Not enough gain?

[S.K]

Hi,

I see... Thanks for your help.

S.K

 

[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

 

[ericgibbs]

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

 

[S.K]

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

 

[ericgibbs]

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

 

[S.K]

Hi,

Instead of becoming amplified, the output becomes smaller.

The swing was from 0 to 1V.

S.K

 

[ericgibbs]

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

 

[S.K]

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

 

[ericgibbs]

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

 

[S.K]

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

 

[ericgibbs]

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

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

 

[S.K]

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?

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

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

S.K

 

[ericgibbs]

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?

https://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

 

[S.K]

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

 

[ericgibbs]

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

 

[S.K]

Hi,

I see... I'll try it out

Thanks.

S.K

 

[ericgibbs]

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

 

[S.K]

Hi,

Thanks alot.

S.K