Hello,
Background
I have a project that I have been working on for a few years off and on. It is basically a wireless DC volt meter that uses a GPS to obtain measurements at a very specific interval and record the data with a timestamp, location, etc. I have been successful with making the GPS work and obtaining somewhat accurate voltage measurements simply using an Arduino mega's ADC. My next step is to switch over to the ADS1115 in hopes to increase my precision as the 8bit ADC in the mega isn't quite enough.
My specific questions today have to do with designing the DC volt meter circuitry. I need to know what impact the presence of an AC signal has on the measurement of the DC signal.
The signal I am measuring is actually a corrosion potential (Cathodic Protection related) between a buried pipeline and the ground (electrolyte). Occasionally, AC interference occurs when a power corridor parallels the pipeline and the pipeline also ends up with a certain amount of AC on it as well.
Typical parameters of what the signal I'm trying to measure are:
+4.0 to -4.0 DCV (it is possible the signal may exceed these and would need to protect the inputs)
0 to 5 VAC@60Hz (it is possible the signal may exceed these and would need to protect the inputs)
The VDC signal will be shifting every few seconds, and the VAC signal will remain constant. I only need to obtain a VDC measurement within 200-500mS after the shift. A very typical reading would be signal "ON" @ 0mS. Wait 200mS and then obtain a measurement (usually -1200mVDC and then wait. Signal shifts to "OFF" @3000mS, wait 200mS and then obtain a measurement (usually -900mVDC). The cycle repeats.
List of Questions I would like answers to:
AC Filter Required?
Does the presence of an AC signal cause a measurement error when measuring the VDC with the ADS1115? If so, can I simply put a passive low pass filter in front of the ADC input to block anything beyond 25Hz? I understand the signal would attenuate slightly if I did this, but could it be managed by compensating in the software or perhaps keeping the required resistor so low that it doesn't much matter?
Over-voltage Protection Circuit
The signal I am measuring is usually low voltage, however it is entirely possible that the VDC or VAC may exceed the maximum 6.144 scale and I need to have the inputs protected against this particular danger. My first thought is to use the 6.144V scale and a 5V Zener diode to clamp the voltage to a maximum of 5V. However, I don't fully understand if this is sufficient protection (does that work with AC?) and does it impact my measurement accuracy? I had the thought that I could simply use one of the Mega's ADC channels and a voltage divider to "check" the approximate voltage before measuring with the ADS1115 on the lower scale, but I'd like to make this project as rugged as possible. Relying on the user not attempting to measure voltages outside the range seems like a bad design.
Voltmeter Input Impedance
The impedance of the meter is important to the particular measurements I am attempting to take. The reason being that the half-cell contact (the connection to ground) is occasionally very high resistance and anything below 10Mohm will cause errors in measurement more frequently. I see that the ADS1115 has different input impedance for the various scales. For example, in differential mode the 6.144 scale is 22Mohm and 4.096 is 15Mohm. There is actually a formula that would be useful to use that requires the same measurement to be taken with two different impedance values and the calculated result would tell me if the measurement is error free. I am therefore considering switching between the scales on the ADS1115 to take advantage of this feature. However, is there a way to externally increase the impedance of an ADC (for example the ADC of the mega itself) just by adding external resistors and does this impact my accuracy?
Measuring low voltage AC
Is it possible to measure 0-5VAC with the ADS1115 or is there a better unit/module/circuit that would be better? It seems like I'd have to rectify the AC to DC to measure it, but would likely have to amplify it if it's under a few volts. I'm inclined to avoid the whole mess altogether but thought it worth asking. My only other thought was that since the ADS1115 can measure both positive and negative voltages, would it be possible to sample fast enough to capture a 60Hz waveform? I understand the ADS1115 is rather slow so again, I'm inclined to leave it alone but would be a nice feature if it would work.
Thank you for anyone that reads this and is willing to shine a light on it. I hope that I have been specific enough, although I'm sure I've somehow left out important information. I will update the post once I am able to take pictures of the circuits.
Background
I have a project that I have been working on for a few years off and on. It is basically a wireless DC volt meter that uses a GPS to obtain measurements at a very specific interval and record the data with a timestamp, location, etc. I have been successful with making the GPS work and obtaining somewhat accurate voltage measurements simply using an Arduino mega's ADC. My next step is to switch over to the ADS1115 in hopes to increase my precision as the 8bit ADC in the mega isn't quite enough.
My specific questions today have to do with designing the DC volt meter circuitry. I need to know what impact the presence of an AC signal has on the measurement of the DC signal.
The signal I am measuring is actually a corrosion potential (Cathodic Protection related) between a buried pipeline and the ground (electrolyte). Occasionally, AC interference occurs when a power corridor parallels the pipeline and the pipeline also ends up with a certain amount of AC on it as well.
Typical parameters of what the signal I'm trying to measure are:
+4.0 to -4.0 DCV (it is possible the signal may exceed these and would need to protect the inputs)
0 to 5 VAC@60Hz (it is possible the signal may exceed these and would need to protect the inputs)
The VDC signal will be shifting every few seconds, and the VAC signal will remain constant. I only need to obtain a VDC measurement within 200-500mS after the shift. A very typical reading would be signal "ON" @ 0mS. Wait 200mS and then obtain a measurement (usually -1200mVDC and then wait. Signal shifts to "OFF" @3000mS, wait 200mS and then obtain a measurement (usually -900mVDC). The cycle repeats.
List of Questions I would like answers to:
AC Filter Required?
Does the presence of an AC signal cause a measurement error when measuring the VDC with the ADS1115? If so, can I simply put a passive low pass filter in front of the ADC input to block anything beyond 25Hz? I understand the signal would attenuate slightly if I did this, but could it be managed by compensating in the software or perhaps keeping the required resistor so low that it doesn't much matter?
Over-voltage Protection Circuit
The signal I am measuring is usually low voltage, however it is entirely possible that the VDC or VAC may exceed the maximum 6.144 scale and I need to have the inputs protected against this particular danger. My first thought is to use the 6.144V scale and a 5V Zener diode to clamp the voltage to a maximum of 5V. However, I don't fully understand if this is sufficient protection (does that work with AC?) and does it impact my measurement accuracy? I had the thought that I could simply use one of the Mega's ADC channels and a voltage divider to "check" the approximate voltage before measuring with the ADS1115 on the lower scale, but I'd like to make this project as rugged as possible. Relying on the user not attempting to measure voltages outside the range seems like a bad design.
Voltmeter Input Impedance
The impedance of the meter is important to the particular measurements I am attempting to take. The reason being that the half-cell contact (the connection to ground) is occasionally very high resistance and anything below 10Mohm will cause errors in measurement more frequently. I see that the ADS1115 has different input impedance for the various scales. For example, in differential mode the 6.144 scale is 22Mohm and 4.096 is 15Mohm. There is actually a formula that would be useful to use that requires the same measurement to be taken with two different impedance values and the calculated result would tell me if the measurement is error free. I am therefore considering switching between the scales on the ADS1115 to take advantage of this feature. However, is there a way to externally increase the impedance of an ADC (for example the ADC of the mega itself) just by adding external resistors and does this impact my accuracy?
Measuring low voltage AC
Is it possible to measure 0-5VAC with the ADS1115 or is there a better unit/module/circuit that would be better? It seems like I'd have to rectify the AC to DC to measure it, but would likely have to amplify it if it's under a few volts. I'm inclined to avoid the whole mess altogether but thought it worth asking. My only other thought was that since the ADS1115 can measure both positive and negative voltages, would it be possible to sample fast enough to capture a 60Hz waveform? I understand the ADS1115 is rather slow so again, I'm inclined to leave it alone but would be a nice feature if it would work.
Thank you for anyone that reads this and is willing to shine a light on it. I hope that I have been specific enough, although I'm sure I've somehow left out important information. I will update the post once I am able to take pictures of the circuits.