Hello, I have a circuit that has an analog-to-digital converter onboard, and is taking 2x voltage measurements from 2 sensors circuits. I have used the PT1000 circuit for some time now and never noticed an issue like this. The flow-meter circuit is new, the ADC128 is new, and the 24V power is new.
The flow-meter is basically generating pulses, which goes through an RC filter to turn it into an analog signal and then continues to the ADC.
When there is no flow/pulses, the analog flow signal is supposed to be 0.00 volts. Some of the time it is. But often the problem I am having is it begins to drift.... e.g. it will start to creep up to 0.050 to 0.080 volts, which yields a significant ADC reading, which shows there is flow when there is not. At the same time when this happens, I notice the temperature voltage also creeps up about the same amount, so those readings get thrown off too. They both drift together.
At one point I had the oscilloscope probe ground connected to the circuit boards ground, and the instant I disconnected the probe the problem went away for both signals. So I thought the scope must have been inducing a small voltage or interference or something. It didn't last long though, it eventually over about 10 minutes creeped back up. It almost seems like a charge or something is building up, and when I discharge it the problem is temporarily fixed. I don't know if that's actually whats going on, but that's kind of what it seems like.
Any help or advice would be greatly appreciated! I definitely want to build a better understanding on what is going on that's for sure.
Some things I have tried and noticed:
The flow-meter is basically generating pulses, which goes through an RC filter to turn it into an analog signal and then continues to the ADC.
When there is no flow/pulses, the analog flow signal is supposed to be 0.00 volts. Some of the time it is. But often the problem I am having is it begins to drift.... e.g. it will start to creep up to 0.050 to 0.080 volts, which yields a significant ADC reading, which shows there is flow when there is not. At the same time when this happens, I notice the temperature voltage also creeps up about the same amount, so those readings get thrown off too. They both drift together.
At one point I had the oscilloscope probe ground connected to the circuit boards ground, and the instant I disconnected the probe the problem went away for both signals. So I thought the scope must have been inducing a small voltage or interference or something. It didn't last long though, it eventually over about 10 minutes creeped back up. It almost seems like a charge or something is building up, and when I discharge it the problem is temporarily fixed. I don't know if that's actually whats going on, but that's kind of what it seems like.
Any help or advice would be greatly appreciated! I definitely want to build a better understanding on what is going on that's for sure.
Some things I have tried and noticed:
- Looking at resistor R76 on the flow meter portion: I can see the drifted 0.080 volts on the right side, but I see 0.00 volts on the left. Does this mean the voltage is coming out of the ADC input pins?
- At one point I accidentally shorted the 24VDC to ground (luckily it has short circuit protection). This emit a spark, and removed the problem. It took almost 30 minutes for the problem to come back after I did this.
- I can see exactly 3.3 volts across the ADC's ground, and reference with the scope. This always stays at 3.3V no matter if its working or not working properly.
- For testing the temperature, I have a 1K ohm resistor connected up to the PT1000 input. 1K reads 32 degrees F. (it reads 25 F when the voltage drifts). For testing the flow meter, I have a short wire connected to ground, and I tap it on pin 2 to generate pulses. I wanted to let you know that I don't have long sensor wires running around the room potentially picking up interference or anything like that, its contained close to the board for troubleshooting.
- It seems like I occasionally move some wires or touch something on the circuit board, and this resets the problem. It will then work for a period of time but will eventually drift back up to the incorrect voltages.
- All of the power rails read exactly the voltage that they are supposed to, when the circuit is working and has not drifted yet, as well as when its drifted and has incorrect voltages.