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# Comparator stability - AD8561 versus LT1711

hello,

I've been using the ad8561 in the following circuit:

IN- connected to OUT- via few resistors (hystersis of few tens of mV) around 0V.

IN+ gets a sine wave signal with frequencies as low as 100Hz. When crossing the 0V point, the output toggles.

There are no oscillations on the output (due to low SR of the sine wave input).

Since the ad8561 is not a rail-to-rail input device, I've taken the LT1711 and I use it similarly (see below).

Vin is the sine wave signal. I've used hysteresis resistors of 50ohm/12.4K (20mV) and even 300ohm/10K (134mV).

On the LT1711 Ioh=10mA.

Both comparators are powered from +5V and -5V.

My problem: The ad8561 outputs are stable in Vin sine wave frequencies as low as 100Hz (and even lower).

With the LT1711 the outputs oscillate when Vin is 90KHz and lower.

Why?

Thank you

Gil

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• Hi gil_he:

Do the oscillations occur when the inputs are near zero cross?   That could actually be a noise problem rather than an oscillation, and might be fixed with a little more hysteresis.

The LT1711 is quite a bit faster than AD8561.  The LT1713 is more similar to the AD8561.  If you didn't want the extra speed of the LT1711, then you could use the LT1713 instead.   The higher bandwidth of the LT1711 will give you more total input noise.

• Hi gil_he:

Do the oscillations occur when the inputs are near zero cross?   That could actually be a noise problem rather than an oscillation, and might be fixed with a little more hysteresis.

The LT1711 is quite a bit faster than AD8561.  The LT1713 is more similar to the AD8561.  If you didn't want the extra speed of the LT1711, then you could use the LT1713 instead.   The higher bandwidth of the LT1711 will give you more total input noise.

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• The oscillations occur near the zero crossing (the circuit works that way).

My understanding is that when the input signal slew-rate is small enough, the passing through the linear region of the comparator is slow. As a result, a very little noise on the inputs and a very high gain of the comparator might cause oscillations. Is my understanding correct?
In addition, to my understanding, when the output slew-rate is high enough and the input hysteresis is too small, interlead capacitance between input and output pins (parasitic output to input feedback) might also cause oscillations.

I used a 20mV hysteresis and a 133mV hysteresis, with no improvement.

In addition, with a square input signal there were no oscillation effects. It happened only with a sine wave input signal with low enough frequencies (low enough slew-rate).

I also realize that the LT1713 has similar gain to the AD8561 and that might do the trick.

Why the higher bandwidth of the LT1711 will give more total input noise?

Thank you !

Gil

• Hi Glen,

I am testing now the LT1713. Same setup.

The oscillations are much "better".

With an input signal of 10Hz, feedback cap of 4.7pF, hsyteresis of 40-100mV (few scenarios), Iout of 10mA (also tested with 7mA):

Output /Q is stable. No oscillations.

However, output Q starts oscillating (at the beginning of the change) at frequencies of 5KHz and lower.

Why is this?

thank you

Gil