From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-01-21 15:01:30
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
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST