Howdy,
I have a sensor that is outputting it's signal in mv eg:
0 - 10mv is it's range.
I need to amplify this into the v range (5v max) in order to get my Arduino to read it.
Anyone got a schematic? I have thought about using opAmps, but am fuzzy on the resistor specs?
Ta.
http://www.uoguelph.ca/~antoon/gadgets/741/741.html
or
http://williamson-labs.com/480_opam.htm
or
http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm
You need a non-inverting op amp with a gain set to 500.
Lefty
might need to cascade multiple op amps for a gain of 500. The cheap op amps I worked with stopped being 'ideal' around a gain of 100
might need to cascade multiple op amps for a gain of 500.
The problem with this is the DC offset of preceding stages gets amplified as well. This can dominate the signal and reduce the range. Better to get a good op-amp and do it in one.
Op-amps can cost between £0.24 and £10.00 so there is quite a range of specifications.
You don't want to use an op-amp for this type of application. You want an
instrument amp. An instrument is laser trimmed for offset, gain and CMRR
errors.
The Analog Devices AD623 was recommended on a dorkbot list for a similar
application. It is a rail-to-rail amplifier, can be run off of a single 5V supply or
+-5V and comes in an 8 pin dip package (or an SO8). 0.2mV offset and 0.1%
gain error. It is probably around $4.
(* jcl *)
Yup, I was just looking at the op amp I regulary use (cause its free!) and it wouldn't work very well for you. If you are interested its the MCP6001 from Microchip. It costs approx 25 cents. The max input offset voltage is 4.5 mV which could cause you some problems (Its half your swing!).
If you do go this path (cause its much cheaper), you are probably going to have to use some sort of trimming resistor going into the input pins. Maybe a pot in conjunction with your normal gain resistor.
If money isn't an issue go with jluciani's op amp. It should work perfectly for you without any issues. Plus my solution isn't sustainable as each opamp will have a different offset voltage. Therefore if you are building multiple copies of your design, you'll have a lot of debugging to do.
Keep in mind that for minimum external components and eliminating external manual tweaking adjustments, it's possible to deal with offset and small gain errors with software trimming values and scaling. As long as the amplified signal is using a majority of the 0-5 volt input range and the offset value output is equal to 0 volts or higher then the rest can be dealt with by calibration constants and code in the setup portion of a program.
Also the advice to use a instrumentation amplifier (can be built with 3 normal op amps) has one tremendous advantage and that is true differential voltage measurement, meaning that the voltage to be measured doesn't have to have share a ground reference with the Arduino. This can solve a lot of tricky analog interfacing and eliminate ground loop problems.
Lefty