I checked both datasheets, the 723 and the LT1762. It was specsmanship that rated the 723 as having lower noise. Here's how:
The internal comparison opamp in the 723 was OK for its day back in Fairchild's history but by today's standards, it is noisy and has poor gain and BW. So in order to make the regulator low noise, a 5uF capacitor had to be placed across the ref comparison input. The low noise, and even the standard linear regulators today are much quieter and few people need 5uV of power supply noise, especially given that the circuitry we place on the power rails will almost certainly spoil that. So no one specs putting a 5uF cap on the ref line. The LTC LT1762 does suggest a Cbyp of .01uF will provide a noise of 20uV broad bandwidth as you correctly stated. I am certain that putting a higher value cap and perhaps even a RC filter on the reference input will lower this substantially. But again, I suggest that it will become meaningless to all but Super-Audio Snake Oil people because any circuitry powered from the produced Vcc line will add more noise than the regulator. Even with the now ancient history LM317 style adjustables, I always put a tantalum cap across the reference to ground. It does wonders for getting rid of traces of hum. Just for your collection I will send John two App notes in PDF form to post. They cover the subject of reducing spikes and switching regulator noise. regards and 73, Jeffrey Pawlan WA6KBL Pawlan Communications _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
