PSN-L Email List Message

Subject: Re: Op amp front end noise - termal variation control
From: "Charles R. Patton" charles.r.patton@........
Date: Fri, 11 Mar 2005 09:51:33 -0800


Hi-stability oscillators use proportional oven control techniques such 
as a copper container that has a heater coil and sensing. Then the ckt 
is inside with some insulation. Very high quality variations of this do 
two containers with separate heaters and sensing  one inside the other 
with insulator between the containers. The idea is to have high thermal 
conductivity containers separated by insulation to reduce gradients and 
thermal transients. A cheaper variation would be to use aluminum pipe 
rather than copper  not as good in the thermal mass dept., but still 
pretty good thermal conductivity. There are lots of examples of these in 
the amateur radio community  see issues of QEX for several examples.

A very simple way to reduce air current variation is to put your circuit 
in a small, open-top box and pour in clean, dry, sand. I like #30 silica 
sand (the fine, white sand you see in hotel lobby ash-tray cans), about 
$4 for a 50 lb. sack at Home Depot. Also helps with microphonics  if 
you have them. Just pour out the sand to work on the circuit.

Regards,
Charles Patton

ChrisAtUpw@....... wrote:

> 
>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
>
>  
>

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