Author Topic: Need help with instrumentation amplifier  (Read 1396 times)

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Offline mitrynicolaeTopic starter

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Need help with instrumentation amplifier
« on: April 09, 2019, 05:25:11 pm »
Hello everyone,

Based on a suggestion of one member I decided that I should look at instrumentation amplifier. For the moment I could not find one that support under unity gain. Let me explain the issue.

I want an instrumentation amplifier that should support the following:
- should be powered by a single 5V power supply
- should support on the positive input a fixed 20V
- should support a negative input between 0 and 20V
-if positive input equals negative input then the output should be 0; if the positive input is 20V but negative input is 0V then on the output I want 2V. for any other values on the negative input the output should be proportional.

Can anybody help me with a schematic that should solve this issue and also with few instrumentation amplifiers ICs examples (in case I could not find one to buy)?
 

Offline RandallMcRee

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Re: Need help with instrumentation amplifier
« Reply #1 on: April 09, 2019, 05:58:42 pm »
Try this:
https://www.analog.com/designtools/en/diamond/#difL=-1&difR=1&difSl=-1&gain=10&l=-6&pr=AD8250&r=6&sl=-6&tab=1&ty=1&vn=-15&vp=15&vr=0

Right off the bat you will see that instrumentation amps want more than a +5v supply--it makes perfect sense, the internal nodes are governed by the common-mode voltage reqts, around 20 volts in your case. So you need to be realistic. Play around and see what you come up with.
 

Offline Zero999

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Re: Need help with instrumentation amplifier
« Reply #2 on: April 09, 2019, 08:26:49 pm »
Hello everyone,

Based on a suggestion of one member I decided that I should look at instrumentation amplifier. For the moment I could not find one that support under unity gain. Let me explain the issue.

I want an instrumentation amplifier that should support the following:
- should be powered by a single 5V power supply
- should support on the positive input a fixed 20V
- should support a negative input between 0 and 20V
-if positive input equals negative input then the output should be 0; if the positive input is 20V but negative input is 0V then on the output I want 2V. for any other values on the negative input the output should be proportional.

Can anybody help me with a schematic that should solve this issue and also with few instrumentation amplifiers ICs examples (in case I could not find one to buy)?
The only way is to power it from a 20V power supply.

Most ICs can only work with their inputs within the supply voltage rails. There are a few exceptions, such as current sense amplifiers, but they don't work with their inputs at the negative rail and some op-amps' inputs work slightly outside the supply rails, but normally less than 0.5V or so.

Instrumentation amplifiers with the 0.01% tolerance you mentioned in the other thread are also quite expensive.
 

Offline RandallMcRee

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Re: Need help with instrumentation amplifier
« Reply #3 on: April 09, 2019, 09:03:08 pm »
I don't think I've groked your requirements.

Will a difference amplifier do it for you?
https://www.analog.com/media/en/technical-documentation/data-sheets/AD8276_8277.pdf

If not, can you tell us why? Which spec does it fail to meet?
 

Offline not1xor1

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Re: Need help with instrumentation amplifier
« Reply #4 on: April 10, 2019, 08:19:38 am »
Hello everyone,

Based on a suggestion of one member I decided that I should look at instrumentation amplifier. For the moment I could not find one that support under unity gain. Let me explain the issue.

I want an instrumentation amplifier that should support the following:
- should be powered by a single 5V power supply
- should support on the positive input a fixed 20V
- should support a negative input between 0 and 20V
-if positive input equals negative input then the output should be 0; if the positive input is 20V but negative input is 0V then on the output I want 2V. for any other values on the negative input the output should be proportional.

Can anybody help me with a schematic that should solve this issue and also with few instrumentation amplifiers ICs examples (in case I could not find one to buy)?

To get real 0V output you need split supply. Rail-to-rail amplifiers just get close to 0V.
Although there are opamp achieving above rails input range via built-in charge pump, AFAIK there is none doing that for the output.

Apart that you can use any INA with x2 gain and /10 dividers at the inputs.
Another option is to use just an high voltage differential amplifier.
Have a look at this AD application note.
 

Offline mitrynicolaeTopic starter

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Re: Need help with instrumentation amplifier
« Reply #5 on: April 10, 2019, 07:09:04 pm »
Hello all,

Correct me if I am wrong, but from my research, buying one of these amplifiers is exactly what I was looking for: https://www.tme.eu/ro/details/ad628arz/amplificatori-operationali-smd/analog-devices/

If you check the bellow diagram you will see that most of the components are already integrated on the chip.

In my case the connections should be made as follow:
- IN+ to +20V
- -VS to GND
- REF to GND
- CFILT a 0.1uF ceramic capacitor to GND
- OUT to external ADC
- RG to OUT (for setting the output amplifier as buffer)
- +VS to 5V
- IN- to the output of the transistor.

Also when I wrote these lines I just got an Idea. I believe that I on the CFLIT pin I should have in my case the 2V output (max) that I am looking for. This means that with the correct resistors/potentiometer I can set on the output amplifier a gain of 2.5 meaning that on the same chip for the maximum readed value (of 20V) I should have 2 volts on CFLIT pin (and also filtered) and 5 volts on OUT pin. Correct me if I am wrong.
 

Offline Marco

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Re: Need help with instrumentation amplifier
« Reply #6 on: April 10, 2019, 07:52:49 pm »
If the +20V input is truly fixed then it is an implementation artifact, putting the cart before the horse to include it in the design requirements.

AFAICS you want a circuit which outputs 2-0.1*input ...
 


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