Correct use of an Instrumentation Amplifier

[Riggy]

Hi all,
I'm new to this forum and hope someone out there can assist me with my miniature project, any help will be most appreciated. I've exhausted what knowledge of electronics I can recall from my schoolings, so hopefully my explanation below is clear and someone can help me out.

I'm building a small and simple instrumentation amplifier circuit to be attached to an existing handheld device to provide an amplified analogue signal out for measurement on a PC via a National Instruments DAQ device. The handheld device has a battery supply.

Specifically, the handheld device is a hydrocarbon analyser, which measures the concentration of hydrocarbons in sample air. Opening up the device, I've found the Input Current Amplifier (National Semiconductor LMC6001) that converts the current signal from the device's ion detector equipment into a voltage. This would normally be converted and passed through an on-board calibration to give a reading on the device display screen. The LMC6001 has a split-voltage supply of +/- 4.9V, but due to the single polarity of the reading, the output voltage is always negative with respect to the device's "ground", which is at the centre of 0V (obviously). The magnitude of this output voltage (sometimes smaller than -2 millivolts) is too low to be measured with any accuracy on my PC's DAQ device due to noise, but I still need to tap into this signal.

Therefore, I've built an amplifier circuit using an Analog Devices AD623 Single Supply Instrumentation Amplifier chip (by "built" I mean I've mounted it on stripboard with a precision resistor of 10 ohm across the gain pins). This is where I've gotten myself stuck (and where my inexperience shows): As I've said, the handheld device is producing a negative voltage with respect to it's on-board ground, but will a single supply instrumentation amplifier with a positive supply work with that ok? I suspect not. Ideally I want a positive voltage out of the amplifier, not a negative one, so could this be inverted by simply ground-referencing not the reference pin but the voltage out of the LMC6001 to the DAQ ground? Could this mean the "floating" handheld device would be constantly adjusting the output voltage through the ground-reference line as the input signal fluctuates up and down? And if so, could this damage the device somehow, or is it perfectly acceptable?

I apologise if the above is not clear, I can attempt to explain everything again differently if anyone would like me to, or perhaps construct some diagrams if that's more helpful?

Thank you in advance for your time!

Riggy

 

[Leftyretro]

Hi all,
I'm new to this forum and hope someone out there can assist me with my miniature project, any help will be most appreciated. I've exhausted what knowledge of electronics I can recall from my schoolings, so hopefully my explanation below is clear and someone can help me out.

I'm building a small and simple instrumentation amplifier circuit to be attached to an existing handheld device to provide an amplified analogue signal out for measurement on a PC via a National Instruments DAQ device. The handheld device has a battery supply.

Specifically, the handheld device is a hydrocarbon analyser, which measures the concentration of hydrocarbons in sample air. Opening up the device, I've found the Input Current Amplifier (National Semiconductor LMC6001) that converts the current signal from the device's ion detector equipment into a voltage. This would normally be converted and passed through an on-board calibration to give a reading on the device display screen. The LMC6001 has a split-voltage supply of +/- 4.9V, but due to the single polarity of the reading, the output voltage is always negative with respect to the device's "ground", which is at the centre of 0V (obviously). The magnitude of this output voltage (sometimes smaller than -2 millivolts) is too low to be measured with any accuracy on my PC's DAQ device due to noise, but I still need to tap into this signal.

Therefore, I've built an amplifier circuit using an Analog Devices AD623 Single Supply Instrumentation Amplifier chip (by "built" I mean I've mounted it on stripboard with a precision resistor of 10 ohm across the gain pins). This is where I've gotten myself stuck (and where my inexperience shows): As I've said, the handheld device is producing a negative voltage with respect to it's on-board ground, but will a single supply instrumentation amplifier with a positive supply work with that ok? I suspect not. Ideally I want a positive voltage out of the amplifier, not a negative one, so could this be inverted by simply ground-referencing not the reference pin but the voltage out of the LMC6001 to the DAQ ground? Could this mean the "floating" handheld device would be constantly adjusting the output voltage through the ground-reference line as the input signal fluctuates up and down? And if so, could this damage the device somehow, or is it perfectly acceptable?

I apologise if the above is not clear, I can attempt to explain everything again differently if anyone would like me to, or perhaps construct some diagrams if that's more helpful?

Thank you in advance for your time!

Riggy

Your new op amp is of a true differenitial input and you are free to simply wire it's + input to your analyzer's common and the negitive increasing measurement signal into the - input, the will result in a more positive output voltage with increasing negitive input signal.

Lefty

 

[Riggy]

Thank you, that's great, I'll try this and see what happens

 

[kaivalya]

Hi all,
I'm new to this forum and hope someone out there can assist me with my miniature project, any help will be most appreciated. I've exhausted what knowledge of electronics I can recall from my schoolings, so hopefully my explanation below is clear and someone can help me out.

I'm building a small and simple instrumentation amplifier circuit to be attached to an existing handheld device to provide an amplified analogue signal out for measurement on a PC via a National Instruments DAQ device. The handheld device has a battery supply.

Specifically, the handheld device is a hydrocarbon analyser, which measures the concentration of hydrocarbons in sample air. Opening up the device, I've found the Input Current Amplifier (National Semiconductor LMC6001) that converts the current signal from the device's ion detector equipment into a voltage. This would normally be converted and passed through an on-board calibration to give a reading on the device display screen. The LMC6001 has a split-voltage supply of +/- 4.9V, but due to the single polarity of the reading, the output voltage is always negative with respect to the device's "ground", which is at the centre of 0V (obviously). The magnitude of this output voltage (sometimes smaller than -2 millivolts) is too low to be measured with any accuracy on my PC's DAQ device due to noise, but I still need to tap into this signal.

Therefore, I've built an amplifier circuit using an Analog Devices AD623 Single Supply Instrumentation Amplifier chip (by "built" I mean I've mounted it on stripboard with a precision resistor of 10 ohm across the gain pins). This is where I've gotten myself stuck (and where my inexperience shows): As I've said, the handheld device is producing a negative voltage with respect to it's on-board ground, but will a single supply instrumentation amplifier with a positive supply work with that ok? I suspect not. Ideally I want a positive voltage out of the amplifier, not a negative one, so could this be inverted by simply ground-referencing not the reference pin but the voltage out of the LMC6001 to the DAQ ground? Could this mean the "floating" handheld device would be constantly adjusting the output voltage through the ground-reference line as the input signal fluctuates up and down? And if so, could this damage the device somehow, or is it perfectly acceptable?

I apologise if the above is not clear, I can attempt to explain everything again differently if anyone would like me to, or perhaps construct some diagrams if that's more helpful?

Thank you in advance for your time!

Riggy

hello...
if i have understood your problem properly then i have a suggetion.
you can apply your negetive output voltage let us say it is in the range of -1 to -10 v then apply a refrence voltage of +20v then the o/p at -1 will be 19v, at -2 it will be 18v and so on. ana at -10 it will be 10v. then you can apply the floated input to into AD202 or any isolation amplifier{in case you need isolation) the output will be obtained in range of 1 to 5 volts and then you can callibrate your scale accordingly in case you need o/p range of 0 to x volts then you can again shift the ground to -1 v and apply the i/p to any opamp for desired gain... i hope i have understood your pblm correctly.. thank you

 

[ericgibbs]

hello...
if i have understood your problem properly then i have a suggetion.
you can apply your negetive output voltage let us say it is in the range of -1 to -10 v then apply a refrence voltage of +20v then the o/p at -1 will be 19v, at -2 it will be 18v and so on. ana at -10 it will be 10v. then you can apply the floated input to into AD202 or any isolation amplifier{in case you need isolation) the output will be obtained in range of 1 to 5 volts and then you can callibrate your scale accordingly in case you need o/p range of 0 to x volts then you can again shift the ground to -1 v and apply the i/p to any opamp for desired gain... i hope i have understood your pblm correctly.. thank you

hi,
Very helpful answer.
But did you see the year.?