interference and noise question

[Boris]

Hello

I'm working on a project using a load cell that needs to amplify a low
signal with a very large gain. I have, for exemple, 2 stages of gain.
The first(using an instrumentation amplifier) the gain is set to 500x.
The second gain (using an opamp) is 20x. The problem is that if I
increase too much the gain, there are a lot of interference. If my hand
gets closer to the device it increases 1V, and Im supposed to have a
precision circuit! I've read about ground loops and stuff, but is there
anything someone can suggest to have a high gain unity without
interference? Is some kind of isolation needed?
Thanks a lot
Boris

 

[Eeyore]

Hello

I'm working on a project using a load cell that needs to amplify a low
signal with a very large gain. I have, for exemple, 2 stages of gain.
The first(using an instrumentation amplifier) the gain is set to 500x.
The second gain (using an opamp) is 20x. The problem is that if I
increase too much the gain, there are a lot of interference.

What sort of interference ?

I hope the first stage is differential and symmetrical impedance on both inputs
! More info is required such as the device and circuit configuration.

Graham

 

[martin griffith]

Hello

I'm working on a project using a load cell that needs to amplify a low
signal with a very large gain. I have, for exemple, 2 stages of gain.
The first(using an instrumentation amplifier) the gain is set to 500x.
The second gain (using an opamp) is 20x. The problem is that if I
increase too much the gain, there are a lot of interference. If my hand
gets closer to the device it increases 1V, and Im supposed to have a
precision circuit! I've read about ground loops and stuff, but is there
anything someone can suggest to have a high gain unity without
interference? Is some kind of isolation needed?
Thanks a lot
Boris

1) how far is the cell from the amplifer (INA)
2) is the cable between the cell and INA screened?
3)the INA is in a differential configuration, is it?
4) what bandwidth do you need?


martin

 

[John Larkin]

Hello

I'm working on a project using a load cell that needs to amplify a low
signal with a very large gain. I have, for exemple, 2 stages of gain.
The first(using an instrumentation amplifier) the gain is set to 500x.
The second gain (using an opamp) is 20x. The problem is that if I
increase too much the gain, there are a lot of interference. If my hand
gets closer to the device it increases 1V, and Im supposed to have a
precision circuit! I've read about ground loops and stuff, but is there
anything someone can suggest to have a high gain unity without
interference? Is some kind of isolation needed?
Thanks a lot
Boris

That's a huge amount of gain. If the load cell has full-scale output
of 5 mv/v, and you excite it from 5 volts, you should get +-25 mv
full-scale, so a gain more like 200 might be reasonable.

Lots of instrumentation amps are sensitive to ambient RF pickup. The
bipolar input junctions make good RF detectors, and rectify ambient
radio stations and such into DC offsets. Offsets caused by
hand-waving, or grabbing leadwires, suggests RF. Your board should
have a solid ground plane, be in a sturdy box (for thermal reasons,
too) and all signal and power paths should be well EMI filtered.

You can expect some hum pickup, and lots of vibration noise, so a slow
integrating a/d converter, or lots of signal averaging, is usually
needed for load cells.

What's your physical construction like?

I could post a known-good load-cell conditioner front-end schematic to
abse if there were huge demand.


John

 

[martin griffith]

That's a huge amount of gain. If the load cell has full-scale output
of 5 mv/v, and you excite it from 5 volts, you should get +-25 mv
full-scale, so a gain more like 200 might be reasonable.

Lots of instrumentation amps are sensitive to ambient RF pickup. The
bipolar input junctions make good RF detectors, and rectify ambient
radio stations and such into DC offsets. Offsets caused by
hand-waving, or grabbing leadwires, suggests RF. Your board should
have a solid ground plane, be in a sturdy box (for thermal reasons,
too) and all signal and power paths should be well EMI filtered.

You can expect some hum pickup, and lots of vibration noise, so a slow
integrating a/d converter, or lots of signal averaging, is usually
needed for load cells.

What's your physical construction like?

I could post a known-good load-cell conditioner front-end schematic to
abse if there were huge demand.


John

I'd love to see the schematic, as I havent really done much with
modulated DC




martin

 

[joseph2k]

Hello

I'm working on a project using a load cell that needs to amplify a low
signal with a very large gain. I have, for exemple, 2 stages of gain.
The first(using an instrumentation amplifier) the gain is set to 500x.
The second gain (using an opamp) is 20x. The problem is that if I
increase too much the gain, there are a lot of interference. If my hand
gets closer to the device it increases 1V, and Im supposed to have a
precision circuit! I've read about ground loops and stuff, but is there
anything someone can suggest to have a high gain unity without
interference? Is some kind of isolation needed?
Thanks a lot
Boris

Use twisted shielded pairs both to and from the load cell, if necessary
drive the shield from the load cell with the common mode signal. Use
separate shielded twisted pairs. Also why such high gain (10,000x)? (100x
to 1000x total gain is usually enough)

 

[Eeyore]

That's a huge amount of gain.

80dB of voltage gain is not unknown for microphone preamps and they can be made
with excellent interference rejection.

Graham

 

[Ancient_Hacker]

You should be using shielded cable for all wires going to the load
cell.

What kind of bandwidth do you need? If it's less than 60Hz, you can
roll off the response of the amplifiers with some capacitors, then
you'll lose all the power-line and higher related noise.

You'll also need really quiet power supplies and some filtering right
at the amps.

 

[John Larkin]

80dB of voltage gain is not unknown for microphone preamps and they can be made
with excellent interference rejection.

Graham

Sure, gain is cheap. Microvolt DC precision and PPM linearity and
stability are not.

If you condition a good load cell right, its stability and linearity
are astounding. Put some, roughly, half-scale weight on a good load
cell system and note the reading. Then put a different one on the
scale, ditto. Now weigh both together and note the indicated weight.
If you do everything right, the two small numbers will add up to the
big number to a few 10's of PPM. Microphones don't work like that.

Load cell conditioning and thermocouple conditioning are similar
problems, with thermocouples being a bit trickier.

John

 

[John Larkin]

I'd love to see the schematic, as I havent really done much with
modulated DC

OK, it's up.

John

 

[Eeyore]

Sure, gain is cheap. Microvolt DC precision and PPM linearity and
stability are not.

If you condition a good load cell right, its stability and linearity
are astounding. Put some, roughly, half-scale weight on a good load
cell system and note the reading. Then put a different one on the
scale, ditto. Now weigh both together and note the indicated weight.
If you do everything right, the two small numbers will add up to the
big number to a few 10's of PPM. Microphones don't work like that.

Load cell conditioning and thermocouple conditioning are similar
problems, with thermocouples being a bit trickier.

I was referring to the interference rejection part of things.

There's no reason it should be a problem.

Graham

 

[John Larkin]

Hello

I'm working on a project using a load cell that needs to amplify a low
signal with a very large gain. I have, for exemple, 2 stages of gain.
The first(using an instrumentation amplifier) the gain is set to 500x.
The second gain (using an opamp) is 20x. The problem is that if I
increase too much the gain, there are a lot of interference. If my hand
gets closer to the device it increases 1V, and Im supposed to have a
precision circuit! I've read about ground loops and stuff, but is there
anything someone can suggest to have a high gain unity without
interference? Is some kind of isolation needed?
Thanks a lot
Boris

Oh, with all that gain, make sure the amp chain isn't oscillating.

John

 

[Boris]

John Larkin escreveu:

That's a huge amount of gain. If the load cell has full-scale output
of 5 mv/v, and you excite it from 5 volts, you should get +-25 mv
full-scale, so a gain more like 200 might be reasonable.

Lots of instrumentation amps are sensitive to ambient RF pickup. The
bipolar input junctions make good RF detectors, and rectify ambient
radio stations and such into DC offsets. Offsets caused by
hand-waving, or grabbing leadwires, suggests RF. Your board should
have a solid ground plane, be in a sturdy box (for thermal reasons,
too) and all signal and power paths should be well EMI filtered.

You can expect some hum pickup, and lots of vibration noise, so a slow
integrating a/d converter, or lots of signal averaging, is usually
needed for load cells.

What's your physical construction like?

I could post a known-good load-cell conditioner front-end schematic to
abse if there were huge demand.


John

Hello John,

First of all, thanks for helping.My first experiment using this load
cell had a gain of approximately 500. The reason I increased the gain
is because I could only notice a small change in the signal when I
tested it. It is for use with a impact machine, so, the load cell is
made up of iron with some other hard material so that it almost
doesn´t deform.After increasing the gain I could notice the signal as
the mass impacts the object.

I don´t have much experience with these projects so I´ll try to
answer your questions the way i´m doing and the way I believe is
right(but it might not be).

The signal caused by the impact lasts approximately 50ms, so I assume I
need a large bandwidth. I put a filter after the AMP in a 40Khz cutoff
frequency. I have a 60Hz noise disturbing my system, thats the main
problem regarding filtering in my opinion. If I get rid of that noise,
It´ll be amost good.
I´m afraid I lose the signal(it gets filtered) if I´d put a 30HZ
cutoff frequency, thats why I haven´t decided what filter to use.

My physical construction is a breadboard, but I´ll change to a printed
circuit board as soon as possible.

You said your schematic is up, how can I see it? Could you send me the
link?

Thanks a lot!

Boris

 

[John Larkin]

John Larkin escreveu:


Hello John,

First of all, thanks for helping.My first experiment using this load
cell had a gain of approximately 500. The reason I increased the gain
is because I could only notice a small change in the signal when I
tested it. It is for use with a impact machine, so, the load cell is
made up of iron with some other hard material so that it almost
doesn´t deform.After increasing the gain I could notice the signal as
the mass impacts the object.

I don´t have much experience with these projects so I´ll try to
answer your questions the way i´m doing and the way I believe is
right(but it might not be).

The signal caused by the impact lasts approximately 50ms, so I assume I
need a large bandwidth. I put a filter after the AMP in a 40Khz cutoff
frequency. I have a 60Hz noise disturbing my system, thats the main
problem regarding filtering in my opinion. If I get rid of that noise,
It´ll be amost good.
I´m afraid I lose the signal(it gets filtered) if I´d put a 30HZ
cutoff frequency, thats why I haven´t decided what filter to use.

My physical construction is a breadboard, but I´ll change to a printed
circuit board as soon as possible.

You said your schematic is up, how can I see it? Could you send me the
link?

Thanks a lot!

Boris

It's in the alt.binaries.schematics.electronics newsgroup. If you
don't get the binary groups, email me. jjlarkin atsign
highlandtechnology dotty com.

A more sensitive load cell, better matched to the mechanical load,
would help. But with proper shielding and signal conditioning, 60 Hz
pickup should be far below the thermal (random) and vibration noise
floor.

John

 

[Rich Grise]

Oh, with all that gain, make sure the amp chain isn't oscillating.

I'm just a techie, but would it make any sense to swap the gain stages?
i.e., the 20X first, then the 500X?

Thanks,
Rich

 

[Eeyore]

I have a 60Hz noise disturbing my system, thats the main
problem regarding filtering in my opinion.

In which case you haven't paid the necessary attention to screening and signal
balance.

Can't you post a schematics in alt.binaries.schematics.electronic or on one of
the free picture hosting sites ?

Graham

 

[Eeyore]

I'm just a techie, but would it make any sense to swap the gain stages?
i.e., the 20X first, then the 500X?

Usually not.

Graham

 

[John Larkin]

I'm just a techie, but would it make any sense to swap the gain stages?
i.e., the 20X first, then the 500X?

Thanks,
Rich

Ideally, about equal gain per stage gives best bandwidth and
stability... 100x100 maybe. But low frequency stuff like this isn't
all that picky.

But with 10k gain and 10's of KHz bandwidth, overall GBW is in the
hundreds of MHz, so a clean layout and a good ground plane are
prudent.

John