Unity gain buffer on AI lines

[johnsold]

VIkash_Kumar, 

Please try one more test. Duplicate the setup of test 2 - both inputs driven by NI-AO channels, except insert a resistor of ~2000 ohms in series between the AO and AI connections. This will simulate the effect of a higher source impedance. By using passive resistors you do not have to worry about any non-linear effects such as current limiting.

Lynn

[Vikash_Kumar]

Hi Sam, 

Here is the data that you asked for. There is not much of a difference so I am attaching 2 frequencies only.

[Vikash_Kumar]

HI Lynn, 

Here is the new test (based on the test2) that you recommended: The one with 2.2 Kohm input impedance (simulated by placing 2Kohm resistor in series between AO and AI). The results are as one would expect - significant ghosting effect is observed.

Below are behaviors with both AI0 and AI1 externally driven by a different card's NI-AO channels with 2.2Kohm resisters in series. One kept fixed at 5v and other with "NI-AO" input (i.e... 10 Hz sine wave from external NI card)

Buffering does improve results a little but still unacceptable.

Note that the buffered waveform obtained here is very similar to the one obtained from real MPS sensor (in the previous page).

[johnsold]

Vikash_Kumar,

I think that test confirms that your sensors really do have a 2000 ohm output impedance. The datasheet was vaguely worded.

I suspect that part of the problem is that the LM2902 is not a very good buffer at higher frequencies.  As the open loop gain reduces at higher frequencies the effect of feedback at reducing the output impedance is reduced. The large signal frequency response begins dropping at ~5 kHz and the open loop gain is about half of the DC gain at that frequency.  While it is not straightforward to estimate the output impedance at higher frequencies, the result is consistent with the buffer impedance increasing with frequency.

Here is how the output impedance a much faster op amp (LF355) behaves over frequency.

If you have a faster op amp, see if that helps.

Lynn

[Vikash_Kumar]

I got followings with me. If  you can point a few you want me to try, it will be super helpful (I'm a maths guy with limited electronics knowledge)

http://www.digikey.com/product-detail/en/TLV2374IN/296-12221-5-ND/413508

http://www.digikey.com/product-detail/en/MAX4222ESD%2B/MAX4222ESD%2B-ND/947990

http://www.digikey.com/product-detail/en/OPA827AIDGKT/296-24280-1-ND/2039705

http://www.digikey.com/product-search/en?vendor=0&keywords=LT1792ACN8%23PBF-ND

http://www.digikey.com/product-search/en?vendor=0&keywords=HFA1412IBZ-ND

http://www.digikey.com/product-detail/en/OPA2277P/OPA2277P-ND/301172

[johnsold]

From that group the OPA827 seems like the best choice. The first application listed on the data sheet, ADC drivers, is exactly what you will be doing. The high speed buffers like the MAX4222 and The HFA1412 would work but it cna be tricky to keep them from oscillating at high frequencies. The other devices are a little slower than I would recommend for your application.

Lynn

[Vikash_Kumar]

And we are back to the MPS sensors on all lines.

Here is the effect of OPA827 as buffer.

Here is zoomed view near the oscillations

[CoffeeBreak]

Good evening Vikash_Kumar,

For the graphs that you answered my previous post, what is the peak-to-peak value for the signals (for both the 9 kHz and 500 Hz sampling rate)? I just want to get a better sense of the exact signal since the scale is 0.1 V on the graphs.

So that I can accurately detail the signal resolution we should see, what is the Total length for your sensors in mm (designated L2 on the MPS Specifications Document)?

[Vikash_Kumar]

AIConvRate = 500 Hz

AIConvRate = 9000.9 Hz

[johnsold]

Can you check the power supply voltages with an oscilloscope, not the DAQ, to see if there is some coupling through the power connections?

I am beginning to think we are looking in the wrong places or at the wrong things.

Lynn