Differential amplifier - Page 1
Forum section: electronics / beginners

author: spintinas
author_rank: Contributor
timestamp: February 21, 2018, 05:47:57 pm
content: Hello, I have a problem with differential amplifier. I use LMC6484 (http://www.ti.com/lit/ds/symlink/lmc6484.pdf) op amp to project differential amplifier (schematic in the picture). Resistors values are R1=R3=100k and R2=R4=1k, so theoretically gain should be about 100. My object for this project is to amplify signal from Honeywell pressure transducer (https://stevenengineering.com/tech_support/PDFs/31DTMAIN.pdf) which voltages are about 0-30mV, to voltages for adc. I connect this circuit on breadboard and connect my pressure transducer signal+ and signal- wires to differential amplifier + and - inputs. Voltage between pressure transducer data pins, when there is no pressure is about 30mV, and in differential amplifier output i get 4,6V, but when I apply pressure to transducer and transducer output voltage is increasing, there is no change in amplifier output - still 4,65V. Maybe someone know solution for this problem?

author: Audioguru
author_rank: Super Contributor
timestamp: February 21, 2018, 06:18:53 pm
content: You do not show a picture of your differential amplifier and its schematic is not in the datasheet of the opamp so we don't know its power supply voltages and if the differential amplifier is made properly.

author: spintinas
author_rank: Contributor
timestamp: February 21, 2018, 06:24:58 pm
content: This is schematic.

author: Benta
author_rank: Super Contributor
timestamp: February 21, 2018, 08:40:47 pm
content: The pressure sensor datasheet you link to contains around 50 different sensors. No way to help you with so poor information.

author: ogden
author_rank: Super Contributor
timestamp: February 21, 2018, 08:49:11 pm
content: Quote from: spintinas on February 21, 2018, 06:24:58 pmDebug it. You need to build test bridge using fixed resistors and pot, with help of multimeter then manually set differential voltage and look how changes opamp output. Using real sensor it is hard to debug, especially if it is not easy to change it's output

author: spintinas
author_rank: Contributor
timestamp: February 22, 2018, 07:32:04 am
content: Pressure transducer is AB model

author: Benta
author_rank: Super Contributor
timestamp: February 22, 2018, 10:37:12 am
content: Full scale output of that sensor is 100 mV, which is outside your output swing at x100 amplification.Make sure the +Signal from the sensor goes to R4, if the sensor leads are swapped it won't work. I'd remove R3, as your input is not ground referenced, but floating.

author: danadak
author_rank: Super Contributor
timestamp: February 22, 2018, 11:20:48 am
content: You have not handled the common mode voltage with biasing fromthe bridge, some ref material -https://www.edn.com/design/analog/4322833/The-right-way-to-use-instrumentation-amplifiershttp://www.ti.com/lit/ug/tidub00/tidub00.pdfhttp://cds.linear.com/docs/en/application-note/an43f.pdfhttps://www.eevblog.com/forum/projects/oscillationinput-issues-in-single-supply-bridge-amp/If you choose not to use a laser trimmed IA keep in mind how critical OpAmp to AmpAol variance and resistor tolerance greatly affect CMR, which you want as high as errorbudget dictates you are seeking. A study in MatLab, attached.Regards, Dana.