ADC0804 Analog Ground and Digital Ground

[umery2k75]

sensor reading adc 0804 *.pdf

I designed a temperature sensor circuit, built with LM35 and ADC0804 and after that EPROM for Voltage->Centigrade conversion after that two BCD decoders and 2 seven segments. This circuits shows accurate temperature from 0'C to 99'C.
At the time this picture was taken, temperature was around 28'C which is quite comfortable.



I came across two grounds as you can see in the PIN configuration, one is Digital Ground(DGND)and other is Analog Ground(AGND), I read somewhere in the datasheet or in the application note of ADC0804, as seperate the two grounds together, don't connect analog ground with digital ground. I am energizing this circuit with bench power supply using +5V and Gnd terminal rated at 1.5Amps. So I am giving +5V and Gnd to circuit.I don't know what to connect with Analog Ground. So I connect the two grounds together. The circuit is complete and checked, it shows temperature with good accuracy, and the job is done.
But I think the story hasn't ended yet, I think as the change in analog value was slow in this design, that's why I might have not run in trouble. What if high speed Analog to Digital conversion was going on, I might have run into a serious problem with Analog and Digital ground connected together, so tell me what to do with the Analog Ground, I can't leave it just unconnected.

 

[House_Cat]

where to get power supply of adc0804

Take a look at the following articles:
http://www.hottconsultants.com/techtips/split-gnd-plane.html
http://www.sigcon.com/Pubs/edn/adcgrounding.htm
**broken link removed**

 

[umery2k75]

adc0804 pinouts

All of these things refer to PCB, I don't have a plane in wire board. Do you mean that Analog Ground must be directly connected to Ground of supply and must not be connected to anything else, where as Digital Ground will also be connected to Ground of supply, but it will also be connected to other grounds like of ICs, transistors, potentiometer,etc. This ground wire will be connected to several points, but I will have a seperate wire for Analog ground and it will be connected to supply ground only. Are all these things also known as Ground loops?

 

[House_Cat]

application note adc0804

I see your problem with a wire board instead of a PCB.

Your temperature sensor is an analog device. Its output is converted to digital by your ADC chip. Any components through which your analog signal passes should be connected to AGND. Any components throught which your digital signals pass should be connected to DGND. Both of those grounds should be connected together at the supply point.

On a PCB, you would connect the two pins to the same plane. However the section of the plane for the analog grounds (AGND) would be segregated from the DGND as shown in the articles I referenced above.

The goal is to have all analog current flow only through analog sections of the ground, and all digital current flow only through digital sections of the ground. The two should not share ground return paths.

Ground loop refers to the ground side of the complete signal path. Keep in mind that all signals flow through whatever conductor you have provided for them - AND THEN have to return via the ground connections. Every signal is a loop - from source to sink, and back to source (via the ground connection). The loops should be kept as small as possible to prevent inductive coupling of noise. The larger the loop, the larger the inductance, and the more coupling you get for undesirable signals. Short, direct, connections are the way to do it with a wired board. You can run a ground wire from the supply to a central point, and connect your grounds to that point - that's called a "star" ground scheme.

In your case, you would want two "stars". One for DGND and one for AGND. Then the two would be joined at the power source.

 

[truong_giang2211]

u can connect AGND(analog ground) with GND,i am used it !