I have a circuit that amplifies pulses and the goal is to measure them with a MCU's ADC in lowest possible power. The pulses width are about 100uS and I am interested about their peak amplitude.
Now I added a HW trigger (comparator) and the plan is to wake-up the MCU, do some conversions, compute peak then sleep. Particularly, I plan to use SAM21L (Atmel) which have some 5uS wake time from a idle mode (not sleep). The speed is not so important, I'll be glad if I can take 10 samples, so 1 at 10uS.
The problem is that I don't know if MCU approach can be realistic, since most of them wakes up quite slowly so pulse can be missed or wrong measured. As can be seen in image, from trigger to peak I have only 40uS or so.
Now I wonder what options do I have?
- use other smaller MCU only for ADC that can wake very fast
- build a sample and hold circuit
- build a precision rectifier (so sample and hold naturally)
- try a HW tiny ADC but no idea how to catch the peak (so convert and accumulate stand alone)
I'd like some opinions, thank you.
The GREEN trace represents (amplified) signal + some induced noise in first stage. The last stage is a non inverting DC coupled OA, with (+) forced to 0.3V reference for maximal swing to 3V3. The BLUE is my comparator reference, little above reference. The LIGHT BLUE is my comparator output which I plan to use as trigger for MCU.
The tolerance for measuring is not so tight, I just want to measure the peak. Since the MCU will consume much more compared to amplifier, the trigger is supposed to wake-up, do some burst (blind) conversions up to 100uS, then store one result as peak found.
Overall, the peaks are non-repetitive (great idea to predict) but luckily, the normal periodicity is slow, one per minute or so (particle detector). However, in practice can appear multiple pulses in bursts and I need to count them, so I presume that pulses will come next to each other.
The S&H idea or rectifier, came up in case MCU isn't able to respond fast enough.