PhotoDiode Output | TIA Output | Comparator Output |
7.38uA | 2V | 5V |
0A | 0V | 0V |
Your circuit differs much to the circuit in theTI application note:
7705, makes one wonder why thats not in some jelly bean OpAmps/Comparators.....Hi,
'But, the comparator outputs square wave (as expected), but it’s only 1V in Amplitude. After revisiting the theory, I found out that one parameter was limiting the output voltage of my comparator, but, being a newbie to circuit designs, I wasn't able to find a solution to my problem. I kindly request someone to help me out to fix this issue.'
What does that mean in reality? Share the secret parameter, please.
I was curious so did a quick simulation with the Microchip MCP6021 Spice model in Tina, and fwiw - because it's only a simulation that lacks the finer details of your actual circuit - these are the results with GND as a trip threshold and 250mV as a trip threshold:
View attachment 172115
I wonder why the simulation does not do what your circuit does: 'the comparator outputs square wave (as expected), but it’s only 1V in Amplitude'. I was so curious about the 1V you get that before simulating I read about Arduino gpio as input impedance and it's supposedly in the Megaohm range so draws no current as that was my initial thought - that no current-limiting resistor might draw op amp full current and lower Vout from expected ~4.95 V to 1V, but apparently not. You haven't set your GPIO incorrectly or something, have you? Is the 1V only issue related to the GBW of the MCP602 at 1 kHz perhaps?
I'll add, a curious IC that might or might not be of interest to you, or the concept behind it, regarding your using ground as a trip threshold is the e.g. LM7705, it creates a ~ -250mV offset on the op amp ground pin, avoiding the need for a dual supply, especially helpful with low voltage devices, it might give you ideas that help to avoid a trimpot or similar horrors and what I suspect is an unwanted offset in the trip threshold.
Hi,
Please:
* Show your exact and complete schematic (hand drawn is O.K.)
* show photos of the circuit and of the wiring
Klaus
No, that plan to use uC to measure waveform will be on future.You using code in uC to measure the waveform ?
Regards, Dana.
Hi,
First, ' I tried Voltage Follower Expt. today, there also I am getting a Square Wave of 1V Amplitude(0V Ref), when the input is a 1kHz, 5V Square Wave. Any thoughts?'
- My first thought is what is a 0V Ref in a voltage follower...? There are no spare input pins for a voltage reference in a voltage follower configuration. I assume you mean the square wave goes from 0V to 5V.
- My second thought is that it's pointless asking questions about circuits no-one can see or other questions that refer to schematics with components referred to in text but not included in pictures. Or giving results/values that change with each subsequent post. You have changed the input voltage of the square wave to the red led from 2.2V to 5V, I think. Remember what you can see and assume others will understand, usually they can't - noone on any forum is a clairvoyant.
- My third thought is that 'expect' is not the same as 'see' and 'measure' in-circuit or in a simulation. In your first post you say that the TIA does output 2V, presumably your second schematic means this by the 'Result [expected]'. 'At the TIA output I expect 1 KHz square wave [0V - 2V]' - that wording leaves me unsure that you do get a 2V square wave at the output of the TIA when the 43uA is present at the input or you don't get it but want to.
- My fourth thought is that - based on the simulation I just did now, and the others from earlier today - if you want a 2V square wave out of that TIA and not a slow rising and falling, curvy RC shark's fin wave that may not even reach 2V at it's peak, I'd reduce the 3nF by an order of magnitude to 330pF.
- My fifth thought is, that from the simulations I did today, you should be able to increase the gain of the TIA to reach the presumably >3V or higher the Arduino input needs to see to sense an input high and improve the square wave rise and fall times if necessary (playing with the capacitor value), ditch the comparator idea completely and use the second op amp as a voltage follower/buffer instead, and you might not even need that second OA if you can shape a decent square wave out of the TIA. Don't Arduinos and so on have Schmitt Trigger inputs anyway?
UNO and Nano board (later version), do. Atmega328 (Nano V 3.0 and beyond).Hi,
"Arduino" uses several microcontrollers from several manufacturers. I assume some have schmitt trigger, some don't .
Thus I think one can not generally say "Arduino have schmitt triggers"
Better is to refer to the microcontroller datasheets.
* Atmel/Micrichip ATmega (various types)
* Espressiv ESPs
* STM32...
* MSP430
* several ARM core types
...and so on
Klaus
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