Would I get true differential ADC signal acquisition with this setup? - Page 1
Forum section: electronics / projects

author: Tungsten
author_rank: Newbie
timestamp: February 03, 2025, 10:49:04 am
content: Greetings,I'm designing a circuit to work with some CTs and VTs that are on the line (overhead electricity lines). My part will only be dealing with reduced signals (after CT and VT). Since there is a cable that will be from the CT (on the pole) to my circuit, I figured there could be a lot of noise pickup there from what's on the spectrum at that time. Thus, I thought it would be good to go for a differential ADC setup for the benefits of CMRR and similar advantages.I found an interfacing circuit for single-ended ADC measurement (meant for Arduino, I believe)online, I've attached it below. I thought I would adapt it to get the benefits of a differential ADC setup.My plan is to get a differential ADC like the MCP3208 and configure 4 differential channels (one for each phase + neutral). Then I would connect each channel across a burden resistor in a circuit identical to the one I attached.. As I understand it, this way I will eliminate negative excursions that might hurt the ADC inputs and I would have found a way to make the voltages reference to ground.My main concern is whether I would still be getting the benefits of a differential approach. Would I be right to say for a true differential approach both sides of the signal should meet or see similar components. Here one side goes straight to the ADC while the other has two resistors and a cap.The final circuit will be used a data acquisition device. So I need to make something that at least can provide a decent reconstructed signal if needed. For this I plan to sample at at least 2.5kSPS

author: Tungsten
author_rank: Newbie
timestamp: February 03, 2025, 10:50:08 am
content: This is the circuit I based my question on

author: moffy
author_rank: Super Contributor
timestamp: February 03, 2025, 11:27:16 am
content: With CTs and VTs the output impedance is quite low, signal level is good, maybe less so for the CT and galvanically isolated, so the need for true differential is normally not an issue. Transformers do a reasonable job of differential to single ended conversion, especially at mains frequencies.

author: Tungsten
author_rank: Newbie
timestamp: February 03, 2025, 12:58:37 pm
content: What about the noise level on the signal. The signal peak to peak will be 3V max, and I think at this level noise could interfere with the "analog" properties of the signal, that is why i wished for differential approach. Would you say I can get that with this?

author: moffy
author_rank: Super Contributor
timestamp: February 03, 2025, 01:37:02 pm
content: For true differential everything has to be symmetric, but that is to discriminate against common mode signals, the transformers in the mains frequency range already do that quite well. As long as you avoid cable induced common mode noise, twisted pair shielded helps a lot, there shouldn't be an issue. But if it is a high noise environment that's a seperate issue and needs to be adressed in detail.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2025, 05:40:52 pm
content: A current transformer may not like 3 V output voltage. At least at low frequency this could be to close to saturation. One may also like to have some for of clamps to limit the voltage seen by the ADC.How much noise would be in the signal will depend on the actual current flow. Unless for very low current, the CT usually does not add much noise. The 10 bit ADC in the AVRs have a limited SNR. So noise is likely not that much of an issue. For low "noise" and low interference it can help to look at the AC signal in the right way, like average over full mains periods.

author: TimFox
author_rank: Super Contributor
timestamp: February 03, 2025, 06:25:18 pm
content: A CT should specify the recommended burden resistor to avoid the problems with a high value discussed above.

author: langwadt
author_rank: Super Contributor
timestamp: February 03, 2025, 07:11:22 pm
content: Quote from: TimFox on February 03, 2025, 06:25:18 pmyep, and if it isn't build in to the transformer you could split it in two to make the whole thing symmetriclike this but with the 10k's much lower value

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2025, 08:22:40 pm
content: The µCs don't need 2 fully differential signals. The ADCs are just single inputs, not differential like many SD ADCs. So the idea in principle is OK. It is just the burden voltage that may be a bit on the high side. So it depends on the CT.

author: jbb
author_rank: Super Contributor
timestamp: February 03, 2025, 08:56:23 pm
content: You mentioned that the CTs would be on overhead power lines. That’s fine but there are two important things to look out for.First, your installation could be subject to voltage surges and huge voltage spikes, so make sure you meet the insulation requirements (I think this is Category IV, I’m not certain) and that your voltage sensing input channel wouldn’t be damaged by a large voltage surge.Secondly, you need to be careful of CTs in power systems. If there is a short circuit somewhere along the overhead line (tree branches push wires together or car drives into power pole) then very large fault currents - literally kilo Amps - can occur. Your CT will try to transform this fault current too. So the upshot is that your CT input circuit must handle high fault currents without damage. I’ve never used them, but something like a Littelfuse SIDACtor might be helpful here.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 03, 2025, 09:17:47 pm
content: The setup shown in that schematic is sometimes referred to as unipolar pseudo-differential, so short answer is "no".But it's unclear what your reason for wanting "true differential" signaling is. It's not just about how you interface a signal to an ADC, but also how you interface the sensor to signal conditioning electronics. One can be differential while the other isn't, vice versa, etc.