0.3V signal to 3 V signal amplifier

[tonyctsiu]

I have to design a circuit to convert the signal picked up by an inductor coil. The signal to be picked up is generated by strobe. The voltage generated by the inductor peak at some where ranges from 0.3V - 5V when viewed in my oscilloscope
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Now the problem is that I need to convert the pulse into a logical one which is fed into a logic pin of MCU, which is at least 70% of the Vdd.
Some other requirement:
The battery CR2032 must operate this circuit for 3 years during idle.
The delay of the MCU receiving the logic 1 must be less than 1 us.

I an struggling between using MOSFET or opamp. For the opamp option the low power opamp does not have enough slew rate for the mcu to detect the local 1 fast enough. So it seems that MOSFET is the only option.

For MOSFET. is there any (very) low power consumption design which can convert 0.3-5 V into logical 1 very quickly?

Thank you
Tony Chun Tung Siu

 

[srizbf]

how about having the lo power opamp as comparator with some hysterisis at the
required threshold voltage and connect the output of opamp to mcu ?

 

[tonyctsiu]

I believe that the low power consumption op-amp does not have fast enough slew rate to send the logic 1 to the mcu fast enough. E.g. NEC opamp which consume 900nA has only slew rate of 2.3V / ms. It means it needs 1 ms to give me a logic 1. It is too slow.

I am now thinking of the solution of adding more indutance to to the inductor so that it can output the voltage high enough to trigger an FET which is in cut of mode during idle. This may be the most power saving solution.

 

[keith1200rs]

Maybe something like the MAX9075 will do the job? 3uA power supply current, 580ns propagation delay at 10mV overdrive, low to high. 250ns high to low.

Keith.

 

[tonyctsiu]

Thanks Keith,

Just one more question. I would like to compare the voltage with a 0.3-0.4V reference. Do you know what is the most power saving way to feed such bias to the Vref?

Thanks

Tony

 

[keith1200rs]

If you have a stable power supply voltage then probably with some very big resistors and a capacitor. Maxim do some comparators with built in references but I didn't look at them in detail. They would give more precision but probably take more power than a simple resistor chain.

Keith

 

[tonyctsiu]

Thanks for your suggestion.
I have already order sample for the comparator you mention. I think I will use a big resistor plus a zener diode to pad the Vref to 0.3-0.4V. I hope it work.
You have suggested to use a capacitor. What is it for?

 

[keith1200rs]

If you use some large resistors to generate the reference then you need a capacitor to keep the noise off it and prevent problems as the comparator switches.

I don't know how you are going to use a zener for 0.3V reference, other than a higher voltage and resistors to reduce it. I am not sure a zener is a good way - you might need quite a bit of current.

You could consider a Schottky diode as a 'reference' although they usually aren't great for leakage, but possibly better than zeners.

Keith

Added after 1 hours:

For a voltage reference you could look at something like the MAX6006A which works down to 1uA. You will still need to pad that down to your 0.3V.

A number of years ago I designed an IC for radiation monitoring which had similar requirements. I designed a 200ns comparator which took 2uA and a 12 bit DAC for setting the threshold which took less than 5uA. Unfortunately it was a multi-channel chip with opamps, regulator and other parts so is not available as individual devices, otherwise some of the building blocks might have been a suitable solution to your problem. With newer, smaller geometries those figures should easily be beaten.

Keith.

 

[tonyctsiu]

Looks like I have found what I want
http://cds.linear.com/docs/Datasheet/6650fa.pdf

 

[keith1200rs]

I thought you wanted microsecond speed not millisecond?

Keith.