Electronic – Measuring VPP and DC offset of a 1Hz – 1MHz signal using MCU

achigh frequencymeasurementmicrocontrollervoltage measurement

I need to measure the peak to peak voltage and DC offset of a 1Hz to 1MHz sine/triangle wave. The signals amplitude varies between 0 and 2Vpp and the offset can vary between -2v and +5v. I will be using an Atmega16 to do the A/D. The Atmega16 has an 8 channel 10-Bit ADC with 50-200KHz sample rate @10-Bit resolution. Taking multiple samples for cycle is obviously out of the question.

What I was thinking of doing was to sample two different versions of the signal eg connect the signal to both a negative peak detector and a precision rectifier and connect those circuits outputs to two different ADC channels and then from result of these conversions (ie the negative and positive peaks) calculate the the vpp and DC offset. Obviously the signals will need to scaled and level shifted before being applied to the controller.

Im not really sure if this will work at all, there must be a far simpler and more sophisticated way to do this that I haven't thought of.

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Several ways to measure peak-to-peak voltage with a MCU:

undersampling: Directly sample with an ADC, even though you know you are getting less than once sample per cycle of the signal. This requires (a) the signal repeats over and over the same waveform, and any changes to that pattern occur relatively slowly, and (b) the actual sample-and-hold part of the ADC has a high enough bandwidth, even though you only get one sample once every few cycles of the signal, and (c) the exact sampling instant of the sample-and-hold doesn't always hit the same point on the repeating signal, but drifts along it, eventually hitting the highest peak and later the lowest valley of the signal. (Often this drift happens without you having to do anything; but to guarantee it will always work, you may need to lower your sampling rate with some signals). High-speed digital oscilloscopes often use this undersampling technique to inspect amazingly high frequency signals.
use maximum peak detector and minimum valley detector to hold the maximum and minimum voltages on sampling capacitors long enough for you to sample them. Either use a resistor on each capacitor with a "long enough" RC time constant that the change in voltage from the actual maximum to your ADC sample is insignificant, or use a transistor on each capacitor to reset the voltage immediately after you sample it. If you can estimate the diode voltage drop in software, the simplest hardware is a diode-capacitor-resistor peak detector and a diode-capacitor-resistor valley detector ... but using an op-amp precision peak detector rather than a single diode may be simpler than trying to estimate the diode voltage drop.
use a DC blocking capacitor, followed by a diode to ground (to clamp the minimum voltage to 0) and a diode to a peak detect sampling capacitor. The voltage on that capacitor gives you the peak-to-peak voltage directly. Somehow estimate and compensate for the diode voltage drops in software, or else use op-amps to compensate for them in hardware, similar to the peak detector.
use a frequency mixer (analog multiplier) to shift high-frequency signals down to an easier-to-handle frequency well inside your ADC's capability. (Like undersampling, this assumes the signal repeats over and over the same waveform, and any changes to that pattern occur relatively slowly).

I'm not sure why you want a precision rectifier. I assume you really meant "precision peak detector" (which is admittedly almost the same circuit).

A RC filter with a long RC time constant will give you the average voltage of the signal. As pingswept says, it's probably best to use a fixed cutoff frequency (for the peak detect, valley detect, and the average detect) to handle the high frequencies with analog circuits, and deal with low frequencies in software.

As long as the MCU is sampling the highest peak voltage and lowest valley voltage, consider also reporting the midway voltage halfway between the two -- this is not always the same as the true average voltage of the signal.

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Electronic – Measuring 0 – 1MHz ( 0.25Hz resolution) Squarewave using an MCU

If possible I'd suggest selecting a microcontroller that supports a counter operation using the timer inputs; rather than manually incrementing a counter inside an ISR (which at high frequencies quickly ends up saturating the microcontroller activity) you allow the hardware to handle the counting. At this point your code simply becomes a matter of waiting for your periodic interrupt then calculating the frequency.

To extend the range and make the frequency counter more generalised (removing the need for multiple ranges at the expense of a little more work for the MCU) you could use the following technique.

Select a periodic interrupt rate that allows for measurement accuracy at the highest input frequency; this should take into account your counter size (you need to select the timer period such that the timer counter will not overflow at the maximum input frequency). For this example I'll assume that the input counter value can be read from the variable "timer_input_ctr".

Include a variable for counting periodic interrupts (should be initialised to 0 at startup); for this example I'll refer to this variable as "isr_count". The interrupt period is contained in the constant "isr_period".

Your periodic interrupt should be implemented as (C pseudo-code):

void timer_isr()
{
  isr_count++;
  if (timer_input_ctr > 0)
  {
    frequency = timer_input_ctr / (isr_count * isr_period).
    timer_input_ctr = 0;
    isr_count = 0;
  }
}

Obviously this rough example relies on some floating point math that may not be compatible for low-end microcontrollers, there are techniques to overcome this but they are outside of the scope of this answer.

Electronic – arduino – Measuring real and apparent power

Real power is based on the difference in phase between the voltage and the current waveforms. You mainly see it in calculation of power factor. From wikipedia:

The ratio between real power and apparent power in a circuit is called the power factor. It's a practical measure of the efficiency of a power distribution system. For two systems transmitting the same amount of real power, the system with the lower power factor will have higher circulating currents due to energy that returns to the source from energy storage in the load. These higher currents produce higher losses and reduce overall transmission efficiency. A lower power factor circuit will have a higher apparent power and higher losses for the same amount of real power. The power factor is one when the voltage and current are in phase. It is zero when the current leads or lags the voltage by 90 degrees. Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle of current with respect to voltage.

In your application I believe they are using shiftedV as a way to see how much the phase has changed since the last value. With a purely resistive load, the power factor would be 1...so the real power=apparent power. So you are probably suppose to use a resistive load to determine your phasecal number(set it so real=apparent). Then you can find the power factor for various loads.

Edit: If you take the time average of the instantaneous product of voltage and current, you will surely have a real power measurement. As I (poorly) implied above, I believe the shiftedV is used to adjust for any reactance elements introduced by the measurement process. As a way to null out the effects of the electronics, so you get the real power of the load. Here good reading and pictures National Instruments

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Electronic – Measuring 0 – 1MHz ( 0.25Hz resolution) Squarewave using an MCU

Electronic – arduino – Measuring real and apparent power

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