RMS voltage calculation is incorrect

[reinto1234]

Hello,
for getting the real VRMS there is a multiplication by sqrt(2) missing. I don't exactly know why but i tasted it with a Waveform generator without resistors and a Fluke. I don't get the correct VRMS values if I use the function written down here i Have to multiply with sqrt(2).
Here's how i changed the code:
ACS37800ERR ACS37800::readRMS(float *vRMS, float *iRMS)
{
ACS37800_REGISTER_20_t store;
ACS37800ERR error = readRegister(&store.data.all, ACS37800_REGISTER_VOLATILE_20); // Read register 20

if (error != ACS37800_SUCCESS)
{
if (_printDebug == true)
{
_debugPort->print(F("readRMS: readRegister (20) returned: "));
_debugPort->println(error);
}
return (error); // Bail
}

//Extract vrms. Convert to voltage in Volts.
// Note: datasheet says "RMS voltage output. This field is an unsigned 16-bit fixed point number with 16 fractional bits"
// Datasheet also says "Voltage Channel ADC Sensitivity: 110 LSB/mV"
float volts = (float)store.data.bits.vrms;
if (_printDebug == true)
{
_debugPort->print(F("readRMS: vrms: 0x"));
_debugPort->println(store.data.bits.vrms, HEX);
_debugPort->print(F("readRMS: volts (LSB, before correction) is "));
_debugPort->println(volts);
}
volts /= 65536; //Convert from codes to the fraction of ADC Full Scale (16-bit)
volts /= 1.19; //Convert from codes to mV (110 LSB/mV)
volts *= 250; //Convert to mV (Differential Input Range is +/- 250mV)
volts /= 1000; //Convert to Volts
volts *= sqrt(2); //Convert to RMS (AC voltage)
//Correct for the voltage divider: (RISO1 + RISO2 + RSENSE) / RSENSE
//Or: (RISO1 + RISO2 + RISO3 + RISO4 + RSENSE) / RSENSE
float resistorMultiplier = (_dividerResistance + _senseResistance) / _senseResistance;
volts *= resistorMultiplier;
if (_printDebug == true)
{
_debugPort->print(F("readRMS: volts (V, after correction) is "));
_debugPort->println(volts);
}
*vRMS = volts;

[PaulZC]

Hi @reinto1234 ,

Thanks for reporting this.

You say you tested it "with a Waveform generator without resistors". Did you remove the voltage divider resistors on the SparkX breakout? Or have you designed your own board? Have you set _dividerResistance and _senseResistance correctly?

Best wishes,
Paul

[reinto1234]

Hello Paul,
i designed my own board. Basically its just the sensor. Divider resistance is set to 0. Sense resistor to 1. Example: Vpp waveform generator: 150mV, FLuke: 53mV AC VRMS, ACS: 0.05V VRMS (with multiplication by sqrt(2).
Greetings Tobi

[PaulZC]

Hi Tobi,

Could this be a grounding issue?

Please post a link to your waveform generator. Is its output isolated? Is GND / 0V common between the waveform generator and the ACS37800 circuit? Does the 150mVp-p output have a DC bias?

Cheers,
Paul

[reinto1234]

That's the Datasheet: https://beyondmeasure.rigoltech.com/acton/attachment/1579/f-001b/0/-/-/-/-/file.pdf
I'm working on a laptop so my ESP32 isn't connected to the same GND as the Rigol i also tried with the charger connected and it still had the same values. I rechecked the DC bias on the rigol it says 0 and the fluke also doesnt recognize any.

[PaulZC]

OK - thanks.

What happens if you add a 75mV DC offset to the 150mVp-p?

I suggest looking again at your GND path. I suspect the voltage going to the ACS37800 is negative for half of each wave, preventing the ACS37800 from measuring it?

Please confirm if these are your connections:

• Laptop (USB?) GND -> ESP32 GND -> ACS37800 GND -> ACS37800 VINN -> Waveform Generator GND (BNC Outer)
• ACS37800 VINP -> Waveform Generator Out (BNC Inner)

[reinto1234]

If I add the Offset i run into a problem of the sensor: it doesnt calculate VRMS if 0V isn't crossed if I add: mySensor.setBypassNenable(false, true);
It seems necessary that the sinus is without offset. This also leads to problems in my other project :-/
Yes the voltage is negative but there is no 0 crossing without negative voltage and the datasheet says -125 to 125mV.
The connections are correct.

Im really confused with the sensor since 3 days so if we resolve this this would help me really much.
Greetings Tobi

[PaulZC]

Do you have an oscilloscope? If you do, probe the voltage between VINP and GND/VINN. Does it look sinusoidal? Or is it clipped?

Maybe try AC-coupling? Add a small capacitor (~10pF) between the waveform generator and VINP. Does that help?

[reinto1234]

I have an oscilloscope, sine looks fine. The values stay the same. There is no offset.

[PaulZC]

Hi Tobi,

OK. Thank you. I will try to replicate this in the next ~2 days.

Best wishes,
Paul