## gumstix-users

 [Gumstix-users] quick electrical question, connecting measuring amplifier to robostix adc From: Esger Abbink - 2006-05-22 11:54:43 ```Hi, I'm working on wiring up a robostix/gumstix combo and hit something I'm not entirely sure about, not being an electrical engineer. the situation is as follows: 1 robostix - gumstix - netstix stack, supplied with 5V 1 measuring amplifier (GSV11), supplied with 24V 5V and 24V are similar but separate power supplies, the 220V going into them is the same. the amplifier has a 10V signal out, the ground/zero of which is the same as the ground/zero of the supplied 24V This 10V needs to be connected to an adc of the robostix, I've done this using two resistors to limit the voltage at the adc to 5V. The question I have is what to do with the ground/zero of the 10V with respect to the robostix adc. Just connecting it doesnt seem right, because the powersupplies might not have exactly the same zero. And if they do, it would be unnecessary anyway? How do I wire this up without causing possible noise or even damage? My thought was to connect the ground/zero's of the powersupplies to make sure they're the same. It also may/should be possible to shut off one powersupply but not the other. thanks, Esger ```
 Re: [Gumstix-users] quick electrical question, connecting measuring amplifier to robostix adc From: ken staton - 2006-05-22 15:38:11 ```On 5/22/06, Esger Abbink wrote: > 5V and 24V are similar but separate power supplies, the 220V going into t= hem > is the same. > > > This 10V needs to be connected to an adc of the robostix, I've done this = using > two resistors to limit the voltage at the adc to 5V. > The resistor divider is ok. You should also add a clamp diode at the A/D in= put so the A/D doesn't see more than 5V if the amplifier output exceeds 10V. Google: diode clamp adc input > How do I wire this up without causing possible noise or even damage? > > My thought was to connect the ground/zero's of the powersupplies to make = sure > they're the same. This is correct. > > It also may/should be possible to shut off one powersupply but not the ot= her. If you need to do this make sure you don't damage the Atmega. Maximum input voltage is relative to its supply (which is zero when unpower= ed). Having the Atmega powered and the amplifier off should be ok. Regards, Ken ```
 Re: [Gumstix-users] quick electrical question, connecting measuring amplifier to robostix adc From: Dave Hylands - 2006-05-22 19:34:53 ```Hi Esger, > The question I have is what to do with the ground/zero of the 10V with re= spect > to the robostix adc. Just connecting it doesnt seem right, because the > powersupplies might not have exactly the same zero. And if they do, it wo= uld > be unnecessary anyway? Ken already answered this, but I thought I would add some emphasis Whether the grounds from the outputs of the power supply is connected or not will depend very much on the power supply. Most power supplies that use transformers provide what's called an isolated ground. Or more correctly they provide two outputs that are some number of volts apart from each other. In your example, you really do want to connect the two ground together, otherwise your ADC readings will be wonly and you could even damage the ATMega. To help eliminate noise, tie the grounds together as close to the power supplies as possibe. --=20 Dave Hylands Vancouver, BC, Canada http://www.DaveHylands.com/ ```