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mosman

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china

comparator with tens of mV hysteresis
Jul 16^th, 2006, 6:06pm

now i design a comparator which should have tens of mV hysteresis, for example, 20mV, but the spread should be less than +- 10%~20% under all supply, temp, process. many schmitt circuits have larger hysteresis and larger spread. i designed some circuits. but the simulated results seems bad.

can anyone help me. i need any advice, circuits, IEEE papers.
tanks in advance!

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sheldon Community Fellow Offline Posts: 751	Re: comparator with tens of mV hysteresis Reply #1 - Jul 17^th, 2006, 12:06am Hunk, Try adding positive feedback to a comparator using resistive feedback. Since the amount of hystersis is controlled by ratioed resistors, it should be repeatable. There is a nice summary at http://www.ecircuitcenter.com/Circuits/op_comp/op_comp.htm Best Regards, Sheldon
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mosman

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Posts: 22
china

Re: comparator with tens of mV hysteresis
Reply #2 - Jul 18^th, 2006, 12:12am

sheldon wrote on Jul 17^th, 2006, 12:06am:

Hunk,

Try adding positive feedback to a comparator using resistive
feedback. Since the amount of hystersis is controlled by ratioed
resistors, it should be repeatable. There is a nice summary at

� http://www.ecircuitcenter.com/Circuits/op_comp/op_comp.htm
Sheldon

sorry, i haven't describe my specs in detail. my comparator is used in a chip. and it used to compare with a reference voltage. i need this compatator should "one side" hysteresis. it means when input rises from low to hign the switching point should equals reference voltage just like a normal comparator, while from high to low the hysteresis should take effect.
for getting "one side hys", i use a MOS switch. but this swtich will intruduce Ron which will vary with supply,Temp,and processes. so if using the resistor positive feedback, comparator will suffer from those. and need more area in my chip.

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jhuang

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Analog/Power

Posts: 2
Boston Area, USA

Re: comparator with tens of mV hysteresis
Reply #3 - Jul 25^th, 2006, 7:33pm

Hunk,

I think you could get your 2nd threshold (Vref2 = V_ref - V_hysteresis) by using a resistor divider. Avoid using a switch in series with the resistor divider (as you mentioned R_on error) instead using 2 tranmission gates to connect either V_ref or Vref2 to the input of the comparator. This way, the Ron of transmission gate has negligible effect due to the high input impedance of the comparator.

The other concern is the offset and its variation over processs/temperature (from the comparartor itself) which could easily mess up your spec (+/-10%, which is +/- 2mV). You need do somthing special there to meet the spec.

Regards, jhuang

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