I am prototyping building blocks on a breadboard for digital clock projects using vintage clock chips (like MM531x) that will have VFD, numitron or Nixie displays. One of my elements is (was) a zero cross detector, to create twice the mains frequency for the purpose of among other things getting seconds out of the chip.
I prototyped a ZCD that does what I need using a diode bridge, BJT and 4N28. However, to reduce part count and eliminate a capacitor I switched to an HPCL3700 AC voltage detector, which has all these parts except the capacitor, which is not needed because it is not a ZCD (I don't have to have pulses at ZC). So I have my needed pulse at twice the mains f. It is powered by the 5V rail, and draws only 1-2mA (it has an O-C output).
The only problem I have is the noise from another block is disturbing the sensitive 3700 voltage detector. For VFD builds there is a filament driver, which is an LM4871 (same as LM9022) running off the 5V rail self oscillating at 25kHz in bridge configuration, generating a square wave. This draws 100mA. At the edges, there are narrow spikes of about 100mV of noise on the 5V rail, which when they coincide with voltage thresholds of the 3700 corrupt its output.
I considered two approaches - use a separate PS for the detector chip, and reducing the amplitude of the noise - something I have little experience in.
Approach 1
I have decoupling caps, both large and small across each block's Vcc and GND. I tried powering the AC detector only from another power supply, with a common ground, and sure enough the issue goes away. The 3700 operates down to 2V, and only draws 1-2mA, so my thought was maybe use a small 4V LDO chip to power just this device. Smallest capacity is 50mA. I have never used an LDO, and don't know anything about noise from the input to output (seems to be largely PSRR), or minimum loads (will it work OK with only 1mA draw?). Might this eliminate or greatly reduce the spikes?
Approach 2
A part of me says "there should be a way to reduce that switching noise". I considered changing to a sine wave, which the LM9022 datasheet (which shows the design I am using) says can be done by modifying the feedback of the first opamp. They say a sine, triangle etc are all possible. This seems like a good approach, except I have no clue how to implement feedback that would me give a sine wave output. I also considered that with my breadboard, wires all over the place, and the friction contact connections, may be a factor in the amplitude I have. Perhaps a circuit board would not have as much.
I should mention that the 100mV noise is not creating any other issues, and the design for VFD is complete and working otherwise.
Any suggestions or comments would be very much appreciated. I was excited that I may be able to greatly reduce my part count with the new frequency generator - and then I zoomed in. Adding a second PS and obtaining a working solution is encouraging, but how to implement a solution within the circuit itself.
Don