Need help with instrumentation amplifier - Page 1
Forum section: electronics / beginners

author: mitrynicolae
author_rank: Contributor
timestamp: April 09, 2019, 05:25:11 pm
content: 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)?

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: April 09, 2019, 05:58:42 pm
content: 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=0Right 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.

author: Zero999
author_rank: Super Contributor
timestamp: April 09, 2019, 08:26:49 pm
content: Quote from: mitrynicolae on April 09, 2019, 05:25:11 pmThe 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 the0.01% tolerance you mentioned in the other threadare also quite expensive.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: April 09, 2019, 09:03:08 pm
content: 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.pdfIf not, can you tell us why? Which spec does it fail to meet?

author: not1xor1
author_rank: Frequent Contributor
timestamp: April 10, 2019, 08:19:38 am
content: Quote from: mitrynicolae on April 09, 2019, 05:25:11 pmTo 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 thisAD application note.

author: mitrynicolae
author_rank: Contributor
timestamp: April 10, 2019, 07:09:04 pm
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: April 10, 2019, 07:52:49 pm
content: 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 ...