Current sensing

[Mosaic]

Hi,
I want to build a current sensing app with opamps....

Can anyone recommend a suitable opamp to amplify DC voltages under 0.1V to output around 2V etc?

I guess precision and the ability to swing to the ground rail s important here.

Thanks!

 

[Reloadron]

The AD623 Instrumentation Amplifier is a popular choice as it is inexpensive and rail to rail working from single or dual supplies. Adding a single 1.02 K Ohm resistor sets the gain at 100. Making it easy to work with.

Also there is Allegro Micro Systems who offer a wide choice of current sensor IC chips. Depending on how you want to go about things.

Ron

 

[Mosaic]

Thx...it's a bit pricey, I have this on hand...
https://www.electro-tech-online.com/custompdfs/2012/02/MC33171-D.pdf
What do u think?

 

[Reloadron]

The AD623 I mentioned cost less than $5.00 USD and is a Rail to Rail Instrumentation Amplifier designed more for what you want to do, I don't see 5 USD as expensive. This is the operational amplifier you have which is not a rail to rail amplifier and not really designed for doing what you want to do. Note the input offset voltage of the AD623 as compared to the MC33171 for starters. For an amplifier to be used in a instrument amplifier capacity the 2 mV of the latter is quite a bit as compared to the 100 -200 uV of the AD623. How high of a current do you plan to measure?

Ron

 

[ericgibbs]

hi Ron,
Depending upon the OP's input configuration [ ie: the CMV input ] the output swing can be limited, especially with a single supply voltage. Ref page #8 of the datasheet
E.

Mosiac,
How do you plan to connect the current shunt sensing resistor to the IA [OPA] .?

 

[MrAl]

Hi,

Microchip makes a couple nice op amps too that fit into this. Alternately you can use an op amp that has an offset null adjustment, but that technique is really becoming more outdated if it isnt already.

 

[Reloadron]

Hi Eric & MrAl

I agree the MC33171 can be used, I just do not see it as the best choice. I do also wish there was a little more information as to the application setup in some detail. I know that in many parts of the world parts can be hard to come by and when they can be had, even what I feel are inexpensive common parts can be quite expensive and hard to get. Memories of my very first transistors just flashed past.....

Ron

 

[Mosaic]

On looking at the range I'd like to monitor (1-50 Amps), it appears that the Allegra link Reloadron offered would be the better solution. It's about $8.00 but has a reasonably linear result and can work with unknown polarities. From what I can tell the resolution is about .05V per Amp which works out to about 10.24 bits per amp (5VDC ref). +/-50A = +/-512 bits (10bit adc) around the 1/2Vcc zero current datum. I should be able to directly feed an MCU ADC input.

I found the IA pricey ...relative to FREE from the OPA I have, of course accuracy and precision will suffer. I wouldn't need a full swing to the Vcc rail, so the 33171 could work in a more limited way. Reloadron gave useful advice , which I will take.

I buy mostly from Newark or Digikey.

Thanks for the help!