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Need advise on 5.1V reference...

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Full Member level 1
May 6, 2001
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Used a DC/DC converter with a KA7805 as supply for an ADC chip.

Measuring the output of the kA7805 I only have 4.90 volt in stead of a nice 5.0V.

I need to have 5.1V reference for a 0-25.5mA / 0-255 step conversion. So each step is ~0.1 mA

Any good idea's for fixed regulators that precise?
Any idea why the KA7805 doesn't regulate at 5.0V according the spec's?
It does if connected to a seperate linear PSU.
The cap's used are 330nF and 100nF. Output voltage supplied by the DC/DC converter to KA7805 is 13V, input supply for the DC/DC is 24VDC.

The KA7805 isn't a voltage reference. It's output may be anywhere between
10% of the nominal 5V. Get yourself a real voltage reverence and try not to load it much. Then make sure to calibrate your ADC.

its possible using an adjustable regulator (like LM317) and build an simple opamp control circuit with an high accurate stabilizer diode to get more precision

Depend on the stability you seek. A system with feedback will not be more stable than the stabilizer diode it refer to. The zener diodes with exception of some 6.2 V references aren't better than 100 ppm/K. The bandgap voltage references usually about 2.5V are around 50 ppm/K. I'm not sure if a feedback through the LM317 will provide better stability than a monlitic buffered voltage reference, because in the later everithing is well thermally matched and trimmed, while trying to build something on discrete components requires serious design efforts and high grade components. Apple per Orange, the ready made voltage reference is cheaper, except if you goal is to learn the vagaries of thermal compensation, thermal matching and all this other sweet thingies.

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