NJM2072D Application Questions for Audio Detection Circuit

Question

I am building an audio switch with channel priorities.

I will use this in car events where we communicate car-to-car on FRS radios. A few of us also use 2 meter amateur radios for longer range communication. The intent will be that, when no radio signal is being picked up by either radio, the audio from my media player will be on. If a signal is detected on the 2 meter radio, the switch will mute the media channel and play the 2 meter audio. If a signal is detected on the FRS channel, no matter what other signals are present, it will be selected.

I have found a signal detection ship I intend to use as part of the circuit: (NJM2072D - "Datasheet")

The above image is extracted from the datasheet for reference.

1. There appears to be a capacitor symbol above the "INPUT" label connected to pin 1 but there is no associated value. I find no indication in the datasheet about how to select a value. Is this just a DC blocking cap? What would be an appropriate value?
2. Pin 2 is labeled "GAIN" but there is no description of what component to attach to vary gain. Does the internal schematic suggest anything?
3. Pin 5 shows what appears to be a 10uF capacitor but the schematic symbol is confusing. This cap is used to adjust recovery time. Does the symbol indicate I should attach this pin to ground if no cap is used?
4. The example enter code here application circuit shows the audio signal being sampled after an microphone amp. My intention is to sample the audio at speaker output signal levels. Does this appear to be a problem and, if so, what signal conditioning would I have to apply?

Answer 1

1. Judging from the block diagram, this is a DC blocking cap (/ AC coupling capacitor, however you choose to look at it). It looks like the input impedance is nominally 20kOhm. You would choose this value to pass frequencies above a certain cutoff. For instance, for audio, maybe you choose to pass above ~100 Hz: going through the RC filter calculations, you would need a 80 nF cap - so a standard value 100 nF cap would probably suffice.

2. The block diagram suggests that the 20k and 100k set the inverting gain of the amp stage. A resistor from pin 2 to pin 3 could be used to decrease the gain of this stage. For instance - right now, the 20k and 100k suggest a gain of -5. By adding another 100k from pin 2 to pin 3, the gain would be halved to -2.5.

3. The capacitor to ground on pin 5 is used to set the dead time from when the channel input goes quiet to when the outputs reflect this change. When the peak input drops below a set level, the internal switch to ground opens up and the 2uA current source starts slowly pulling up the voltage on the pin 5 cap. With a 10 uF cap, this is nominally about 2 seconds. (But with significant variation, as the charge current varies from 1 uA to 3 uA). With a 1 uF cap, this would be reduced to about 0.2 seconds.

4. The input to the NJM2072 is rated to a absolute max voltage of Vsupply - 1V. Whether this is a problem depends on the supply voltage used for the NJM2072 and the peak voltage of the speaker output levels. If it ends up being a problem, you could attenuate the signal before coupling it into the NJM part.