Creating buffer amp & Bessel LPF for PC baseData Acquisition

[frainsquest]

I've developing acquisition program use ADC MAX186 using
parallel port interface.
my program is able take sample in every 1 millisecond (1K Sample/sec).

my objective design is to reads voltage value in graphical (of AC or bipolar and DC, or unipolar input) and have amplification factor from 1 to 100 times (for low value voltage input,approximately 10mV) by connecting the input ADC using amplification op-amp. And can divide the analog input by factor 1/10 and 1/100 for input voltage bigger than VREF but less than 20-30 V for hardware safety.
my program is kind of digital pc oscilloscope but just using low time sampling.

the programs is run perfectly but the problems comes from the hardware.

i using ordinary op-amp (such as TL 084 or LF 347) because this
op-amp is commonly very easy to finds in electronic parts market
on my country (Indonesia). And its very hard to finds MAXIM parts on my country.

in attachment image1.png file shows the op-amp buffer and voltage multiplier (op-amp 1&2) and 20 Hz bessel low pass filter (op-amp3) from the references of http://dsp.com
using bessel LPF, because this filter design it provides a gentle edge smoothing for low frequencies signal while removing the high frequencies signal.
And the AC-DC switch i use DC relay controlled from software.
I also use input resistor 10k in every input channel for balancing input resistance.
analog input using CH1, CH3, CH5, CH7, and the other input connected to ground. and using single ended mode.
i use the default VREF (4.096V), and the default ADC schematic design as shown on data sheet.
After i create this design the results is very disappointing me,
The DOUT value shows only 0 - 20 mV, just showing noise signal

before i creating the hardware i use electronic simulator design, it runs normally without problems.

I once try not using buffer amp. Just directly give analog input to CH0-CH7, after running the program then is give the correct conversion value.

my question :
are this hardware doesn't support/compatible using ordinary op-amp (TL081,TL082,TL084 and LF347)? or just compatible for others MAXIM op-amp parts ? or caused from incompatibility of input resistance ADC and output op-amp ?

why the results value of CH1 to CH7 have an interference value (or cross-talk), from each input. are this caused from high frequencies sampling. or from the resistance input is too little ?
but i saw from data sheet this ADC support using FCLK up to 2 MHz. i check the ADC it doesn't broken. What's wrong?
i've check the interference value it can be 20-70% to other channel value.
i try to set to ground of channel that not used, it gave the better performance to minimize the interference.

can some one gave me the correct design for this application and the Bessel LPF design using fc(cutoff frequency)=500Hz, and to minimize the interference problems.
if there other parts needed,i hope just using common parts.

are there other discussion forum of from official MAXIM for these case ? and for other topics, such as MAXIM microcontroller, sensors, etc ?

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