From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-21 17:41:28
Dear Andreas:
The gain of the input amplifier is about 100 therefore all the noise you are calculating should be referred to the input and divided by 100. The 20 uV would be the equivalent of 0.02 uV of input noise and therefore insignificant.
Your main concern should be leakage current of the op amp and changes with temperature due to the high resistor values. I do not know the op amp. Inthe dark ages when I used the 709 and 741 it was important to put equal source impedance for both the inverting and noninverting inputs so that the bias currents would be cancelled. This may not be needed with this op amp,but I though you might like to know.
The biggest noise source that I see is the 5.6 K input protection resistors. You had asked earlier why a carbon composition resistor was specified inthe Linear Literature. It has to do with high voltage breakdown capability. A carbon resistor is a hollow insulating core filled with a mixture of carbon and mud. Different mixtures have different ohms per square inch. But it is a relative homogenous mix and you will have a linear voltage gradient through it. Today's resistors are mostly carbon film or metal film. These are made with a ceramic core and end cap with wire leads. The resistor is made by coating or evaporating (sputtering) a thin conductive film onto the ceramic core. Then a spiral groove is cut through the conducting film to make a longer and larger value resistor. They are actually adjusted to value by this cutting wheel. This insulating groove may only be 0.001 inches wide and any high voltage applied to the resistor will jump and arc across. So there is a major difference when used in high voltage protection.The surface mount parts are usually not cut with a wheel but rather a laser does the cutting. Same breakdown problem though.
The other problem with carbon composition resistors, is that they are the "most" noisy of all resistors. You can build a good noise source by simply running current through a carbon resistor.
I hope to argue with you later about this resistor. Right now it has no bearing on getting the project to move forward.
No, you cannot substitute an electrolytic for a tantalum! Not all capacitors are created equal. Capacitors are as different as cars. Not all cars are the same. You could not "substitute" a VW for a Porsche in a race.
I worked for a capacitor manufacturer for a few years so if you want to know more just ask. Each type of capacitor covers a different part of the frequency spectrum.
If the A/D sampling frequency is 128 Hz, the cutoff should be much lower than 124 Hz. I see very little benefit of sampling faster.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
----- Original Message -----
From: sleeper75se
To: buildcheapeeg_at_yahoogroups.com
Sent: Monday, January 21, 2002 10:01 AM
Subject: [buildcheapeeg] The input stage: almost done!
Hi all,
this is a question for Jim or Joerg or anyone who has the know-how:
I have finished selecting resistors and capacitors for the amplifier
and filter. Because I'm using NP0 capacitors, and they are so
expensive, I've decided to use very small values in the filter. The
capacitors are just 2.2nF and 1nF. In return, large resistors are
needed - 680Kohms.
I've calculated the noise in one filter stage using a program called
Noise. It's available here:
http://www.linear.com/software/
The design note describing it, and the calculation it is actually
performing, is here:
http://www.linear.com/pub/document.html?pub_type=desn&document=39
The resulting noise per opamp is 6.6uV(p-p) for 0.1 - 100Hz. For all
three filter amplifiers, the total is about 20uV noise peak to peak.
Is this value reasonable? If I've made a mistake here, it would cost
quite a bit of time and money, so a second opinion is greatly
appreciated! :-)
This is how I did it, so you may retrace my steps
(optional reading) ---------------------------------------
>From the data-sheet at http://www.linear.com/pdf/lt1112.pdf I got
these values:
voltage noise: 15nV/sqrt(Hz), corner frequency 2.5Hz
current noise: 8fA/sqrt(Hz), corner frequency 140Hz
I selected 5 in the main menu of Noise and entered these parameters
(as they are written, without my comments of course):
R1 680000 (ohms, inverting input resistor)
R2 0 (ohms, feedback resistor)
R3 680000 (ohms, non-inverting input resistor)
FL 0.1 (Hz, Lower frequency)
FH 100 (Hz, Higher frequency)
VN 15 (nV/sqrt(Hz), voltage shot noise)
IN 0.008 (pA/sqrt(Hz), current shot noise)
VFC 2.5 (Hz, voltage noise 1/f corner frequency)
IFC 140 (Hz, current noise 1/f corner frequency)
After typing in the values, hitting the return key gave the result.
-----------------------------------------------------------
Another question: Is it OK to replace the 10uF tantalums used for
decoupling, with aluminium electrolytes? They are half the price, and
not nearly as sensitive to reverse voltages...
The updated schematic and a parts list is in the files section.
http://groups.yahoo.com/group/buildcheapeeg/files/hardware%20design/
The amplifier layout will be updated and I'll consider it finished as
soon as these last few things are settled. The layout for the ADC is
also complete, but still needs a parts list and some tweaking.
The cost is $73 for two channels, not including taxes and shipping.
The ADC module will probably be another $40, but that is just a guess.
Regards,
Andreas
Yahoo! Groups Sponsor
ADVERTISEMENT
To unsubscribe from this group, send an email to:
buildcheapeeg-unsubscribe_at_egroups.com
Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST