In a message dated 10/03/2005 16:11:50 GMT Standard Time, jpopelish@........
writes:
Jack Ivey wrote:
> Bret Nordgren wrote:
>> Another factor that you may want to consider is thermal variation. At
very
>> low frequencies, below 1Hz, the effects of micro-variations in the device
>> temperature can add additional "noise".
> I've seen this effect with thermocouple amplifiers, where moving your hand
> near the circuit would move the air enough to create low-frequency noise.
> It can be almost eliminated by pressing the circuit board between pieces
> of foam rubber.
It also helps a lot to keep the internal temperature rise of the front
end opamp to a minimum. Reducing the opamp supply voltage as much as
possible without degrading the performance of the amp helps keep the
chip cool and reduce the thermal effect of changes in air currents.
For this reason, if two amp choices have similar noise specs, but one
may be operated at lower supply voltage or draws less supply current,
its lower self heating may allow it to out perform its hotter
competition in the low frequency realm.
Hi John,
Assuming that you are using a 16 bit ADC with a range of +/-10V, one
count is 305 micro volts. Normal amplifier gains can result in very significant
count drifts with temperature unless great care is taken in the design and
construction.
There are two different factors operating here. One is the
temperature sensitivity of the opamp input circuit in micro V / C Deg. Remember that
this relates to temperatures on the IC chip itself, so it is effected by the
chip dissipation.
The CAZ type opamps have very greatly reduced thermal input drifts and
1/f noise.
The other is the signals derived from external thermo electric junctions
and are rarely less than a few micro V / C Deg. These can be between the
chip header and the socket or the wiring, or between cables and the input
clamps, or even between different cables or connections. You will see differences
across the circuit board, if there is a thermal gradient across it.
Some resistors, like the metal oxide types, generate high EMFs if there
is a temperature difference between the two ends. Don't even try to use
carbon resistors, either composition or film.
It can be an advantage to stick a strip of soft Al or Cu to the top, or
even to both sides, of the input amplifier chip and bolt this onto the outer
Al Screening Case. Another alternative is to use double sided circuit board.
This greatly reduces temperature variations across the board. You can bolt a
Cu chip cover strip onto the board. This is preferable to trying to reduce
the dissipation by reducing the supply voltage. Having said this, it may be
desirable to use separate IC regulators for the input opamp supply, to give low
noise and drift and high AC supply rejection. The first amplifier does need
very good supply noise decoupling.
Seismometer amplifiers often have two distinct gain stages, with a high
pass filter set to maybe 20 to 30 sec in between. This will greatly reduce
thermal error signals and 1/f noise at the output. For geophone circuits, the
filter maybe set to 1/10 the resonant frequency.
The seismometer amplifier case is preferably made of metal and earthed.
It should be kept dry, screened from drafts and any temperature variations
should be minimised. It can be an advantage to fill the case with glass wool to
inhibit convection.
You might include the LF412 for second amplifiers. They have quite low
drift.
The INA118 is very useful as a low noise true differential input opamp.
For information, noise calculation and selection of your photo diodes
See
_http://usa.hamamatsu.com/assets/applications/SSD/photodiode_technical_information.pdf_
(http://usa.hamamatsu.com/assets/applications/SSD/photodiode_technical_information.pdf)
Regards,
Chris Chapman
In a message dated 10/03/2005 16:11:50 GMT Standard Time,=20
jpopelish@........ writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Jack=20
Ivey wrote:
> Bret Nordgren wrote:
>> Another factor that y=
ou=20
may want to consider is thermal variation. At very
>> low=20
frequencies, below 1Hz, the effects of micro-variations in the=20
device
>> temperature can add additional "noise".
> I'=
ve=20
seen this effect with thermocouple amplifiers, where moving your hand
&=
gt;=20
near the circuit would move the air enough to create low-frequency=20
noise.
> It can be almost eliminated by pressing the circuit board=20
between pieces
> of foam rubber.
It also helps a lot to keep=20=
the=20
internal temperature rise of the front
end opamp to a minimum. =20
Reducing the opamp supply voltage as much as
possible without degrading=
the=20
performance of the amp helps keep the
chip cool and reduce the thermal=20
effect of changes in air currents.
For this reason, if two amp choi=
ces=20
have similar noise specs, but one
may be operated at lower supply volta=
ge=20
or draws less supply current,
its lower self heating may allow it to ou=
t=20
perform its hotter
competition in the low frequency=20
realm.
Hi John,
Assuming that you are using a 16 bit ADC with a=
=20
range of +/-10V, one count is 305 micro volts. Normal amplifier gains can re=
sult=20
in very significant count drifts with temperature unless great care is taken=
in=20
the design and construction.
There are two different=
=20
factors operating here. One is the temperature sensitivity of the opamp inpu=
t=20
circuit in micro V / C Deg. Remember that this relates to temperatures=20
on the IC chip itself, so it is effected by the chip dissipation.
The CAZ type opamps have very greatly reduce=
d=20
thermal input drifts and 1/f noise.
The other is the signals derived from external=20
thermo electric junctions and are rarely less than a few micro V / C De=
g.=20
These can be between the chip header and the socket or the wiring,=20
or between cables and the input clamps, or even between different cable=
s or=20
connections. You will see differences across the circuit board, if there is=20=
a=20
thermal gradient across it.
Some resistors, like the metal oxide types,=20
generate high EMFs if there is a temperature difference between the two=20
ends. Don't even try to use carbon resistors, either composition or=20
film.
It can be an advantage to stick a strip of soft=
Al=20
or Cu to the top, or even to both sides, of the input amplifier chip and bol=
t=20
this onto the outer Al Screening Case. Another alternative is to use double=20
sided circuit board. This greatly reduces temperature variations across the=20
board. You can bolt a Cu chip cover strip onto the board. This is=20
preferable to trying to reduce the dissipation by reducing the supply voltag=
e.=20
Having said this, it may be desirable to use separate IC regulators for the=20
input opamp supply, to give low noise and drift and high AC supply=20
rejection. The first amplifier does need very good supply noise decoupl=
ing.=20
Seismometer amplifiers often have two distin=
ct=20
gain stages, with a high pass filter set to maybe 20 to 30 sec in between.=20
This will greatly reduce thermal error signals and 1/f noise at the out=
put.=20
For geophone circuits, the filter maybe set to 1/10 the resonant=20
frequency.
The seismometer amplifier case is preferably ma=
de=20
of metal and earthed. It should be kept dry, screened from drafts and any=20
temperature variations should be minimised. It can be an advantage to f=
ill=20
the case with glass wool to inhibit convection.
You might include the LF412 fo=
r=20
second amplifiers. They have quite low drift.
The INA118 is very useful as a=
low=20
noise true differential input opamp.
For information, noise calculation and=20
selection of your photo diodes See http://usa.hamamatsu.com/assets/applications/SSD/photodio=
de_technical_information.pdf
Regards,
Chris Chapman
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