Proper Wiring

[rwskinner]

Can someone here give some pointers on grounding? My adc's are jumping around a bunch. I think the grounding is correct but I like to hear from the experts.

I have a 24 vdc battery source, a 24 vdc to 5 vdc DC/DC converter.

The 24 vdc Battery voltage powers the dc converter module as well as 16 transmitters (sensors). The 5 volt output powers the microprocessor and a couple of other electronic items, and also the loop power for digital inputs.

I have the following grounding scheme:

24 vdc battery common to DC/DC Converter 24 volt common pin, chassis, and to the transmitter common terminal blocks.

5vdc common from DC/DC to Chassis, Processor, and to ADC Common, transmitter common.

Basically, I've taken all commons and tied them together, attached to chassis, and to earth ground.

Transmitter outputs are nice and steady, DC converter outputs are nice and steady with a very small ac on top 20mv from the switching of the dc/dc converter.

Even if I ground all ADC inputs the readings still jump around by 40 mv.

If I disconnect the wiring from the adc inputs to the terminal blocks, the adc calms down and reads zero.

Any pointers?

RIchard

[Melanie]

The two obvious things that spring to mind are...

(1) If you have a 20mV or so ripple on your DC/DC converter, and this is your +5v Vdd, and in turn this is also your +VRef for the ADC's... your ADC's will immediately jump about by the very fact the VRef isn't steady.

(2) You didn't say if you had a filter in-line with your ADC inputs. You can't just take a Sensor voltage coming in off-board from some distant location and plumb straight into a raw ADC pin. A simple RC circuit usually suffices.

Melanie

[rwskinner]

Thanks for replying Melanie,

The sensors output a 0-5vdc signal and are mounted about 5 feet away. Sample rates vary depending on user settings from 1 hz to 5 hz. A Filter would be a good idea but I'll have to figure out the values as I have no clue.

I still feel it's something different. It doesn't matter if sensors are hooked up or not, I still see the jumping. I run 20 conductor ribbon cable from the adc to the terminal board which is about 6" away. When I hook up the the terminal board, That is when the numbers start moving around (even without any sensors tied in).

As soon as I disconnect the ribbon cable, it all reads a steady zero.

Power supply, I see the small ringing in all wiring from the dc//dc converter. I've tried several including my 24 volt regulated bench top supply.

I agree, these items all need to be addressed, however, I have a question. If it it does it without sensors hooked up, then we know it's not noise from the sensors, so we can rule out the filter fixing it, although it's an excellent idea.

Next, if it were the power supply causing it, wouldn't I still see the flutter even when I disconnected from the terminal board?

Terminal board is a pretty simple board. PCLD-880, basically a row of screw connectors going straight to a 20 pin ribbon cable. The ground from the ADC goes to the terminal board as well. I have it tied to the sensor commons and to chassis ground.

[rwskinner]

I just installed a couple of good size caps on the DC/DC Converter and now all the way to 1 us there is no apparent noise on the scope.

Also installed (by guessing) a 10k and a .47mf cap on the input as signal conditioning and it didn't seem to make any difference.

I still have the jitter but it has been reduced. I'm trying to figure out what values are needed for the RC Network to filter properly for my application. I will continue to search.

[picnaut]

Hi,

You may want to try using a different voltage reference than VCC.
If you have an ADC pin that you're not using, you can configure it as Vref+. Then you simply connect the output of a stable voltage reference IC to it. You can probably even connect the input of the voltage reference IC to your 24V battery. These voltage reference IC's are generally very accurate and very stable. They can't source much current though (they're not designed to) so don't try and use them as a regulator.

I think that this will significantly reduce the noise you're seeing on your ADC inputs.


Cheers!

[Dwayne]

Hello RWSkinner,

This may sound crazy... But I will say it anyhow... I do not know the environment you are in, but if you are in a Radio/electromagnetic noisy environment... try using a "Coax" type cable for your AD.

Dwayne