Hi The ADC0808 is a (relatively) slow multi-input 8 bit ADC. It’s a good candidate for built in test chores on a board that does not have a MCU with built in ADC capability (or where you have used all the pins). They are cheap and easy to use.
Current limiting the antenna supply is a pretty good idea. They do short from time to time. Monitoring the voltage going to the antenna is a quick way to see if the supply is limiting. So far a great idea. Get a bit of over voltage on the antenna and the ADC probably isn’t going to last very long …Over voltage might be lightning, it could be other things. Bob > On Dec 24, 2014, at 4:58 AM, Azelio Boriani <[email protected]> wrote: > > An ADC0808 directly connected to the GPS antenna? Not even a DC > blocking capacitor? So it receives also the 5V antenna supply? > > On Wed, Dec 24, 2014 at 7:42 AM, Chuck Harris <[email protected]> wrote: >> It's Alive!!! >> >> The MGPS module was saying there was an A-to-D fault >> and a Feedline fault. Suspicious of the common element, >> I traced the signal path from the GPS antenna connector >> to the input of the ADC, and it was a straight DC path. >> Sure, there were a few chokes, and capacitors to ground >> to filter out any stray 1.5GHz signal, and to keep the DC >> circuitry from loading the antenna signal. >> >> I ordered up a new ADC converter, which was an ADC0808 by >> National and TI. A small 28 pin quad J lead surface mount >> package. It finally came, and I swapped it out this >> afternoon. >> >> I was rather worried, as the failure mode could have been >> simply a faulty converter, or it could be that something >> happened on the antenna line that killed the converter. >> >> It was not evident how the GPS module knew there was an >> A-to-D fault. It could have been something like a lack >> of an end of conversion flag, or they could have been >> using some extras of the 8 analog inputs to measure things >> like power, ground, resistor dividers... I really don't >> know... >> >> Powering the GPS module on the bench showed an immediate >> improvement: The A-to-D converter fault condition was >> no longer set! It was still showing an antenna line fault, >> but since I didn't have an antenna connected, that was to >> be expected. >> >> I put the GPS module back into the MGPS chassis, and >> installed it in the MFS-209, and as soon as I connected >> a good antenna, the antenna line fault indicator went >> away.... So far so good. >> >> Some satellites started appearing on the display, and after >> waiting for what seemed like forever, and the almanac was >> finally updated. >> >> And... >> >> The system status changed to GPS Lock! And reports normal >> operation. >> >> Now to let it cook for a few weeks and see how well this >> ancient GPS RbDO performs. >> >> Thanks to all that offered tips and moral support. >> >> -Chuck Harris >> >> Chuck Harris wrote: >>> >>> And as I feared, the MGPS display is telling the truth, there >>> is something wrong with the GPS module's ability to check the >>> feed line for shorts and opens, and something wrong with the >>> motherboard's ADC module. >>> >>> At least the communication between the MGPS and the GPS engine >>> is working properly... >>> >>> -Chuck Harris >> >> _______________________________________________ >> 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. > _______________________________________________ > 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. _______________________________________________ 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.
