ADC/Op-Amp/Temp. Sensor design help wanted
parsko
Posts: 501
Hello all,
I need help with the design of a circuit.
What I know:
ADC:· MCP3001
OpAmp: 741
Input resistance: 50-100,000 ohm
Input supply: 3.3V
What I don't know:
How to calculate gain (from the information I DO know)
I would like to have everything running on 3.3v.· I do know enough that my output from the temperature sensor will be too small for my ADC to read accurately.· My temp. sensor has two pins, I assume one is the goes-into, and the other the goes-outta.· Does it mean that it always draws the same current, regardless of resistance?
First, I'm an ME, not an EE, as this is likely found in an EE 101 class, so bear with me...
I know, from V=iR, that:
50ohm @ 3.3V = 0.066A
100,000ohm @ 3.3V = 0.000033A
Conceptually, at 0ohms, I would get 3.3V, and at infinity ohms, I would get 0V.· But what is my value at 1000ohms?·
I have always had a tough time with this concept, and don't know how to design using the info I do know.
Any thoughts or solution to how I calculate gain?· I know how to set gain on the op-amp, just don't know what to set it to...
Thanks,
-Parsko
I need help with the design of a circuit.
What I know:
ADC:· MCP3001
OpAmp: 741
Input resistance: 50-100,000 ohm
Input supply: 3.3V
What I don't know:
How to calculate gain (from the information I DO know)
I would like to have everything running on 3.3v.· I do know enough that my output from the temperature sensor will be too small for my ADC to read accurately.· My temp. sensor has two pins, I assume one is the goes-into, and the other the goes-outta.· Does it mean that it always draws the same current, regardless of resistance?
First, I'm an ME, not an EE, as this is likely found in an EE 101 class, so bear with me...
I know, from V=iR, that:
50ohm @ 3.3V = 0.066A
100,000ohm @ 3.3V = 0.000033A
Conceptually, at 0ohms, I would get 3.3V, and at infinity ohms, I would get 0V.· But what is my value at 1000ohms?·
I have always had a tough time with this concept, and don't know how to design using the info I do know.
Any thoughts or solution to how I calculate gain?· I know how to set gain on the op-amp, just don't know what to set it to...
Thanks,
-Parsko
Comments
Why 3.3V supply (the op amp is a dual supply op amp)
What are you using to read the ADC output?
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think outside the BOX!
The sensor is a standard issue temp sensor used to measure Engine Coolant temp:
http://www.injector.com/sensors.php?PHPSESSID=6487ba5767ffd3f2661d7db1a045ddd7
I know it's dual supply, but I figure that if I could solve how to interface with 3.3V, it would be a matter of scaling to 5 for more accuracy.
I have a routine written for the Propeller to read the ADC out, which I know works, as I have it running already. BUT, I am pretty sure that it also is not hooked up correctly (I have a 10k pot hooked up directly to the input of the ADC right now).
-Parsko
I'm an EE I'll try to help but I'm not sure exactly what your asking. Could you just give me the big picture what you want this ckt to do?
The 741 is a dual supply ckt but you don't need to use it that way. You can put GND on the neg supply and 3.3V on the positive supply. The output will then be a range between 0-3.2V or so.
That sensor you linked to is just a resistor that changes value with temp. It has a large range, 100K to 50. Since you only want to work with 3.3V your range will be 3.3/(100,000 - 50) = 3.31 microvolts per ohm. that's probably not going to work. Then you have the issue of resolution, do you want +/- 10 degree or what do you need? How many bits is that ADC?
Gain is just (output)/(input)=Gain If you have 3.3microvolts and want 3.3millivolts you need to design the 741 for a gain of 100.
There's always a way to do what you want. Give me more specifics and maybe I'll whip you up a ckt on ORCAD.