Smoothed power supply DC-DC for GPS receiver
I have a GPS receiver board (Hemisphere P200) which I wish to use for a PTP time server. The board requires 2 x 3.3v input and 1x 15v input for the antenna. The board will be mounted inside a computer and powered from its power supply. This will provide a 12v power from which I intend to power the board and antenna. I was planning to use two DC-DC converters (TRACOPOWER TEC 3-1210WI, TEC 3-1213WI) to get from the 12v to the 3.3v and the 15v. My question revolves around how to connect the grounds. Do I just attach everything to a common ground or does the 3.3v GND need to go back through the 3.3v DC-DC converter for it to work? Ditto for the 15v. What is the best practice for instruments? The antenna obviously works with RF what considerations does this involve in the circuitry.
Apologies for the basic questions. Just starting out.
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Hi Richard, that is not really a basic question, but one that often requires some careful consideration. I could not find anything useful while searching for the Hemisphere P200, do you have a good link to a data sheet?
In general the best method is to have a 'star point'ground, i.e. one point where the 3.3V gnd and 15V gnd are commoned, this is with the Traco's the P200 and the PC Gnd. The +V supplies should route to their appropriate destination P200 connection and decoupling capacitors applied to each +ve supply pin to the appropriate Gnd pin close to the P200 module. WIthout a datasheet it is difficult to know if the P200 has all the GND'd connected together on the module. If they are, apply local decoupling from the supply to the nearest Gnd and run a track back to where you have commoned the supplies.
If supply decoupling is 'tricky' in any respect the manufacturer will normally specify what is required, if not probably a tantalum in parallel with a ceramic will probably be OK.
@Boss Hi Boss, the data sheets I have are the following: http://www.hemgps.com/dl/eclipseII-875-0275-000_A1.pdf http://www.hemgps.com/dl/875-0344-0_Crescent%20P206_P207_%20IntGuide_A1.pdf The second is the integrator guide for the newer model but the pinouts are the same. I think I made the wrong choice in DC-DC converter as it is the isolated version. Reading through the datasheet, it appears that there are four DC inputs (p12 or19), however the four GND outputs don't appear to be specifically assigned to the respective inputs. Is there any specific phrase I should look for that would indicate GND'd connected in the module? Richard
@Richard Pearce I have only had a fairly quick look and agree with you that all the GND pins look like they are common on the PCB. If I was performing the design I would presume this is the situation and wire each supply to their respective supply pin and a nearby GND. I would also provide some external decoupling capacitors close to the module and also check that I meet the regulator requirements for any external decoupling. The isolated DC-DC converter is probably a good feature as there is no DC route back to supply and the output will have a single point of ground at the module. Back to the GND pins, can you visually inspect the board to see if they appear commoned. It may be difficult as they could all connect to an internal GND power plane, which in reality is probably what they would do for a good low impedance GND.