transimpedance mode, so the opamp noise voltage
quadratically adds to the .9 nV/sqrt(Hz) of the 50 ohms.

Later on I will put a C-R high pass filter below 1 Hz, and a non-inverting
stage with gain=10 or so. At present the AD797 seems the right choice, but
I'll make noise calculation with my two stages.

Jean-Pierre Coulon

 

[Spehro Pefhany]

...meaning not much of a market..but OK for DIY?

Okay, but it would be nice (as in I'd be happy to pay for it) if it
was available as a fairly small hybrid (fully spec'd and tested, of
course, with guaranteed voltage noise and input capacitance).


Best regards,
Spehro Pefhany

 

[John Devereux]

Cool. I have a few suggestions for you. At the noise levels you
are talking about you will need to take extraordinary
precautions to prevent external noise ruining
your measurements.

1) Heat sink the photodiode. Even a very small change
in temp can affect the leakage current. Even somebody walking
by can make enough of a draft to affect the reading.

2) Place the photodiode and first amplifier in a heavy
metal box and keep it at a controlled temperature.
If you are going to allow light to enter the box onto
the photodiode make sure no light can get past
into the rest of the box.
There are several reasons for this.
a) any temperature change between readings will
cause the leakage currents in the photodiode to change
and change your readings. It might be interesting to vary
the temp and see how much the photodiode leakage is
affecting your readings.

Also an alternative could be to run the photodiode in photovoltaic mode
(zero bias). Then there will be no leakage current since there is no
voltage across it.

This does increase the PD capacitance, may not be a problem at the low
frequencies being mentioned here?

 

[Jean-Pierre Coulon]

Cool. I have a few suggestions for you. At the noise levels you
are talking about you will need to take extraordinary
precautions to prevent external noise ruining
your measurements.
[big snip]

I've made spectrum comparisons with the photodiode without any light source,
and without the photodiode,

I obtain the same spectrum. And the spectrum values agree with what I
calculate for a particular op amp.

Jean-Pierre Coulon

 

[Jean-Pierre Coulon]

Also an alternative could be to run the photodiode in photovoltaic mode
(zero bias). Then there will be no leakage current since there is no
voltage across it.

I seems the photodiode series resistance is pretty high - I'm still
researching this - so if my incident light varies between curves (2) and (3)
of fig.2-2 of JK17PWGBDR's Hamamatsu document, even if my opamp does its job
of forcing its negative input to 0V, the voltage at the "true" photodiode
will vary between VDC and VDC', and the current through the series resistance,
which I can read, will vary in the corresponding proportion, which is too low
to my taste.

Jean-Pierre Coulon

 

[Rene Tschaggelar]

Is there any opamp better than Linear Technology's LT1028 regarding low
frequency noise? i.e. voltage noise about 1 nV/sqrt(Hz) and current
noise about 1 pA/sqrt(Hz), and 1/f corners below 10 Hz if possible.

It doesn't matter if the GB product is as low as 1 MHz.

Regards,

Jean-Pierre Coulon (here "cacas.pam" is what others call "nospam")

Better ? in terms of ? I notice the so called low noise
OpAmps, where some sub-nV per root-hertz are specified,
are having a rather high bandwidth. Also observe that
these low noise specifications are tied to a rather
low impedance, usually below 100 Ohms, or less. This
is physics though and cannot be cheated. However what
can be improved is the bandwidth, if not really required.

Rene

 

[kflynn11]

Depends on the definition of "better". The LT6200 offers nearly the voltage and current noise of the LT1028, but with rail-to-rail input and output and single supply down to 3V. It won't be lower noise than the LT1028 (physics limit ever bettering that part), but the LT6200 might be more usable in a modern single supply system.