Simple Instrumenatation Amplifier Circuit

[keane2097]

I'm trying to debug a larger circuit of mine by breaking it up into smaller parts and dealing with them one by one. One of the parts of the circuit is an instrumentation amplifier.

The chip is using is the INA118 for TI (datasheet here).

The pin-out of the chip is shown here:-

**broken link removed**

The chip is capable of operating with a single supply, or at least that's what the datsheet has led me to believe. however, to this point I have only been able to get it to do anything meaningful when I use a dual supply of +/- 2.5V. At the moment I'm trying to get it to work witha single supply of +5V & 0V and I'm having no luck at all.

The setup i have at the moment is as follows:-

Pin 1: Connected through two 40kOhm resistors in series to pin 8. Sets the amplifier gain.
Pin 2: Connected through a 390kohm resistor. This is the inverting input pin.
Pin 3: Connected through a 390kohm resistor. This is the non-inverting input pin.
Pin 4: Connected to the negative port of the power supply to give -Vcc = 0V.
Pin 5: Grounded to give 0V offset.
Pin 6: Output.
Pin 7: Connected to the positive port of the power supply to give +Vcc = 5V.
Pin 8: Connected through two 40kOhm resistors in series to pin 1. Sets the amplifier gain.

Using a probe (with the negative end connected to the negative power supply port) I've checked and found that Pins 4 & 7 are at 0V and 5V respectively as desired. Pin 5 is at 0V as desired. Pins 2 & 3 for some reason show a voltage of around 0.48V that I don't know the reason for. Finally, pins 1 & 8 each show a voltage of around 3.5V which i also am not sure is desirable.

I've tried grounding one of the inputs and connecting the other to both a small DC (~20mV)voltage and also to a small amplitude (~20mV p-p) sine wave. In each case I failed to see the expected, amplified and uninverted, output signal.

Can anybody see any issues with how I'm supplying power to my chip? Or any other issue that might be causing my problem?

Thanks...

 

[ericgibbs]

hi,
The cct looks OK,

How do you have the inputs of the Inst amp connected to the signal source.?

Why do you have the 390K's in the inputs.?

 

[keane2097]

hi,
The cct looks OK,

How do you have the inputs of the Inst amp connected to the signal source.?

You mean the inverting and non-inverting inputs yeah?

On each of these I have a 390kOhm resistor and then just a wire coming out of the breadboard that I connect up to my function generator with crocodile clips. So I've got the positive wire from the coaxial cable connected to the non-inverting input of the INA, and the negative wire of the coaxial cable is conencted to the negative on my power supply (0V).

I have the inverting input grounded at the moment...

 

[keane2097]

If it helps, this is the overall circuit I'm trying to build:-

 

[ericgibbs]

You mean the inverting and non-inverting inputs yeah?

On each of these I have a 390kOhm resistor and then just a wire coming out of the breadboard that I connect up to my function generator with crocodile clips. So I've got the positive wire from the coaxial cable connected to the non-inverting input of the INA, and the negative wire of the coaxial cable is conencted to the negative on my power supply (0V).

I have the inverting input grounded at the moment...

That type of amp dosnt like working with the inputs close to/at 0V.
Look at the datasheet for a 0V input.

Make up a test bridge, using resistors, so that [static] bridge output is half Vsupply. In that way you can experiment.

Looking at your circuit, you will see the 'bias voltage' of 2.5V is fed back to the amp inputs.

 

[ericgibbs]

If you measure/look at the voltages at the 390K ends you will find that they are at +2.5V,,, not 0V.!

Thats supplied by the leg connection

 

[q5101997]

Do you honestly think you can build an instrument amplifier. This has been the plight of many an engineer. We are talking about very small input voltages, comparable to the thermal noise. If, somehow, you can amplify very small voltages, without also amplifying noise, you are doing well.

 

[keane2097]

That type of amp dosnt like working with the inputs close to/at 0V.
Look at the datasheet for a 0V input.

Make up a test bridge, using resistors, so that [static] bridge output is half Vsupply. In that way you can experiment.

Looking at your circuit, you will see the 'bias voltage' of 2.5V is fed back to the amp inputs.

Sorry, that schematic is not 100% accurate - I'm actually using an INA118.

The chip is designed for use in biomedical applications as far as I know so the small voltages I wouldn't have expected to be a problem...

Does the bias voltage affect the operation of the INA as well as the three op amps it's directly connected to?

Having measured the voltages at the end of the 390k resistors (with resect to the negative of my power supply) I'm only seeing ~0.5V on each pin - is this because I need to have the Vbias feeding in somewhere?

 

[keane2097]

Do you honestly think you can build an instrument amplifier. This has been the plight of many an engineer. We are talking about very small input voltages, comparable to the thermal noise. If, somehow, you can amplify very small voltages, without also amplifying noise, you are doing well.

The entire circuit shown in the schematic is designed to amplify the signal of interest while eliminating offset and common mode noise.

Thanks for your concern.

 

[ericgibbs]

Sorry, that schematic is not 100% accurate - I'm actually using an INA118.

The chip is designed for use in biomedical applications as far as I know so the small voltages I wouldn't have expected to be a problem...

Does the bias voltage affect the operation of the INA as well as the three op amps it's directly connected to?

Having measured the voltages at the end of the 390k resistors (with resect to the negative of my power supply) I'm only seeing ~0.5V on each pin - is this because I need to have the Vbias feeding in somewhere?

Im not talking of low level voltages, Im talking of low level voltages NEAR zero.

The bias voltage sets the common mode input voltage at half Vsupply, which is +2.5V in that circuit.

To test the circuit on the bench, complete the 'loop' by connecting from the 390K on the output of A2, a 40K to each of the 390K inst amp inputs.

This should set the output of the inst amp at +2.5V
OK.?

 

[q5101997]

Build it and see if it realy does what you claim. I can garantee that it will not work as expected.

 

[keane2097]

Build it and see if it realy does what you claim. I can garantee that it will not work as expected.

Clearly it's not working as expected or i wouldn't be asking for help.

You're not helping tbh.

 

[keane2097]

Im not talking of low level voltages, Im talking of low level voltages NEAR zero.

The bias voltage sets the common mode input voltage at half Vsupply, which is +2.5V in that circuit.

To test the circuit on the bench, complete the 'loop' by connecting from the 390K on the output of A2, a 40K to each of the 390K inst amp inputs.

This should set the output of the inst amp at +2.5V
OK.?

I'll give that a bash so and let you know how it goes - thanks for your help...

 

[q5101997]

It may work better with ±15V. Try it.

 

[q5101997]

Double post. Get a power supply from funked computer, this will give you ±12V

 

[keane2097]

Im not talking of low level voltages, Im talking of low level voltages NEAR zero.

Would you mind explaining the difference here? I'm not sure exactly what you're getting at - analog electronics is not my strong point I'm afraid...

The bias voltage sets the common mode input voltage at half Vsupply, which is +2.5V in that circuit.

Understood. I didn't realise the reference was important for the INA as well as the other op-amps in the design.

To test the circuit on the bench, complete the 'loop' by connecting from the 390K on the output of A2, a 40K to each of the 390K inst amp inputs.

Ok, here's what i've got built now - excuse my shocking MS Paint skills:-

**broken link removed**

As you can see I've now built the voltage follower and driven-right-leg circuitry. I've connected the 390kOhm resistor at the O/P of A2 through two 40kOhm resistors to each input of the INA. Is this what you were suggesting?

This should set the output of the inst amp at +2.5V
OK.?

With this setup I get the following voltage readings (again measured with the negative end of the probe connected to the negative of my power supply):-

Pins 1 & 8 are at ~2.5V.

Pins 2 & 3 are at ~3V.

Pins 4 & 5 are at 0V.

Pin 7 is at 5V.

Pin 6 (the output pin) is at ~40mV but this without any inputs connected to the inverting and non-inverting inputs. All I have connected to them at the moment is the resistors and the right-leg feedback you suggested. Do these values look ok or are they unexpected?

FYI, the source for my bias voltage is a voltage divider. I'm using a voltage regulator to insure that the voltage from the power supply is at 5V, then I've connected two 100kOhm resistors between the 5V and ground, and I'm tapping the bias voltage from the centre point...

 

[Roff]

Would you mind explaining the difference here? I'm not sure exactly what you're getting at - analog electronics is not my strong point I'm afraid...



Understood. I didn't realise the reference was important for the INA as well as the other op-amps in the design.



Ok, here's what i've got built now - excuse my shocking MS Paint skills:-

**broken link removed**

As you can see I've now built the voltage follower and driven-right-leg circuitry. I've connected the 390kOhm resistor at the O/P of A2 through two 40kOhm resistors to each input of the INA. Is this what you were suggesting?



With this setup I get the following voltage readings (again measured with the negative end of the probe connected to the negative of my power supply):-

Pins 1 & 8 are at ~2.5V.

Pins 2 & 3 are at ~3V.

Pins 4 & 5 are at 0V.

Pin 7 is at 5V.

Pin 6 (the output pin) is at ~40mV but this without any inputs connected to the inverting and non-inverting inputs. All I have connected to them at the moment is the resistors and the right-leg feedback you suggested. Do these values look ok or are they unexpected?

FYI, the source for my bias voltage is a voltage divider. I'm using a voltage regulator to insure that the voltage from the power supply is at 5V, then I've connected two 100kOhm resistors between the 5V and ground, and I'm tapping the bias voltage from the centre point...

You're showing an INA326. I thought you had an INA118.

 

[keane2097]

You're showing an INA326. I thought you had an INA118.

I'm using an INA118 alright, sorry about the picture.

 

[audioguru]

The INA118 instrumentation amplifier works pefectly from a 5V supply. But the inputs (the patient) must be biased at half the supply voltage which is +2.5V.
The opamps and +2.5V reference voltage in the schematic biases the patient at +2.5V.

The function of pin 5 on the INA118 and INA326 are completely different so you need to modify the original circuit for it to work with the INA118.

 

[ericgibbs]

As you can see I've now built the voltage follower and driven-right-leg circuitry. I've connected the 390kOhm resistor at the O/P of A2 through two 40kOhm resistors to each input of the INA. Is this what you were suggesting?

With this setup I get the following voltage readings (again measured with the negative end of the probe connected to the negative of my power supply):-

Pins 1 & 8 are at ~2.5V.

Pins 2 & 3 are at ~3V.

Pins 4 & 5 are at 0V.

Pin 7 is at 5V.

Pin 6 (the output pin) is at ~40mV but this without any inputs connected to the inverting and non-inverting inputs. All I have connected to them at the moment is the resistors and the right-leg feedback you suggested. Do these values look ok or are they unexpected?

FYI, the source for my bias voltage is a voltage divider. I'm using a voltage regulator to insure that the voltage from the power supply is at 5V, then I've connected two 100kOhm resistors between the 5V and ground, and I'm tapping the bias voltage from the centre point...

hi,
This is in line with what I would expect, the inst amp 'cancels' the common mode input voltage of ~ +2.5V and so the amp output should be near zero.
This is what you have measured.

Taking Ron's point, which inst amp are you working with.???
Please advise and if necessary post the correct datasheet.

If you look at the ECG cct you have posted, you will see that on the patients ankle is ~ +2.5Vdc.
This is the same voltage at the junction of the 40K's on the circuit.

The 40K's you have added, simulate the resistance of the patients body, between the ankle point
and the other two points on the patients chest.

Ideally the 'bridge' formed by these 4, 40K's should be in balance and both ints amp inputs should be identical, ~ near +2.5V.

Your task now is to 'inject' a low level voltage into the bridge, so that the amp senses and amplifies
the 'difference' on the inst amp INV & NI inputs.

You could simulate this by having a variable resistor in place of one of the 40K that you have added.
eg: a 39K and a 2K multiturn pot in series.

This will give ONLY a dc level change in the output of the amp..

The remainder of the circuit expects a 'pulse' [ac] signal from the patients heart...
To fully test it you will need to inject low level pulses into the input bridge.

DONT use +/-12V on this circuit as some has suggested.

Do you follow.?

Why have you taken on such a complex project.?