Poul-Henning Kamp wrote: > In message <[EMAIL PROTECTED]>, "Dave Brown" writes: > > >> John >> You won't need much of a cap for DC blocking as inputs are hi-Z....too >> big and the chargeup may well trigger the overrange condition. >> > > You need to put a shunt resistor after the capacitor if the input is hi-Z > but an even better way is to offset the gnd clip with a couple of fresh > batteries so that you can avoid the capacitor. > > Batteries have very low, but not zero noise, so what you do is: > > Put six fresh 1.5 volt batteries in series, so that three of them > is the "wrong way" and the resultant voltage is zero and then you > measure their noise. > > Then you put three of them "the wrong way" on your 5v supply, so that > the output voltage is only .5V and then you measure the noise. > > That worked fine for me using a HP6885B > > This technique only offsets the voltage to within 3/4V of zero when measuring an arbitrary voltage regulator output. With a discrete regulator it is usually easy enough for test purposes to adjust the regulator output so that it is within a few tens of millivolts of the battery stack voltage.
A low noise preamp with perhaps 40-60dB gain is still required before the spectrum analyser. Even residual dc voltages of a few tens of millivolts may saturate such an amplifier. A simple dc servo can be used to remove the residual offset (<1V) without using electrolytic capacitors with their attendant leakage and noise. A low frequency cutoff well below 1Hz is possible without adding significant noise. Alternatively for measurements in the 10Hz to 100kHz range a preamp like that used in Linear Technology's AN83: http://www.linear.com/pc/downloadDocument.do?id=4172 may be useful. It uses electrolytic coupling capacitors together with low value resistors. For ultralow (milliHertz) frequency noise measurement AC coupling is best avoided altogether, if possible. Bruce _______________________________________________ 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.
