How to tie system to noisey floating ground? - Page 1
Forum section: electronics / projects

author: sidA30
author_rank: Newbie
timestamp: August 30, 2024, 09:34:46 am
content: Hi,I want to measure the high-voltage cells of a battery, such as the battery voltage, which is hundreds of voltages. So I float my measurement circuit to the location of the cells I want to measure - what I dont understand, is one I tie my floating transformer secondary ground to the battery ground, how do I stop the noise on the battery system messing up my measurement?I've attached a diagram, the ADC part is not a full circuit, just an idea, but the front end, what I dont understand, for example, is the In-amp has a high CMRR, but the ground noise couples straight into it via the REF pin. Also my ADC and its references would be floating to this noisy ground.How can I tie my system to the potential of the battery without the noise messing up my differential cell measurement?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 31, 2024, 03:36:02 pm
content: Start by cleaning up your mess.* Looks like D2 is backwards.* Or else, if you are using zener diodes, use a symbol for a zener diode, and add the zener voltage.* Your shorting out the stuff around the "ADC" thing.* You are putting 30Vdc over an transformer winding.* Those two "RG" terminals probably need a resistor to set the gain.* That AD8421, Is that U3 or U1?* Your picture is much to big, and does not fit properly on a web page. (pictures are still horrible on this forum).* Add the battery voltages to your power sources.On a schematic that is such a mess you have to make so many assumptions that the original question also gets lost in the assumptions.Apart from that, batteries do not have much noise. You would have to try really hard to get a source that has less noise then a battery.And last:This looks like a spice schematic. So add a noise source to model the noise source you want to know more about, and then simulate it.

author: SCSKITS
author_rank: Regular Contributor
timestamp: August 31, 2024, 07:58:48 pm
content: You show a DC to DC converter followed by a transformer. This does not make sense to me. Transformers do not couple DC.A transformer with a rectifier/filter followed by a linear regulator can be used to "float" measurements, Tek did that in their DMM plug ins.I would set up for +/-15V for the input amplifier and the output buffer before the ADC, or better yet, +/-5V so the Diff amp output cant go too high vs the ADC input.According to the data sheet, the AD8421 can drive an ADC directly, no need for a buffer.The split supply also eliminates the need for R3, D2, and U3.If the input can go negative, some protection may be needed between the diff amp output and the ADC.You can also add some filtering on the inputs using series resistors and a capacitor.If the capacitor is large I might add some 1K resistors between the cap and the inputs on U1Since you are measuring battery voltage a filter might not be needed (batteries are quiet) unless the battery stack is being charged or there are noise sources nearby.Also of the load is attached and it is noisy.How are the batteries going to be multiplexed to the differential input? Relays?As mentioned in another reply, the RG terminals need a resistor to set gain unless a gain of 1 is desired..What is the cell voltage max?Just some thoughts...ed