Ac bridge

[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.

 

[Nigel Goodwin]

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

 

[ronsimpson]

Precision Full Wave Bridge

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

 

[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?

 

[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.

 

[vinke]

It's not clear to me why you think you need a bridge rectifier.

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.

 

[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.

 

[Nigel Goodwin]

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?

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

 

[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

 

[Nigel Goodwin]

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

 

[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?

 

[vinke]

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

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?

 

[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.

 

[JimB]

Sorry sorry, i should have given specs about the sensor,

It does help to know the whole story.

but i thought it wouldnt bother as i said that it needs to be excited with ac ,

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 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.

But now it all makes sense.

JimB

 

[vinke]

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.

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.

 

[vinke]

One more thing.I will implement the AC bridge in a stand-alone system with a 12V battery(12V because i have other loads which require that).Since i will already have a +5V reference to operate the other ICs in the system,Im thinking of converting that 5V dc into AC by making use of a 555 timer and two transistors (NPN and PNP) and a suitable transformer to get the required voltage source for the AC Bridge. Follow that link to get a schematic of what im talking:
**broken link removed**
Of course ill modify the circuit to get an AC output of about 2V pk-pk with a frequency of 1Khz. Before going into it i will be grateful if i could have some comments about my methodology.Is there any other easier way of achieving what i intend to do?
Thanks in advance

 

[vinke]

Please give me some critics. I need the opinion of other persons,cause i have very little time to implement that and i want to know if at least what im thinking to do it is right..
Is my Avatar causing some reticence? Like :Oh this dude is a manchester fan,i wont give him a clue.. just kidding

 

[ericgibbs]

Please give me some critics. I need the opinion of other persons,cause i have very little time to implement that and i want to know if at least what im thinking to do it is right..
Is my Avatar causing some reticence? Like :Oh this dude is a manchester fan,i wont give him a clue.. just kidding

hi,
Are you planning to drive/power the bridge with an 'ac' sinusoidal waveform or a rough square wave...?

Dont worry about being a Manchester supporter, its a tough job, but somebodys got to do it!...

 

[vinke]

hi,
Are you planning to drive/power the bridge with an 'ac' sinusoidal waveform or a rough square wave...?
!...

I would for the better option.What you think is better? It would be preferable to have a sinusoidal waveform ,but at the same it will be more difficult to achieve.What you advise me?

Dont worry about being a Manchester supporter, its a tough job, but somebodys got to do it

hahahaha..I know

 

[ericgibbs]

hi vinke,
A sinewave drive of constant amplitude and form factor is not easy to achieve, the closest low freq 1KHz osc I would look at the Wien osc.

As you are going to measure the Vpk of the rectified signal, the form factor may not be too much of a problem, but any amplitude variation would affect the signal amplitude and hence the calibration.

What exactly is the Sensor element and what parameter are you measuring?