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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 Harry,

    I stitched the device through a 1-2mm wires to pcb pads (that occupied the ad8561). Bypass caps (on the pcb) are 10uF on each rail. The pcb is a layout pcb board.

    However, 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), so I assume the stitching is not the cause.

    I put a 4.7pF and also a 1nF on the feedback resistor, with no effect.

    Thank you !

    Gil

  • Hi Gil.

    Not sure what is so magical about AD8561 compared to LT1711.   LT1711 seems at first glance to be a faster part.  

    I believe Harry and others may have talked about supply bypass.   In addition to trying a cap across the hysteresis feedback resistor (which helps to couple the output into the input very quickly), you should make absolutely sure that your bypass capacitor is VERY close.  You can try adding 100nF also.  10mA or other output current turning on suddenly will generate a big L*dI/dt spike, so you need your bypass capacitor close.  

    Note, the AD8561 datasheet shows Voh and Vol with only 3.2mA.  Less current.  Less twitchy behavior.

    Sorry if this does not exactly answer your question...