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Part Number: LM741
Hi! I am currently working on designing a Compactor Circuit for a pulse signal from GM Tube as shown in Oscilloscope! It would be grateful, if you could help me to figure out the best Part number for the Op-Amp IC and the Zener Diode for the circuit below. The specialties I am looking from the Op-Amp IC is it can detect 500 microseconds pulses and getting a TTL pulse for the computer with ultra low noise (Since TTL pulse output, I am wondering to supply +5V for +Vcc of the IC ). The purpose of the Zener Diode for shapping the pulse for TTL output. I worked with 741 series ICs, But I was suspecting about the noise and setting +5V for +Vcc!
I would appreciate any advice.
Thank you very much in advance for your help!
Hello Lakshitha Lathpandura,
Welcome to the forums. Your circuit is stressful and harmful to the op amp. I added some protection and changed to a comparator.
I'll let the comparator team refine this circuit.
Regards,Ronald MichallickLinear Applications
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In reply to Ron Michallick:
Thank you very much for your reply and I really appreciate your time! I examined the suggested circuit and I read the Datasheet of TLV3401IP.
However, I need to detect the negative peak of the input pulse (between - 2.0V to -4.0V) (please see the Oscilloscope Image) for the comparator circuit! Because I am going to improve the input pulse by a RC Circuit and and after it the positive tale will be diminished. Since TLV3401 IC works in 0 to 16 V supply voltage, I am in a doubt of using the suggested IC for a comparator with negative reference voltage (Vref = -2V)!
Further, this GM Tube is not for a portable device and it is for an IPE (Inverse Photo-Emission) spectrometer! Therefore, ultra low noise also I am expecting! The output of this Comparator circuit (TTL pulses) will be fetched to a NI USB-6009 DAQ device for LabVIEW software in the Computer!
I look forward to hearing from you back!
In reply to Lakshitha Lathpandura:
You could also configure a comparator in a split supply configuration so that your -2V signal will still be within the common mode range of the device. Something like the TLV1701 could work for your application. It is an open drain comparator with 300uV input offset.
Looking at the datasheet of your DAQ, it seems that it can take +/-10V single ended measurements, so the output of the TLV1701 with a split supply configuration should be compatible with your DAQ.
Do you have the option to power the comparator from split supplies? We have comparators such as LM311 that can be powered from split supplies for the input stage and still level shift the output with respect to ground.
In reply to Chuck Sins:
There has not been any activity on this thread for a while, so I will be closing it.
If you need further support, feel free to reply or open a new thread.
are you sure that only -2V...-4V are coming from the Geiger Müller Tube? Your scope plot shows a much larger pulse.
Can you post a schematic of your Geiger Müller tube showing the HV supply voltage, "sense" resistor, "load" resistor, caps for decoupling and AC coupling, etc.?
I am sorry that I could not reply until now! Wednesday and Thursday our detector had a leak and then we had to get it back working!I really appreciated your reply by suggesting LM311 and I read the datasheet too. I think it's Fast Response time is important for us. We have a "Canberra 1400 NIM Bin" which provides +24, +12,-12,-24 Voltage supplies!
Here I have attached a circuit diagram that I suppose to build using LM311. Since I do not have +5V supply, I can use +24V (or +12V which I already plan to use for powering up the IC) to make a voltage divider with the ground using 2 resistor network. i hope it is not make problem due to the current drop! If you think I did not understand the point you mentioned in the first message, please kindly refine the circuit diagram!
Meanwhile, I built a circuit using 741 IC which gives a basic idea (attached below.) The purpose of the Zener Diode is for limiting the hight of the square pulses for TTL output!
So my plan to replace the 741 IC and the Zener Diode using a LM311 by expecting fast response and low noice interference for TTL pulses! Thank you very much for your time and kind attention!
In reply to kai klaas69:
Thank you very much about your reply! I am sorry about the confusion. I meant that I want to detect negative peak by using a comparator with reference voltage around -2V for the 3rd pin of the Op-Amp IC. We are developing electronic read-outs for a GM Tube to get counts for NI USB-6009 DAQ device which is linked to LabVIEW software in the Computer!
Here is the full circuit, I have developed so far! I have attached Oscilloscope Images of some connection points to make it sense! This GM Tube is consisted with a gas mixture of He and Iodine Vapor (Total ~20 torr) with 43pF inherent Capacitance. This detector is capable of detecting VUV photons (10eV) with the help of SrF2 window and gas mixture cut off! So my intend is to reduce noise and other interferences to read real counts. Therefore my 1st question was which IC (Op-Amp, Comparator) is suitable for this purpose instead of general 741 IC. My 2nd question (If you can help me) is what are the good values I should use for Cc and Cs Capacitors! Sometimes I feel Cs Capacitor is not required too! It seems it keeps low count rate!
I feel Rb is around the right value! Because when it lowered to 2MOhms, I noticed multiple peaks in a count (see the last separate Oscilloscope Image). It may result of multiple Geiger Discharging in a count since it does not quenching well externally. Higher Rb values reduce the count rate!
Thank you very much for your time and kind attention! I look forward to hearing from you back!
have you thought about inserting a 100k resistor, or similar, between cathode of Geiger Müller tube and signal ground? This would give you a positive pulse and eventually simplify the circuit.
Thank you very much for the reply! Actually, I could not mention you that our Cathode is always grounded with the Ultra High Vacuum (UHV) Chamber! Therefore we can not introduce any other circuit for it! Onetime I tried by setting negative Bias Voltage to the center wire (converting into a cathode instead of Anode). Then I got a pulse. But it is not good for center wire, because corrosive Iodine (Quenching Agent) will be deposited on it and it will reduce the count rate with long period of time! Do you have any suggestion for Cs and Cc Capacitor Values?
Thank you very much for your kind attention! I look forward to hearing from you back!
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