Sine to square wave conversion using LT1016 comparator

Thread Starter

bt316

Joined Aug 17, 2015
4
Hi All,

I am trying to convert a 500khz sine wave signal to square wave using the LT1016 comparator. I have just built a basic comparator circuit with the inverting grounded(pin3) and applying a 2v sinusoidal signal to the non-inverting(pin2). The latch and ground pins are grounded (pins 5 and 6). I am using it with a single +5v supply with the negative supply (pin 4) grounded too. However, in the output(pin I am seeing a large offset of ~2.6v. I am not ure what is wrong with the circuit. Can any1 help please?
 

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ronv

Joined Nov 12, 2008
3,768
Hi All,

I am trying to convert a 500khz sine wave signal to square wave using the LT1016 comparator. I have just built a basic comparator circuit with the inverting grounded(pin3) and applying a 2v sinusoidal signal to the non-inverting(pin2). The latch and ground pins are grounded (pins 5 and 6). I am using it with a single +5v supply with the negative supply (pin 4) grounded too. However, in the output(pin I am seeing a large offset of ~2.6v. I am not ure what is wrong with the circuit. Can any1 help please?
I suspect the sine wave goes below ground, which means you also need a negitive supply.
The alternitive is to AC couple it and bias the comparator to 1/2 the supply voltage.
 

OBW0549

Joined Mar 2, 2015
2,952
You say you're using the LT1016, but the data sheet you posted is for the LT1116. They are not the same. Which are you actually using? Either way, your input is extending outside the allowable input voltage range and will give you unpredictable results; that needs to be fixed.
 

Thread Starter

bt316

Joined Aug 17, 2015
4
Thanks alot that seems to work!
I suspect the sine wave goes below ground, which means you also need a negitive supply.
The alternitive is to AC couple it and bias the comparator to 1/2 the supply voltage.
 

Thread Starter

bt316

Joined Aug 17, 2015
4
You say you're using the LT1016, but the data sheet you posted is for the LT1116. They are not the same. Which are you actually using? Either way, your input is extending outside the allowable input voltage range and will give you unpredictable results; that needs to be fixed.
Sorry my bad! I attached the wrong one! I am using the LT1016!
Thanks for the reply!
 

OBW0549

Joined Mar 2, 2015
2,952
Sorry my bad! I attached the wrong one! I am using the LT1016!
Thanks for the reply!
For an LT1016 operated on a single 5V supply, the input voltage range is from 1.25 volts to 3.5 volts. This applies to both inputs, at all times. Go outside this range on either input, and your comparator output becomes "who the heck knows?"
 

ronv

Joined Nov 12, 2008
3,768
Sorry my bad! I attached the wrong one! I am using the LT1016!
Thanks for the reply!
I think @OBW0549 is right. You would be safer to reduce the input voltage slightly. Having said that I think it still works as a zero crossing detector since the signal is back inside the common mode range where it crosses the threshold.
 

OBW0549

Joined Mar 2, 2015
2,952
...I think it still works as a zero crossing detector since the signal is back inside the common mode range where it crosses the threshold.
Yeah, but what about when the signal is outside the common mode range? Opamps and comparators have been known to do some pretty bizarre things sometimes (like momentary phase inversion, for example) when you go outside the limits. Best to stay inside them.
 

SgtWookie

Joined Jul 17, 2007
22,201
ronv, the op is using a single +5v supply, so the input range is 1.25v to 3.5v as has already been stated. Go outside that range, and the output gets dicey. Reducing the input voltage is not a correct option.
 

ronv

Joined Nov 12, 2008
3,768
ronv, the op is using a single +5v supply, so the input range is 1.25v to 3.5v as has already been stated. Go outside that range, and the output gets dicey. Reducing the input voltage is not a correct option.
I still don't see the problem with either option. He has a two volt peak to peak sine wave. If he biases it to 2.375 he doesn't go outside the common mode range. If he were to reduce the amplitude to say 1.5 volts he would have more room.
Even if he goes outside the common mode range but stays above the low, and below the high side supply the spec says there is no damage or latch up.
Where did I go wrong, go wrong?:)
 

MikeML

Joined Oct 2, 2009
5,444
It also talks about running it on a single 5V supply, and what the common mode range is...

Lots of info about bypassing, layout, etc...
 

OBW0549

Joined Mar 2, 2015
2,952
You're right: I didn't see the "1.25V above the negative supply" part and was only going by the "+/- 11V" value in the table for +/- 15V operation.
AN72 is about the LT1394 comparator, and doesn't discuss the LT1016 except in the introduction. AN13 discusses the LT1016 but doesn't have much to say about common mode range other than the need to keep signal voltages inside it to avoid spurious outputs.

In the end, I think ronv is right: if the TS biases his inputs up to a 2.375 volt center voltage (instead of ground, as he has it now) and doesn't exceed 2 volts peak-to-peak on his non-inverting input, he should be OK. 1.5 volts peak-to-peak would be better, as it would give him a bit more safety margin.
 
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