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| OM222O:
Hello I've designed a PCB using the ADS1219 (24 bit ADC) and the ADR440BRZ (2.048V Reference) which are controlled using the Atmega 328P (Arduino) I separated the digital and analog grounds as recommended by the data sheet (one side is digital only, one side is analog only, there's just a plated through hole connecting the two together) I'm also experimenting with input filtering and whatnot which is why you see one side has a ton of RC filters while one side is directly connected to a header. My main question is should I use star grounding (similar to what I did) or should I just use a ground plane for the analog section? Pictures of the PCB are attached |
| DaJMasta:
You can kindof do star grounding with a flood fill, but it really only becomes important when you have multiple devices on the analog side (but do keep the digital ground on its own trace/plane until the board grounding point), so with just an ADC and a reference generator, it may not make much of a difference, but if you had an amplifier chain ahead of the ADC or a DAC onboard too, then it would become more beneficial. For this sort of board, I always like the idea of an LC filter and some bulk capacitance on the power input. Even better than a simple inductor on the power rail is a common mode choke at the input - keeps out some of the EMI picked up on the power cable. Another easy and nice addition is local regulation, so if your digital and analog sections needed 5V, for example, you'd have a power input of 6V or more, then an LDO for the digital rail and an LDO for the analog rail. Further isolates the two sections from each other in terms of power noise and offers some noise rejection from the supply input as well. The images are sort of small and I didn't take a close look, but do you have an input buffer for the ADC? A lot of high speed or high resolution ADCs like a close buffer amp to supply enough current to sample, and having one means you can hook up even very low supply current devices or sensors to your board - seems like the default for the ADS1219 includes one too, so it's good to have if it's not already there. |
| OM222O:
I do not have one indeed but I'm planning on using it as a DC voltmeter ... how much does the input op amp matters? I don't want to go to negative supply stuff and even rail to rail op amps have way too much offset for my taste |
| OM222O:
for example here there are no input op amps (just a current sense amplifier) and it seems to be ok? |
| DaJMasta:
Your right, I mistook that as just part of the standard circuit, from the input current specs, as long as below 5nA is fine for the device you're measuring, there's no need for an amp. If you wanted a very high input impedance, then you would still want one, but that's low enough as to not be an issue for most things you'd want to measure. You've got that input filtering section and I expect it will be required for good noise performance, 1kS/s sample rate means your Nyquist frequency is 500Hz, so anything above that could cause aliasing, and more importantly, is extra noise in the system. Narrowing the input bandwidth with a low pass filter will improve noise performance - even a filter designed for 10s of kHz will likely make a notable improvement over nothing. The narrower the bandwidth of the part you're measuring, the lower noise. |
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