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Subject: Re: Op amp front end noise
From: John Popelish jpopelish@........
Date: Thu, 10 Mar 2005 21:03:10 -0500

> ChrisAtUpw@....... wrote:
> jpopelish@........ writes:

>> 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.
> 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.

On this, we agree.
> The CAZ type opamps have very greatly reduced thermal
> input drifts and 1/f noise.

No argument here, either.  Eliminating the 1/f noise is essentially
the reason they were invented.

(snip advice on wiring, circuit board and resistor thermoelectric
> 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.

How so?  Preferable for what reasons?

> 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.

Makes good sense to me.
> 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.

I am looking at using an integrator as negative feedback from the
output back to the second or third stage to actively null the output
drift, instead of putting a large capacitor in the forward signal
path.  This reduces the net offset to the offset of the integrator,
instead of being the amplifier offset of the input where a series
capacitor would normally be used.  I think it is also easier to get a
low noise, low frequency high pass corner frequency this way.  Have
you any criticism of this technique?

>     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.

Agreed, but keeping all internal heat sources to a minimum should also

>     You might include the LF412 for second amplifiers. They have
> quite low drift.

I have included the equivalent LF411 in the survey.
>     The INA118 is very useful as a low noise true differential input
> opamp.

I'll add that.
> For information, noise calculation and selection of your photo
> diodes See

I'll give it a read.  This is the sort of thing I am designing at

John Popelish

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