vinke

Hi,
I have a resistive sensor which needs to be excited with a small ac voltage(preferably not more than 2V pk-pk) at 1khz.I am thinking of using an ac bridge. I need to send the output of the bridge to A/D module of a microprocessor(PIC16F877A). Surely ill have to convert the ac output in dc and amplify that dc into an appropriate level with an instrumentation amplifier.But how do i convert that ac output(considering that at times it may be a very small value which is less than the forward voltage of a diode) into dc?
Any help will be greatly appreciated.
Thank you.

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Nigel Goodwin

Use a precision rectifier, which uses an opamp to remove the voltage drop across the diode.

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ronsimpson

From the LM3915 data sheet. Used for measuring audio signals.

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vinke

Thanks guys,it helped a lot.
So i need two of this circuit above , and i connect them to each arm of the bridge.I am right?

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Roff

You should post a simplified schematic of what you have in mind. You could just use a diode bridge in place of the precision circuitry, and we can tell you what we think of the technique. It's not clear to me why you think you need a bridge rectifier.

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Ron

vinke

well in the schematic ive attached,V1 is the ac source voltage,V2 and V3 are the outputs,my sensor is R4 which can exhibit resistances from 100ohms to about 40Kohms.(The values of R1, R2,R3 are for compensating the sensor non-linearity) For instance when my sensor has a resistance of 25Kohms,V2 would be about 67mV and V3 =100mV.If the supply source was dc,i would have connected V2 and V3 to an instrumentation amplifier and the output of the inst. amplifier to my A/D,but since they are ac i doubt that it works.So ,i was thinking that i need to find a way to convert the ac outputs into dc first.Using simply diodes for that purpose wont be a good idea since V2 and V3 are very less than the forward voltage of a diode.

My goal is to get my sensor interfaced with an A/D module of my microprocessor.The precision rectifier seems a good idea.If you have any other idea how to achieve that ,you are most welcomed to suggest.

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ericgibbs

hi vinke,
When you use Ron's FW precision rectifier, remember, it has only a Gain of about 1.

So I would suggest you amplify the small signal from the ac bridge before you rectify it.
As the FW rectifier input is ac coupled, try to keep the amplified ac signal within the limits of the FW rectifier.
That is, stay within the linear region of operation of the FWRect.

Its also possible to amplify the dc out from the FWRect, so that you get the full 0 thru +5v input to the PIC's adc,
assuming you are using the internal +5Vref.

The output of the FWRect is the peak 'dc' level not the rms value.

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Nigel Goodwin

No, you use a differential amplifer on the bridge, and a single rectifier circuit on it's output.

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JimB

Have I missed something here?

If the bridge is just resistive, why use AC excitation?
What is wrong with DC? No need for a rectifier, just a differential amplifier.

JimB

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Nigel Goodwin

Well he says "needs to be excited with a small ac voltage", but he doesn't give us any details (as usual).

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Roff

What's wrong with amplifying with an instrumentation amp (maybe adding some offset) and then feeding the AC to the A/D? Can't the PIC's A/D keep up with a 1kHz sine wave?

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Ron

vinke

Sorry sorry, i should have given specs about the sensor,but i thought it wouldnt bother as i said that it needs to be excited with ac and that the question is how to interface an ac bridge with an A/D.But to answer your question its a gypsum block,a resistive type soil moisture sensor.Inside the block there are ions due to the block being slightly dissolved when exposed with water.Using a dc voltage to excite it will cause electrolytic effects,gassing will occur and the readings will not reflect the actual one.

Roff i dont know if the PiCs A/D will be able to convert the 1Khz ac?Im quite a newbee in PIC.Will it be able?

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Nigel Goodwin

So you're measuring IMPEDANCE and not RESISTANCE, it makes a huge difference.

You could do it entirely in the PIC, but it would be a lot simpler to have an external rectifier.

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JimB

It does help to know the whole story.


Yes I noticed that and seeing "resistive sensor" and "needs to be excited with AC" set alarm bells ringing in my head. This is either something out of the ordinary, or you are barking up the wrong tree.

But now it all makes sense.

JimB

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vinke

I will go for simplicity cause its only part of my project and have lots of other things to do.
I think ill proceed as follows,(please correct me if im wrong):
1. Just after the ac bridge ill put an instrumentation amplifier(a rail-rail one with single supply voltage (+5V and ground)).
2.Then at the output of the instrumentation amplifier ill put the precision rectifier and send the rectified AC to the PIC A/D module.

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