http://forums.leaflabs.com/topic.php?id=1301 A place to share, learn, and grow... en-US Fri, 22 Jan 2016 00:17:30 +0000 http://bbpress.org/?v=1.0.2 q http://forums.leaflabs.com/search.php http://forums.leaflabs.com/topic.php?id=1301#post-7857 Tue, 24 Jan 2012 14:38:56 +0000 gbulmer 7857@http://forums.leaflabs.com/ <p>Zouk - Here's my way of thinking, I hope it helps.</p> <p>Table 47 explains the relationship between the output impedance of the upstream circuit (the signal source and voltage divider), and the sampling time.</p> <p>How fast do you need to measure the signal?</p> <p>Once you know that, you can estimate the maximum sample time, ts(us), in Table 47, and from that you can get the maximum impedance of the upstream signal, Rain(max). </p> <p>This impedance is a combination of the output impedance of the signal source plus the voltage divider. </p> <p>As a first approximation, treat the voltage divider as a resistance in series with the source signal.</p> <p>If the signal is just a battery, it's impedance is likely negligible, so then ensure the voltage divider is low enough to be less than Rain for the sampling frequency you need.</p> <p>If the upstream signal has a non-negligible output impedance, subtract that from Rain(max) (for the corresponding sampling rate you need), and you have an estimate for the total impedance of the voltage divider.</p> <p>Note, by default, the Maple ADC is driven by a 12MHz clock, and it is configured to sample for 55.5 cycles. So multiply the ts(us) 14MHz/12Mhz. </p> http://forums.leaflabs.com/topic.php?id=1301#post-7856 Tue, 24 Jan 2012 12:46:59 +0000 Zouk 7856@http://forums.leaflabs.com/ <p>I've been poking around for this figure and haven't had much luck, and I've been reluctant to poke at the board with a DMM because I'm not sure what voltage I'm going to be exerting on the IO pin. Paranoid, I know.</p> <p>I've looked at the datasheet (<a href="http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00161566.pdf" rel="nofollow">http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00161566.pdf</a>) and found two figures of note:</p> <p>Rain - External Input Impedence - This is listed as 50kOhms, but based on other things in the datasheet, this seems like this is the maximum external signal impedance to ensure ADC accuracy.<br /> Radc - Sampling Switch Resistance - This is listed as 1kOhm, but I've no earthly idea what that means.</p> <p>I'd looked into measuring voltage across a capacitor that theoretically was charged to 9V (so that I can notice a drop and save settings before power is lost), but when I calculated what values I'd probably need, I wouldn't get results that made any sense (4095 even if I twiddled the adjustment pot so that I'd get less than 3.3V).</p> <p>So, when I'm trying to put together a voltage divider to move the range of a signal voltage to something that the Maple can survive, should I be taking into account the input impedance of the ADC? Am I just going about this completely wrong? </p>