Electronics > Beginners
Measure reversible voltage without PD drop?
David Hess:
--- Quote from: coppice on August 10, 2019, 04:02:41 pm ---
--- Quote from: David Hess on August 10, 2019, 03:49:12 pm ---Some ADCs can measure negative voltages directly while operating only on a positive supply but few if any differential ADCs can operate this way because of input common mode limits.
--- End quote ---
There are a number of ADCs designed specifically to interface to current shunts, which run from a single rail but allow the ADC input pins to go slightly below ground. Typically 0.1V, which is enough to allow a well laid out shunt circuit to function properly. Most electrical energy measurement chips have a front end like this.
--- End quote ---
The LTC1609 with an input range to -10 volts with only a +5 volt supply is an example of the kind of part I was referring to.
But referencing the input divider to a positive voltage like the positive supply voltage or positive reference voltage allows a negative input voltage to be measured with an ADC which only accepts positive voltages. A more common way is simply to use an inverting amplifier however this requires an additional operational amplifier.
coppice:
--- Quote from: David Hess on August 10, 2019, 04:19:43 pm ---
--- Quote from: coppice on August 10, 2019, 04:02:41 pm ---
--- Quote from: David Hess on August 10, 2019, 03:49:12 pm ---Some ADCs can measure negative voltages directly while operating only on a positive supply but few if any differential ADCs can operate this way because of input common mode limits.
--- End quote ---
There are a number of ADCs designed specifically to interface to current shunts, which run from a single rail but allow the ADC input pins to go slightly below ground. Typically 0.1V, which is enough to allow a well laid out shunt circuit to function properly. Most electrical energy measurement chips have a front end like this.
--- End quote ---
The LTC1609 with an input range to -10 volts with only a +5 volt supply is an example of the kind of part I was referring to.
But referencing the input divider to a positive voltage like the positive supply voltage or positive reference voltage allows a negative input voltage to be measured with an ADC which only accepts positive voltages. A more common way is simply to use an inverting amplifier however this requires an additional operational amplifier.
--- End quote ---
It would be interesting to see how they do the input protection in that device. Its only single ended, though. Do you know what causes the common mode limits you referred to? In the sigma delta converters typically used for shunt sensors the CMRR is usually very good. It has to be when you are looking for a small differential signal from the shunt. Most use switched cap discrete time front ends, which can have an extremely good CMRR figure. Some use continuous time front ends, which are not as good, but still have a fairly respectable CMRR.
MosherIV:
--- Quote ---So I find a micro-controller with a fully differential ADC. It has AN+ and AN- inputs. I connect these across the the brushed DC motor terminals, via a suitable voltage divider, and the difference between the terminal voltages may be read by a suitably referenced ADC as the motor runs down.
What then happens when I want to measure the motor's terminal voltages difference, when the motor spins the other way, and now the AN- input is at a higher voltage than the AN+ input?
I ask this because I'm basically clueless, and because you've mentioned something about the differential ADC being capable of measuring voltage, but not the polarity (?).
Do you suppose I might alternate the AN+ and AN- input polarities to the differential amplifier without damage, and reliably get sensible conversion magnitude data as a result?
--- End quote ---
ADCs in differential mode only measure the voltage between the pins AN+ and AN-
This is what I meant about voltage only but not polarity.
You must always obey the input limits of the ADC device or processor ADC.
The others are correct that you may not be able to put a true negative voltagr to either AN+ or AN-.
If you use a pick a ADC decive/chip that allows seperate ADC Vref and ground point to the device power then you can measure true negative voltage.
The other are correct.
You must still obey the input limits of the device though.
I believe that in differential mode, the ADC does not mind if AN1- is higher voltage than AN+
The ADC should have a non signed mode and a signed (polarity) mode.
Non signed mode will only tell you the voltage difference (that was the mode I was thinking about)
The Signed mode will tell you the polarity of the voltage as well (I forgot this mode - though not all ADCs have this, you will need to check)
Can you measure the speed and direction of a motor by measuring the motor terminal voltage?
Yes. It is not simple as you are finding out.
Why (how) would a motor suddenly change direction if the micro is controlling the speed and direction of the motor
12407622:
Thanks everyone for the super support.
For just now I'm going to go with 2 x full-wave rectifiers (and voltage dividers) for each of the two motors, with low forward voltage drop germanium diodes. My present setup uses an 328P controller, and I can spare the requisite 2 x single-ended ADC channels for now.
If there's a better choice than a commonly available 1N60P (Vf ~0.24v @ 1mA), then please lemmy know.
If that doesn't give me the low rpm control needed for the application, then I'll be back for a much closer look at all the excellent links and suggestions provided. I suspect that I'll have to study op amps a little further, and implement one of these precision op amp rectifier circuits to feed my present ADCs.
Sorry, I'm not too lazy to go through all contributions in full depth, just a bit dotty nowadays, and time is becoming very precious.
Thank you all again, so much.
12407622. ;D
langwadt:
--- Quote from: 12407622 on August 11, 2019, 12:45:07 am ---Thanks everyone for the super support.
For just now I'm going to go with 2 x full-wave rectifiers (and voltage dividers) for each of the two motors, with low forward voltage drop germanium diodes. My present setup uses an 328P controller, and I can spare the requisite 2 x single-ended ADC channels for now.
If there's a better choice than a commonly available 1N60P (Vf ~0.24v @ 1mA), then please lemmy know.
If that doesn't give me the low rpm control needed for the application, then I'll be back for a much closer look at all the excellent links and suggestions provided. I suspect that I'll have to study op amps a little further, and implement one of these precision op amp rectifier circuits to feed my present ADCs.
Sorry, I'm not too lazy to go through all contributions in full depth, just a bit dotty nowadays, and time is becoming very precious.
Thank you all again, so much.
12407622. ;D
--- End quote ---
if everything is on the same ground you will short half the diodes. How about two diodes one from each terminal of the motor, when measuring turn on low side fet on the low voltage side
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