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op-amp comparator

Hi,

I was doing the sine to square converter with op-amp comparator LM339. During the course i was stuck with a problem. The configuration of the circuit is the sine wave (+/- 10V) was given to the inv. i/p and the reference voltage of around 0 V was given to non-inv, i/p. To bring the hysterisis option i have added feed back resistor to the non.inv input. The op-amp was powered with +10V and Ground.

When i was checking the input signals at inv and non.inv i/p through CRO, the observation was my reference signal was also follows the input sine wave during the negative signal period of the input sine wave. Also my -10V sine wave was clamped to around -0.5V automatically. Also i have got the high and low transition two times per cycle of the input sine wave meaning that my square wave output frequency was also double that of the input sine wave frequency.

Here my doubts are 1. Why was the clamping at the input sine wave had happened during -ve period? 2. Why was my ref. input followed the input sine wave during the -ve period alone. 3. Also why was the double the time frequency have come?

I hope this is general basics of op-amp itself. Could any one please clarify my basics.

Thanks in advance.

Regards, M.Nagarajan.

Reply to
nag
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Have you looked at the data sheet for the comparator?

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The absolute maximum input common mode range is -.3 volts with respect to the negative supply rail (zero volts, in this case) to the positive supply. The functional common mode range is 0 to Vcc-1.5 volts.

There is an internal diode junction between the inputs and the negative supply pin that turns on and conducts if the inputs are driven negative. Look at the internal schematic and see the base to collector junctions of the input transistors.

When this diode turns on, lots of internal functions go to hell, including the high impedance between the two inputs.

To make your system work, you will have to add resistance between your +- 10 volt signal and the input and clamp that input with a diode that conducts before the voltage reaches

-.3 volts. This would take a germanium or Schottky diode.

Of you change the negative supply rail to -10 volts or more negative.

Reply to
John Popelish

Or use a resistor network to change the input signal to be in the right range. Perhaps a 10k between signal and input, and another 10k between input and +10v, giving you a 0-10v sine centered at 5v? You'd need to adjust the IN- reference too.

Reply to
DJ Delorie

If you drive either input of the 339 below its ground voltage, internal substrate diodes conduct, spray charge everywhere, and all sorts of very strange things happen.

John

Reply to
John Larkin

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