nchemlani:
I tried to filter through the available instrumentation amplifiers available on our supplier's website, and I narrowed it down to 27. Assuming I used the correct search terms, will any of these 27 work, or is there something more I'm looking for?
The INA331 looks ideal - in fact, you should leave out the MCP602/602 buffers when using that chip, because the INA331 has an input bias current of only 0.5pA.
[EDIT: I just noticed that the common mode input voltage of the INA331 doesn't include ground. So you'll need to connect the return side of the 1G ohm resistors to a voltage source providing about +0.7V, instead of to ground. I suggest you generate such a voltage source using a resistor and a forward biased silicon diode.]
nchemlani:
I tried to filter through the available instrumentation amplifiers available on our supplier's website, and I narrowed it down to 27. Assuming I used the correct search terms, will any of these 27 work, or is there something more I'm looking for?
The INA331 looks ideal - in fact, you should leave out the MCP602/602 buffers when using that chip, because the INA331 has an input bias current of only 0.5pA.
[EDIT: I just noticed that the common mode input voltage of the INA331 doesn't include ground. So you'll need to connect the return side of the 1G ohm resistors to a voltage source providing about +0.7V, instead of to ground. I suggest you generate such a voltage source using a resistor and a forward biased silicon diode.]
And I'm confused again...how do I do that last bit?
The chambers are going to be a complete circuit in themselves - the chamber itself will be connected to one terminal of the battery, and there is a probe within the chamber that connects to the other terminal of the battery. This creates an electric field within the chamber that attracts the ions from the ionisation.
And if I leave out the MCP602 buffers, that would mean I connect a 1G ohm resistor in each chamber circuit...and then connect the INA331 accross those resistors? And feed the output from there into the Arduino board?
PS - you will probably want to feed the voltage on the INA331 REF pin into another Arduino analog input pin, so that you can subtract it from the main reading.
Are those two circled capacitor like things supposed to be the ionisation chamber?
The setup we had in mind is as attached. We need two separate 9V batteries, one for each chamber. The unknown circuit is circled.
Really was hoping to keep this as simple as possible, but it's turning out a bit complex. Don't want to use an additional PCB or breadboard, because the electronics setup is already a bit clunky, won't give us much freedom in designing the actual chamber. Initially thought it was going to simply be an additional component added into that 'unknown circuit' bit. Is there really no other way to do this?
nchemlani:
Are those two circled capacitor like things supposed to be the ionisation chamber?
Yes.
nchemlani:
The setup we had in mind is as attached. We need two separate 9V batteries, one for each chamber. The unknown circuit is circled.
The "unknown circuit" could comprise a suitable op amp with low input bias current, configured as a non-inverting amplifier. See attached schematic. If you want to run the op amp from the 9V battery, then you should use an op amp with very low current consumption, in order to get good battery life. The MCP603 has a supply voltage limit of 6V, so you can't use that unless you feed it from a micropower voltage regulator (e.g. MCP1702). The TS271 would be a good choice, although it needs an additional resistor connected between the Rset pin and ground. LMC662 is less suitable as it draws a couple of mA.
The 10K resistor in series with the output is there to protect the Arduino input if the output voltage goes above 5V.
Have you seen the price of 1GR resistors? They do not come cheap.
Also have you considered how you are going to build this? These are extremely small currents and high resistance values. Your fingerprint is likely to be a lower resistance than 1GR so it requires special construction techniques. Try not to touch the components and make sure your PCB is clean of all dirt and contamination. Definitely not a beginners project in my opinion.
You could wire the chip dead-bug style, that way there would be no pcb for moisture to collect on - although it could still collect on the chip. If you do use a PCB then I suggest that after assembly, you de-flux it, warm it to drive off moisture and then spray it with PCB lacquer - although I don't know for sure that this will provide a resistance greater than 1G ohm. Either way, putting the amplifier inside either the chamber or a separate shielded compartment will be necessary.
Trying to design the PCB on Eagle atm (Freeware version). One of the Mech labs in uni has the full version on it, gonna try to meet with the technician and get his help in designing in correctly - there are like 15 different symbols for resistor with different sizes and I am not sure which one I should be using.
Trying to arrange a meeting with one of the Elec professors here as well, to get their input on the circuit. Maybe they can suggest something simpler, or help me out and have it made for me by the technicians in the Elec department.
The 1G resistors have reached (around $1 USD per piece), LCD display should be coming in the next couple of days, and the Op Amp by next week. Got the Arduino already, will start playing around with that once I get the LCD display.
there are like 15 different symbols for resistor with different sizes and I am not sure which one I should be using.
You use the one that matches the size of resistor you are getting. The odds are that there will be none suitable and you will have to make your own symbol and footprint.
Here's my eagle schematic for the circuit that dc42 posted up for me.
Looks okay I reckon - mirrored it to get two, one for each chamber. I've included the schematic for a switch, but I don't think that's so necessary? 2 junction points with some gap between them would work just as well wouldn't it, as I would have to solder the switch in anyways...
In our design we are going to lock this circuit and the arduino board into a separate box from the actual chambers. Hopefully that will work.
Still waiting on a professor to get back to me; keeping this schematic just in case they don't reply, in which case we'll go ahead and make it on a PCB.
nchemlani:
Here's my eagle schematic for the circuit that dc42 posted up for me.
In both sides, the negative side of the decoupling capacitor is shorted to the output. It should be a crossover, not a 4-way junction.
I don't think the MAX430 is such a good op amp for this. It has an input bias current of 10pA typical, 100pA maximum @ 25C, which will cause a voltage drop of 10mV typical across the 1G resistor. The bias current is likely to be temperature-dependent, so you will get a temperature-dependent offset voltage. The reason I suggested the TS271 is because its typical input bias current is only 1pA. You want the op amp input bias current to be lower (preferably much lower) than the ionization chamber current you want to measure.
The other advantage of the TS271 is that you can program it (via the Iset pin) to use only a few tens of uA of current, making an on-off switch unnecessary. You will presumably have a snap-connector for the 9V battery, so you can disconnect the battery when the device won't be used for long periods.
btw in the schematic I posted, I chose feedback resistors of 10K and 100K arbtrarily, giving a gain of 11. I don't know what gain you will need, it depends on the ionization current. So you will probably have to adjust one of the resistors. If you go for the TS271 and low current consumption, then you probably want to scale up the feedback resistor values to reduce the current consumption, e.g. use 1M or even higher instead of 100K.
nchemlani:
In our design we are going to lock this circuit and the arduino board into a separate box from the actual chambers. Hopefully that will work.
Bad idea. The wire from the ionization chamber to the op amp input will be extremely sensitive to interference, leakage and stray capacitance. I suggest you put the amplifier inside the chamber, or in a separate screened chamber that is part of the same assembly (as in the article that you linked to). The Arduino can be somewhere else, the wires carrying the amplifier outputs to the Arduino will be much less sensitive.
nchemlani:
Here's my eagle schematic for the circuit that dc42 posted up for me.
In both sides, the negative side of the decoupling capacitor is shorted to the output. It should be a crossover, not a 4-way junction.
I don't think the MAX430 is such a good op amp for this. It has an input bias current of 10pA typical, 100pA maximum @ 25C, which will cause a voltage drop of 10mV typical across the 1G resistor. The bias current is likely to be temperature-dependent, so you will get a temperature-dependent offset voltage. The reason I suggested the TS271 is because its typical input bias current is only 1pA. You want the op amp input bias current to be lower (preferably much lower) than the ionization chamber current you want to measure.
The other advantage of the TS271 is that you can program it (via the Iset pin) to use only a few tens of uA of current, making an on-off switch unnecessary. You will presumably have a snap-connector for the 9V battery, so you can disconnect the battery when the device won't be used for long periods.
btw in the schematic I posted, I chose feedback resistors of 10K and 100K arbtrarily, giving a gain of 11. I don't know what gain you will need, it depends on the ionization current. So you will probably have to adjust one of the resistors. If you go for the TS271 and low current consumption, then you probably want to scale up the feedback resistor values to reduce the current consumption, e.g. use 1M or even higher instead of 100K.
nchemlani:
In our design we are going to lock this circuit and the arduino board into a separate box from the actual chambers. Hopefully that will work.
Bad idea. The wire from the ionization chamber to the op amp input will be extremely sensitive to interference, leakage and stray capacitance. I suggest you put the amplifier inside the chamber, or in a separate screened chamber that is part of the same assembly (as in the article that you linked to). The Arduino can be somewhere else, the wires carrying the amplifier outputs to the Arduino will be much less sensitive.
I can fix that crossover bit. Will do so later and post the new diagram up.
I just used the symbol of the MAX430 cause the TS271 wasn't there and this matched it the closest. Will be using a TS271 in the real circuit.
How do you calculate the gain? I would ideally like the voltage we are feeding into the Arduino to be something large enough to be read and translated accurately, but not so large that it exceeds the Uno's limit. So somewhere between 7 to 12V. I am not sure of the exact range of currents the chamber will give as of yet, since we haven't built or tested the chamber, but it's expected to be around 10^-12 to 10^-15 amps. I guess we'll need a value of that before choosing the right resistors for the circuit?
And thanks for that last bit of info as well; I've passed it onto my groupmates who are designing the housing and chambers.
One more thing...we're expecting a range of 10 Bq/m^3 to 1000 Bq/m^3 for our detector. Our calculations show that this can result in a range of 10^-14 to 10^-16 amps.
If the arduino can only take in 5 to 20V...can such a large range of currents be accurately converted and amplified to the required range of voltages?
We're rechecking the data and calculations now, and will do some practical tests ASAP to confirm this...but yea. Incase the current actually does have such a large range, how should we account for it?
Is there really no other way to do this?
Will get onto acquiring all these components and actually building the circuit, and will see how it goes.
