# Searching for a comparator

Discussion in 'General Electronics Chat' started by Padderick, Nov 23, 2012.

1. ### Padderick Thread Starter New Member

Jan 7, 2012
11
0
Hallo,

1)

I search for a fast comparator with the following parameters:
- response time smaller than 10 ns,
- VCC =+/- 5V
- VOH = +3.3V, VOL = 0V
- VIN = +/- 5V
- Output current @ VOH >= 20 mA

If somebody knows such a comparator or comparator circuit from one of
his projects, it would be nice if he could share this information with me!

2)

In the datasheet of the suppliers of comparators they give the output
characteristics I have added as Output_characteristics.png.
Can somebody tell me what this parameters mean?
Does it mean that the e.g. the output at VOH is 0.81 V smaller than the
positive supply voltage?
Why they do not write anything about the possible output current?
I have added the whole datasheet as ADCMP561_562.pdf

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2. ### MrChips Moderator

Oct 2, 2009
17,060
5,276
Most comparators are open collector.
Hence the output voltage VOH will depend on the supply voltage to which the pullup resistor is connected.
The current will depend on the value of the pullup resistor.

3. ### crutschow Expert

Mar 14, 2008
20,029
5,634
A Google search turned up several different high speed comparators which may work for you.

The ADCMP561/ADCMP562 are high speed comparators that have PECL (Positive Emitter Coupled Logic) output characteristics. The output does not go to ground as it does with general purpose comparators. Thus the output current is not specified, but the output voltage is given with a 50Ω load to a voltage 2V below the supply voltage, as is typically done when using these devices to drive a transmission line (which you need to avoid ringing if the output path is more than several inches long). The output voltages are then given as PECL logic levels for a high and low signal, for example Vdd-1.15V and Vdd-1.95V, since PECL levels are referenced to the plus supply not ground.

4. ### Padderick Thread Starter New Member

Jan 7, 2012
11
0
I want to drive a optical isolated gate driver directly from the output of the comparator, so I can only use a open collector comparator!?
But they are much slower than the comparators with a push-pull circuit...
or do you know a circuit which makes it possible to use a Push Pull logic to
control the current through gate drivers primary diode?

Thanks

5. ### crutschow Expert

Mar 14, 2008
20,029
5,634
You can use a comparator with push-pull output if it has enough current drive to drive the opto-isolator.

Since opto-isolators are generally somewhat slow, why do you need such a fast comparator?