Hi all,

I have a question for anyone with experience with the LMC567 tone decoder. I have one connected in the following way:

0.47 uF cap on pin 1
0.0047uF cap on pin 2
signal fed through a 0.47uF cap to pin 3
0.47uF decoupling cap on supply
4.6K ohm and 0.20uF timing components (1% cap and 1% resistor)

I'm trying to make a 500Hz decoder. This circuit works wonderfully, but the problem is that I had to determine the timing resistor empirically. The free-running frequency of this circuit is down at 773Hz, which is dead-on what you would predict from the datasheet. However, the datasheet would have you believe that the free running frequency should be twice the center frequency of the desired detection band. When I set the free-running freq to 1000Hz (twice 500Hz), the detection band spans 593-673 Hz. Currently, it spans 490-526 Hz with the free run freq at 776Hz. These are all for 1V pk-pk square waves (measured directly at pin 3). The range is a little broader and lower for sine input but still not what you'd expect from the datasheet.

Swapping in a different LMC567 does not make much difference.

I'd like to know what is going on because we are going to use this in a production product.

Any hints?

Thanks,

Sean


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