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how to design zero crossing comparator with single supply?

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ddt694

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In a digital circuit board design, the high speed global clock may be from a TCXO or others. The output of the TCXO is sine wave sometimes. we must transform the sine wave to a square wave in order to clock some digital device, for example, FPGA or DSP.

We can use a zero crossing comparator to accomplish the process. With dual supply operation amp or comparator , the zero crossing comparator is very easy to design, but how to design it with a single supply system?

Can this circuit work well?

Thanks
 

Re: how to design zero crossing detector with single supply

Maybe we can use a capacitive level shifter and then see the zero crossing as a level crossing.
 

Re: how to design zero crossing detector with single supply

Can you explain and give the circuit of the capacitive level shifter
 

Re: how to design zero crossing detector with single supply

I have noticed when making a zero cross detector for a dimmer, using the low voltage side of a transformer, that there will be a slight phase shift which you will need to compensate for in hardware or software. You must also filter out unwanted frequencies, making the phase shift even worse.

MrEd
 

Re: how to design zero crossing comparator with single suppl

You could try a simple arrangement like the one shown in the figure.
 

If you connect -VEE to ground , you'll have a positive output comparator. Or use only a opamp with a high gain. This will also work.
 

Re: how to design zero crossing comparator with single suppl

You can make comparator referred to dc level of sinusoid.. for example Vin-=Vdd/2.
 

Re: how to design zero crossing comparator with single suppl

ddt694 said:
In a digital circuit board design, the high speed global clock may be from a TCXO or others. The output of the TCXO is sine wave sometimes. we must transform the sine wave to a square wave in order to clock some digital device, for example, FPGA or DSP.

We can use a zero crossing comparator to accomplish the process. With dual supply operation amp or comparator , the zero crossing comparator is very easy to design, but how to design it with a single supply system?

Can this circuit work well?

Thanks
Click to expand...

Maybe not. Because the noise might cause some unwanted glitches. Amplify the sine wave then use Schmitt trigger.

For example, if you are makeing a communication transceiver, and you are waveshping your clock signal for digital circuits, usually this clock is in MHz range. But this clock is easily affected by radio frequency local oscillator usually in GHz and hundreds MHz. Your clock signal now looks like a MHz sine wave carrying a GHz "ripple". If you don't use Schmitt trigger, instead you just simply compare the signal with a fixed DC, extra pulses occurs near zero crossing point.
 

Thanks to all friends.

To design a good comparator with single supply, I should:
1) set the right voltage bias point, for example, vcc/2, to avoid the noise trigger the comparator sometimes.
2) design comparator with hysteresis, like schmitt trigger to also avoid the affect of the noise.

Are these right?

Thanks
 

the offset is important at the beginning of the comparison period. Due to the structure of a latched compartor (positive FeedBack), a small difference in its input should give an almost rail-to-rail output.
 

Re: how to design zero crossing comparator with single suppl

I have no graphic editor so I will only describe my idea.

2 NMOS (M1 and M2) current source connected to GND and to common gate bias.
Drains of these NMOS transistor are connected to 2 PMOS transistor configured as a current mirror, that connected to VCC.
Now disconnect the source of NMOS transistor that is connected NOT to PMOS diode. This disconnected source wil be input. The drain of this transistor is output.
It is just idea. You can add hysteresis and additional inverter to output to increse gain. And may be resistor divider to GND to reduce 1V amplitude of input signal to 0.3V or so.
Good luck!
Fom
 

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