Measuring small AC current and AC voltage?

[Spoerle]

I have a 30-year-old construction in which I use classic panel ammeters for 200uA to measure AC voltage and AC current.The measured current is 0-200uA on the order of 100kHz. Voltage 0-20mV, both AC more or less sine symmetrical around 0.
I will not state any requirements for accuracy, in the old design it was like this: the operator watched the meter and turned the potentiometer until the needle showed what the operator wanted.
In the new design, the operator is replaced by an MCU and, depending on what the ADC measures, it turns a digital potentiometer.
What I'm solving is how to complicate life as little as possible. The MCU has a 12bit 1MSPS ADC and so theoretically it's enough to separate the measured signal, amplify it and shift it from +- to 0-2.5V and solve the rest with SW, ADC. Another option is to convert AC to DC, True RMS, etc.This is not commonly done and so I don't know what the path of least resistance is.How would you solve it?

 

[barry]

Wait, what? "convert AC to DC....This is not commonly done". This is done all the time.
But since you didn't state any accuracy requirements, I would suggest you just sample the AC signal directly and let software calculate the RMS value. Maybe average over a number of cycles.

 

[Spoerle]

I don't even dare to guess with what resolution the operator reads the data from a 5cm ammeter 8 bits, 10 bits max

 

[Easy peasy]

One would normally use a CT directly on the wire carrying the HF current to get a decent sized signal out ( 20mV is quite small ) and then feed that to an ADC - usually inside the uP - then the ave or rms can be calculated as desired using standard software techniques - else the CT signal could be rectified and averaged ( 1N4448 should be good enough here ) using linear discrete circuit, averaged and then fed as essentially a DC signal to the uP, say 0 - 500mV, or larger as required.

 

[Spoerle]

Yes, a current transformer seemed like a logical choice to me, however, if we are measuring in the range of, say, 1uA to 1mA, I am not entirely sure that it can be commonly implemented.

 

[KlausST]

What I'm solving is how to complicate life as little as possible.

What does this mean?
I am the person who does the simple analog way ... and does the math in software:
I --> amplifier --> ADC --> software RMS --> control loop

But others do it the simple digital way:
I --> amplifier --> RMS_to_DC --> ADC --> control loop

both AC more or less sine symmetrical around 0

More ore less... around 0. This means there may be DC
Now you have to tell us if you are interested to know this DC value or not.
I´m the friend of including DC input the RMS calculations .. but some DVMs exclude DC before they do the RMS calcualtion.

it's enough to separate the measured signal,

What does "separate" mean here?
Galvanic isolation?
Frequency filtering? (to focus on one frequency only)
Filtering to get rid of noise?

******
Or maybe you need help in conditioning the analog signal only. Please tell us.

******

Or maybe you need help to achieve galvanic isolation.

*****
I just assume you don´t need galvanic isolation ... and you want to do V and I (quasi) simultaneously.
(Which you need to do (perfect simultaneously) in case you want to calculate phase shift between V and I or if you want to calculate true power ....)

thus my approach for the current path is:
0...200uA AC --> shunt --> voltage --> amplifier + LPF --> ADC --> software.

This way you have low analog / harware effort, but you are most flexible and most accurate in software.
If you want to calculate RMS ... you don´t even need to care about nyquist.
You may do under_sampling ... as long as you can ensure: f_signal / f_sample is not an integer number. (include all frequency inaccuracies)

The higher your sampling frequency the less critical the situation gets. Absolute frequencies matter.

Klaus

--- Updated Saturday at 7:54 AM ---

if we are measuring in the range of, say, 1uA to 1mA,

please don´t change your ranges / requirements from one post to the next.
--> post#1 says: 0...200uA AC 100kHz

So why now 1mA .. which is 5 times the initial specification?

Is there a range of "set point" we should know?
I mean: is it really a use case where the operator needs to focus on 5uA RMS value? or is the "target" always somewhere between 180uA and 200uA RMS?

Klaus