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I've been trying to figure out the lower input bound for the LM567/NE567 tone decoder. Supposing the pin 7 is connected to ground, is the minimum input voltage 0V or can it go negative?

In the datasheet (poorly written BTW) the input pin is always AC-coupled using a capacitor, so I guess this suggests the voltage can indeed go negative. The datasheet says the upper bound is Vcc+0.5V, I was wondering if the lower bound is GND+0.5V.

The input signal I'll use on it has a DC offset level. So if there's no coupling capacitor, could the IC get damaged by the DC offset (when no AC component)?

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From the datasheet, it can take ±8.5V on the input.

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And the most it can take is -10V:
enter image description here

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  • \$\begingroup\$ LOL I didn't see these tables on the datasheet I've downloaded (but they are there), I feel so embarassed now. Thanks! \$\endgroup\$ – strange_bakery Feb 22 at 17:49
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damage?, probably not. Perform well? Probably not.

The datasheet schematic indicates Vin is just below 3 diode drops above ground and is self-biased so DC input is out of the question but limiter action is expected for large AC inputs, whereas small inputs control a linear Gilbert cell multiplier mixer. 250mV rms seems to be the threshold for large signals that cause saturation and thus BW is determined by the RC filter. Below this, the BW reduces by the square root of Vin.

The text and tables and graphs indicate Sensitivity below 50mV rms requires tuning so not recommended.

But for damage the Absolute Maximum specs are clear for pin 3.

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  • \$\begingroup\$ Very interesting observations! I'll use a coupling capacitor then. The AC component has amplitude higher than 250mV, so I guess I won't have problems with the RC filter. Thanks! \$\endgroup\$ – strange_bakery Feb 22 at 17:53
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    \$\begingroup\$ You can test the capture range vs Vin vs SNR if that matters. \$\endgroup\$ – Tony Stewart EE75 Feb 22 at 18:10

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