Power sensor signal conditioing

[charbel89]

For the benefit of others viewing this thread here is Erics response:

Hi Eric,

I've been going over the circuit of the CA3140 to amplify the AD736 signal and I drew a blank on how the cicuit below was decided upon (Please ignore question in the image ):

What I would like to understand is why did we go with a 4k7 resistor at pin2 and Why the 10k at pin 3.
The gain of the CA3140 is set by the 4K7 and 20Kpot
Gain = 20K/4K7 = 4.25
In order to vary the gain to suit the application the 20K can be varied, eg: sy we set the 20Kpot to 14.1K, then the gain is 3

The 10K is to protect the +NI input of the CA3140 against excess current in the event of a failure in the driving circuit

I understand the voltage at 4k7 will be 4.5v so applying a 4700 ohm resistance will generate 0.96mA.
And for pin 3 at a maximum of 1VDC we get 0.1mA.
The curents are not usually assessed in this way.

So what is the significance of choosing those particular ones as opposed to say 10k at 2 and 20k at 3 for example, how would this have impacted the output. For instance.. would i get the same results provided i adjust the 20k VR correctly?
You adjust the 20Kpot to give the signal levels that suits your ADC input range.


Mainly pages 2 and 3.

The whole diagram (proposed is what was built):
Thanks a lot for your time.
Regards
Charbel

 

[charbel89]

Thanks.

I think I understand. my issue was that I was revising the circuits and for the AD736 it is pretty clear because it is out of the datasheet mainly, however for the CA3140 it is more custom and it was hard for me to deduce the circuit from the datasheet.

So I was pretending that the circuit is not there and I need to build it, so how would I go about doing that just by looking at the datasheet. i.e. to arrive at the same circuit I currently have...

Which is why I was asking why the choice of 10k resistor at +NI and not 15K or 1K, and why the 4k7 at 2 and why is 2 connected to 6 and what is the datasheet diagram equivalent that justifies the circuit i have..

Those were my questions ....



I have built the cicuit and tucked it away in a plastic box with a switch and all.. very crude (ehm I am mean professional) it is doing its job.

I will post some pictures in the next few weeks.. I am just a bit busy now..

Thanks a lot for everything..

Charbel.

 

[ericgibbs]

hi Charbel,
Pleased hear you project is working OK.

With regard to your questions, I have posted a pdf of OPA basics.

If you plan other projects using operational amplifiers, it would be useful to read thru the pdf.