hi,
The datasheet states that the linear region for the sensor is 0.6 thru 3.0 Oe, so I would imagine the magnetic field from the heart muscle would fall well below this Oe level.
I would expect stray electromagnetic fields from nearby equipment to be a cause of detection problems in the system

You would just get a very weak signal, try to amplify by 2000.

What is the OPA output connected to?, why must it be 1Vpk?

Eric

 

Hi,

I see... ok... I'll try 2000 gain... The OPA is connected to a µC. From the µC datasheet, the min input voltage to the port should be 0.9V and max should be 2.1V.

S.K

 

hi,
It appears that the mcu is just detecting the signal as logic low or high, or are you using a mcu with an ADC input?

Eric

 

Hi,

The mcu is detecting falling edges of the input signal.

S.K

 

You can use the 'Ref' pin on the OPA to give a positive offset to the output signal, say by 0.5V.
So the 'ac' output signal of 2Vppk, would appear to the mcu input as +0.5V to 2.5V

 

Hi,

I see... Thanks for your help.

S.K

 

Hi,

Will it make a difference if i use a 2 stage amplifier circuit? I have tried using a gain of 2000 but in vain. Or do i have to redesign the amplifier circuit?

S.K

 

hi,
You can add a further non inverting OPA to the output of the 1st instrumentation OPA. The more stages of amplification you add for a very weak signal, will also amplify any external/system electrical noise.

A simple noise R/C filter between the 1st and 2nd OPA's would be a good idea

What Vpeak to peak signal output do you get with a Gain of *2000?
Is the Vsignal swinging from Gnd/0V and have you added the Vref voltage to the instrumentation OPA?

If the PIC input is looking for a low going 'TTL' edge, ie: +5V to 0V, then you should consider using a Comparator as the
2nd stage connected between the output of 1st stage and the PIC input.

Eric

 

Hi,

The output got saturated at a gain of 2000. About the Vref thing, i have not tried it yet. Because the PCB board they provided does not make use of the vref pin. It is shorted to ground.

So if i use the Vref, i have to connect Vref to a voltage source of 0.5V?

S.K

 

hi,
What do you mean 'the output got saturated', are you saying in reached almost +5V. So the swing was from 0v thru +5V??

Tell me the minimum/maximum voltage swing on the OPA output.

Eric

 

Hi,

Instead of becoming amplified, the output becomes smaller.

The swing was from 0 to 1V.

S.K

 

hi SK,
I would recommend that you use a 2nd stage Comparator after the 1st stage OPA.[1st Gain *1000]
Set the Comparator, using a pot, to switch over when the input reaches say, +0.5V, this will give a +5V edge for the PIC input. [you said the PIC was looking for a low going edge]

Do you follow this explanation
Eric

 

Hi,

Using a op-amp without feedback? So the 1st stage op-amp gain will be 1000? Whats a pot?

Sorry, I do not understand.

S.K

 

Hi,
A 'pot' is a shorthand way of writing 'potentiometer' [ variable resistor]

You can configure a standard OPA to work as a comparator.
Problem is, some OPA's will 'latch up' when used this way.

What OPA's do you have?
EDIT: Look at this pdf

 

Hi,

I see...

I only have the OPA that I showed you. INA118.

Do I have to purchase another type of OPA or can I use the INA118 as the 2nd stage OPA?

S.K