3.3v robustness

[Daveb]

Hi,

Does anyone have any experience/thoughts on the robustness of a 3.3v
CMOS Opamp configured as a differential amp with the inputs connected
via 10m cables to a strain gauge bridge in an electrically noisy
environment ? If I add clamp diodes from the opamp inputs to the rails
is there any difference between a 3.3v device and a 12v device in
terms of susceptability to damage from external transients, etc ?

Thanks
Dave

 

[Leon]

Hi,

Does anyone have any experience/thoughts on the robustness of a 3.3v
CMOS Opamp configured as a differential amp with the inputs connected
via 10m cables to a strain gauge bridge in an electrically noisy
environment ? If I add clamp diodes from the opamp inputs to the rails
is there any difference between a 3.3v device and a 12v device in
terms of susceptability to damage from external transients, etc ?

Thanks
Dave

Why don't you put the op amp close to the strain gauge?

Leon

 

[Daveb]

Why don't you put the op amp close to the strain gauge?

Leon

Leon,

The strain gauge is in a high temperature environment which is too hot
for signal conditioning electronics. Also, the mechanical part doesn't
allow easy location of a circuit board so we've elected to put the
conditioning remotely. The signal can be heavily filtered as the
sampling rate will be low. It seems there's a better choice of single
rail opamps with good DC performance with low voltage rails than with
higher voltage rails. Hence the question regarding the suitability of
the low voltage amps in such an amplication.

Regards
Dave

 

[Daveb]

A CMOS op amp is a horrible choice for a strain gauge signal
conditioner, unless it's a chopper.  Bad drift, bad offset, really
putrid 1/f noise, electrically fragile, and you don't care about the
high impedance because the strain gauge impedance is very low.

Cheers,

Phil Hobbs

Phil,

Thanks for your post. Looking at the opamp table on the Analog
website, for example, sorting by Vos shows many CMOS/FET devices with
very low figures for input offset, drift and noise but they tend to
have low voltage rails. My concern was their robustness (but with
external series resistor/clamp diodes) when connected to outside world
signals and how they compared with higher voltage technology
especially bipolar.

Regards
Dave

 

[Daveb]

A CMOS op amp is a horrible choice for a strain gauge signal
conditioner, unless it's a chopper.  Bad drift, bad offset, really
putrid 1/f noise, electrically fragile, and you don't care about the
high impedance because the strain gauge impedance is very low.

Cheers,

Phil Hobbs

Phil,

Thanks for your post. Looking at the opamp table on the Analog
website, for example, sorting by Vos shows many CMOS/FET devices with
very low figures for input offset, drift, noise but they tend to have
low voltage rails. My concern was their robustness (but with external
series resistor/clamp diodes) when connected to outside world signals
and how they compared with higher voltage technology especially
bipolar.

Regards
Dave

 

[[email protected]]

Phil,

Thanks for your post. Looking at the opamp table on the Analog
website, for example, sorting by Vos shows many CMOS/FET devices with
very low figures for input offset, drift and noise but they tend to
have low voltage rails. My concern was their robustness (but with
external series resistor/clamp diodes) when connected to outside world
signals and how they compared with higher voltage technology
especially bipolar.

Regards
Dave

"sorting by Vos shows many CMOS/FET devices with
very low figures for input offset, drift and noise but they tend to
have low voltage rails."

I've noticed this also, I'm far from a device guy, but I think that
some of these specs might track with the supply voltage. Double the
voltage and double the offset? So 36 V opaamps look like they have
worse offsets. But hey if you are worried about siganl to noise then
you want to crank the gain up to get the signal 'near' the rails. And
then the relative error is the same.

Is the strain gauge a bridge? if so why not use a nice bjt instrument
amp?

George Herold

 

[[email protected]]

Hi,

Does anyone have any experience/thoughts on the robustness of a 3.3v
CMOS Opamp configured as a differential amp with the inputs connected
via 10m cables to a strain gauge bridge in an electrically noisy
environment ? If I add clamp diodes from the opamp inputs to the rails
is there any difference between a 3.3v device and a 12v device in
terms of susceptability to damage from external transients, etc ?

Thanks
Dave

You may want to consider diode clamps from the rails back to ground.
You can't insure a device is powered when a transient hits.

Lower voltage generally goes with finer geometry processing, but that
doesn't necessarily translate to a more fragile part. Electrical
overstress depends a diode doping profiles, assume a reverse bias. The
gentler doping profile is less prone to hot spots. Some protection
devices used a fet breakdown to snub the current. This could be better
in finer geometry since the breakdown will occur at a lower voltage.
Basically, I don't think you can make the blanket claim that a 12V
chip is more rugged than a 3.3V chip. Generally when you scale a chip,
the protection devices aren't scaled much at all.

 

[whit3rd]

Does anyone have any experience/thoughts on the robustness of a 3.3v
CMOS Opamp configured as a differential amp with the inputs connected
via 10m cables to a strain gauge bridge in an electrically noisy
environment ?

Small strain gage signals and a 10M cable? Put your bridge
amplifier right next to the strain gage, run the 10M cable
to the amplifier output. Cables can be antennas, thermocouples,
transformers, triboelectric sources (maybe not for a low impedance
signal), etc. Don't do that to your precious not-yet-preamplified
signal.

 

[JosephKK]

Leon,

The strain gauge is in a high temperature environment which is too hot
for signal conditioning electronics. Also, the mechanical part doesn't
allow easy location of a circuit board so we've elected to put the
conditioning remotely. The signal can be heavily filtered as the
sampling rate will be low. It seems there's a better choice of single
rail opamps with good DC performance with low voltage rails than with
higher voltage rails. Hence the question regarding the suitability of
the low voltage amps in such an amplication.

Regards
Dave

Call me weird, but there are fundamental reasons why a split supply
opamp will always work better at DC than a single rail opamp. If you
are willing to go to +/- 3.3 V or +/- 5 V for the first stage you can
easily get much better DC performance. Much of it is a result of
circuit topology. Just to push the point can you put the strain gauge
bridge in the same thermal environment? It will save you much trouble
if you can.

 

[JosephKK]

Phil,

Thanks for your post. Looking at the opamp table on the Analog
website, for example, sorting by Vos shows many CMOS/FET devices with
very low figures for input offset, drift, noise but they tend to have
low voltage rails. My concern was their robustness (but with external
series resistor/clamp diodes) when connected to outside world signals
and how they compared with higher voltage technology especially
bipolar.

Regards
Dave

IME bipolar DC offset and drift of bipolar opamps kick CMOS opamps
tush up and down the block. For really sensitive situations; bridge
the strain gauge sensor inside the thermal environment, and use split
supply opamps, perhaps driving the sense wire twisted pair shield with
the common mode voltage. BTDT and i even had load cells (bridge
included) to work with.

 

[JosephKK]

You're weird ;-) Your statement is only true if the OpAmp
architecture (internal circuitry) contains a ground pin... such as my
venerable MC1530.


Maybe you are arguing for getting the signals ground-referenced?

...Jim Thompson

Not really, the output from the bridge is inherently differential, and
with split supplies and sits nicely close to 0 V for best CMRR. CMRR
does degrade as the common mode voltage approaches the rails. Second
i want the entire bridge in the same thermal environment to keep
thermal mismatch as much out of the problem as possible.

 

[JosephKK]

[snip]

CMRR
does degrade as the common mode voltage approaches the rails.

Maybe with the OpAmp you are using, but not universally.

Why do so many "engineers" have preconceived notions ?

...Jim Thompson

It has been an issue with every opamp that i have ever used. If you
have made some that do not have that problem, send me the data sheet
and some samples. It will enlighten me. Just because i have not seen
one or heard of one does not mean that they cannot exist, but that
they are outside my experience. And i have seen testing of opamps
where this not supposed to be a problem, but it still is.