Op Amp Alternative

A

ac246

New Member
Hi
I am not too good at reading datasheets and comparing components. I am trying to make a very accurate oxygen meter for underground air monitoring. I am using the winsen ME2-O2 sensor and i have a circuit diagram from the manufacturer for the amplifier to be able to read a value into the ADC on the picaxe.

at the moment i have a small oxygen sensor board which i bought from seeed studio which is giving me varied results. so i am going to make the manufacturers board and see if it helps.

I am trying to find an equivilent of an AD8572 op amp. Preferably on www.maplin.com as i can go there today. I like the idea of the AD8572 having nearly zero drift thrugh the temperature range and full swing from rail to rail. another problem is that the device is powered by batteries and i am having trouble keeping the readings constant as the battery runs down.


I have included my code and the datasheet for the oxygen sensor showing the circuit diagram. The code just accumilates 64 adc readings and then displays it on the LCD. i then plan on converting this value into a percentage of oxygen in the air.. the result of ADC in fresh air should then be calibrated as 21.9% and an ADC reading of 0 would be 0%. I can manage the calculations but just need help to find an op-amp.

I would appreciate any help atall

Datasheet for O2 sensor View attachment ME2-O2-�¤20.pdf

Thanks
Code: 
[color=Black]Display_Oxygen:[/color]

[color=Blue]Pause [/color][color=Navy]1000[/color]
[color=Blue]serout [/color][color=Navy]6[/color][color=Black], [/color][color=Blue]N2400[/color][color=Black], [/color][color=Blue]([/color][color=Navy]254[/color][color=Black],[/color][color=Navy]1[/color][color=Blue])   
symbol readings [/color][color=DarkCyan]= [/color][color=Navy]64[/color]
[color=Black]Main:[/color]
[color=Blue]setfreq m16[/color]
[color=Purple]b0 [/color][color=DarkCyan]= [/color][color=Navy]0[/color]
[color=Purple]w16 [/color][color=DarkCyan]= [/color][color=Navy]0
      
      [/color][color=Blue]do
            readadc10 [/color][color=Navy]6 [/color][color=Black],[/color][color=Purple]w15
            w16 [/color][color=DarkCyan]= [/color][color=Purple]w16 [/color][color=DarkCyan]+ [/color][color=Purple]w15
      
            [/color][color=Blue]inc [/color][color=Purple]b0
      [/color][color=Blue]loop until [/color][color=Purple]b0 [/color][color=DarkCyan]= [/color][color=Blue]readings      
      
            
setfreq m8
serout [/color][color=Navy]6[/color][color=Black], [/color][color=Blue]N2400[/color][color=Black], [/color][color=Blue]([/color][color=Navy]254[/color][color=Black],[/color][color=Navy]192[/color][color=Black],[/color][color=Red]"O: "[/color][color=Black],#[/color][color=Purple]w16[/color][color=Black],[/color][color=Red]"        "[/color][color=Blue])
Goto [/color][color=Black]Main[/color]
 
hippy

hippy

Ex-Staff (retired)
http://uk.mouser.com sell a range of AD8572 op-amps.

I would have expected the Seeed Studio board to have used an AD8572 or something of similar quality so inconsistent reading may be some other issue than op-amp quality. Have you checked with a scope that the O2 sensor is delivering a stable output and checked what the op-amp output into the PICAXE looks like ?

To avoid issues with falling battery power it is probably best to test using a bench supply, move to batteries once you have everything else resolved.
 
B

BeanieBots

Moderator
I concur with hippy & g6ejd.
The op-amp drift will be insignificant compared to that caused by temperature changes at the sensor.
I've not seen your circuit but don't forget that any amplification will suffer from hum and hiss just like an audio amp does. Use correct layout, leads and screening just like you would if setting up a high quality audio system. Cheap carbon resistors generate lots of noise.
 
T

tmfkam

Senior Member
In a circuit used to measure pico-amps, I used LTC1050 op-amps. These have self zeroing offset capabilities.

I must admit that I initially built the prototype using some 741's selecting by substitution those amps with the least offset. I also used a TLE2426 as the rail splitter (not the MAX660 used in the circuit on the datasheet) as I found it easier to implement, but again for prototyping I used another 741 as a rail splitter.

The whole thing worked well, being capable of reading the bias current of silicon transistors and FET's. About 10-15 pico-amps was the lowest obtainable reading, beyond this the induced current in the wires swamped the readings.
 
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