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audioguru
Nobel Prize Winner
Canada
4218 Posts |
 Posted - Jun 08 2010 : 08:53:37 AM
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quote:
Originally posted by codingplanet
Already have done. I'm using an LM3900, could that be the problem?
The schematic posted shows a "normal" opamp.
An obsolete LM3900 is a very strange quad opamp that has completely different biasing. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Jun 08 2010 : 09:34:37 AM
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| Aw crap. I'll need to get some new amps then. Cheers audioguru. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Jun 10 2010 : 11:03:08 AM
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So I'm using a TL081 setup as a voltage follower. Whilst the input is between 0v & 0.7v, the output is 10.5v. The output is also 10.5v whilst the input is above 2v. between 0.7v & 2v, the output is input + 0.4v. Why is this happening?
Also, a quick question. Can I use a pull down resistor on the ADC input to stop it floating? If so, what value? 1K? |
Edited by - codingplanet on Jun 10 2010 11:04:05 AM |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Jun 11 2010 : 03:30:30 AM
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| So I read the datasheet and it seems that the when the input voltage is within 3v of the negative rail, the output goes high. Or something to that effect. So I guess that means that I need negative voltage rail, or get different op amp. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Jun 12 2010 : 11:41:02 AM
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| I now found an LM358. It works as a voltage follower (woot!). But I have a question. With a single supply to the op amp, can I feed negative voltage to the input and everything will be well? I'm not expecting a negative voltage out of the op amp. I would give it a go but I can't right now, no negative supply. Cheers. |
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BEatonNo1
Nobel Prize Winner
USA
1133 Posts |
Posted - Jul 29 2010 : 11:55:14 PM
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Check the data sheet for your parts, in the absolute ratings section they will list how far you can go beyond your rails. If I had to guess the -2 would be bad.
Try a differential amp (http://en.wikipedia.org/wiki/File:Op-Amp_Differential_Amplifier.svg) supply the op-amp with +/-5 and use -2 as your V1 to put your output from 0-2.5. could also tweak the gain to get 0-5 if you were ambitious.
Also on the output of the op-amp stick a schottky diode such that the op-amp output is tied to the cathode and the anode is tied to ground that way if you do get a negative output from the op amp you will not exceed the ratings of the pic.
Have you figured out anything on the thermocouple? I would recommend a thermistor or diode to measure the temperature. Most CPUs now have diodes in them to measure temperature I think. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Jul 30 2010 : 09:57:35 AM
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Thanks for the info but I think the project has died.
All CPUs now have a temp sensor in them (quite possibly a diode), but next to none go down to the temps I need. Also I can't constantly monitor them, like I can with a K-Type and meter. Are there thermistors that go down to -200c? |
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BEatonNo1
Nobel Prize Winner
USA
1133 Posts |
Posted - Jul 30 2010 : 11:45:04 AM
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Yeah to get those temperature ranges you would be better off with a thermocouple. Microchip has a good app-note about using one of their I2C ADCs and I2C temperature sensor to measure a thermocouple. Both of the parts they use are only in SMT packages though. There are other parts out there to do this with though.
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Jul 30 2010 : 3:58:23 PM
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| Thanks. SMD is no problem BTW. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Aug 06 2010 : 08:19:37 AM
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| OK so Im having another look at the AD595. Looks very easy. Maybe Im wrong? Anyway, how do I go about getting a 2v Ref? |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Aug 06 2010 : 08:39:03 AM
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| How about a potential divider using 12k and 8.2k resistors with 5v input? Accurate enough for a op amp if I use 1% resistors? |
Edited by - codingplanet on Aug 06 2010 08:40:43 AM |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Aug 06 2010 : 11:38:18 AM
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| So the potential divider won't work with the op amp (doh). So how do I get 2v reference? I have found the LM4140 but nothing else. |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Aug 06 2010 : 3:39:28 PM
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Sorry about the post spam. Please disregard previous posts. Am I better off using an op amp with a gain of 2 and offset of 2 with 0-5v going into my PIC using VDD as the vRef for ADC. Or, do I use a gain of 1, offset 2 opamp with 0-2.5v into the PIC, using a 2.5v vRef on the ADC?
Is one better than the other? |
Edited by - codingplanet on Aug 06 2010 3:40:04 PM |
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codingplanet
Mad Scientist
United Kingdom
195 Posts |
Posted - Aug 15 2010 : 10:14:29 AM
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| Nevermind Ive sorted it now. |
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RRITESH KAKKAR
Apprentece
India
23 Posts |
Posted - Aug 20 2010 : 04:16:05 AM
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HELLOW friends,
I am not know how to make microcontroler project plz tell somebasic of it......! |
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