Hi,
I have never tried using a instrumentation OPA as a Comparator, as I dont have a INA118 on the bench I cannot try it.
Comparators are quite cheap, you should be able to buy one for less than £1GBP, that would be my suggestion.

Eric

 

Hi,

I see... Thanks... But can will it work theorically or practically if i use the non-inverting OPA method as u suggested?

If i use the comparator as the 2nd stage, the 1st stage gain of the OPA can be of any value? So can I use this type of comparator? I have to connect another supply to power up this IC or can I use the original 3V to power up this IC? Also, the 'pot' have to be connected at the Vout? What is the latch pin use for?

http://www.farnell.com/datasheets/64060.pdf

Sorry for asking so many questions at a time...

S.K

 

hi,
I would keep the Gain at 1000 for the INA118 OPA.
Connect the AD8561 as per Fig17 of the datasheet, the output of the AD8561 is connected to the PIC, [low edge detect]
By adjusting the Gain potentiometer on the OPA, you should be able to set comparator detect level.

You may have to select the components for the Comparator to give the best signal detection.

The LATCH pin should be connected to 0V/Gnd, you will not be using the Latch function.

Dont worry about asking questions, if I cannot answer, I am sure other members will be pleased to help out.

Eric

 

Hi,

So the pot have to be connected at the 1st stage OPA to control the gain?

Where does the Vref comes from?

And by using R1 and R2, i can control the Vhi and Vlo?

S.K

 

Hi,
I would recommend that you have a pot on the 1st stage to control the Gain of the instOPA while you are experimenting with sensor/circuit.

The Vref comes from a resistor potential divider [Ra.. Rb], choose say, to have 1mA flowing thru the divider from the +3V supply [Ra+Rb=3K].
You can then calculate the value of the low end resistor Rb, so that you have +0.5V at the junction of Ra/Rb, with respect to the 0V.[Rb=0.5/0.001=500R] You may have to use preferred values of resistors. Ra=2K7 , Rb=560R, will give +0.51V..

You will have to calculate a resistor divider for the Vref on the Comparator.
If for example, you wanted the Comparator to switch at about +0.25V it would require a Vref of about +0.25V

For the initial test you can leave off R2, until you get the comparator working.

Your problems are due to having to many unknowns in the project.
eg: you dont know the strength of the hearts magnetic field and the direction of the magnetic field relative to the sensor.

I would connect the mag sensor to a high sensitivity oscilloscope to determine what magnetic field strength/direction,
I would expect to see close to the heart.
From that, I would know what level of Gain I would require and what the compartor threshold would have to be.

REMEMBER: If you do carry out trials on humans with your project, get qualified advice on any potential electrical hazards before working close to the human heart.

Eric

 

Hi,

I see... I'll try it out

Thanks.

S.K

 

hi,
This comparator circuit will give you an idea of the component values.

Eric

 

Hi,

Thanks alot.

S.K