Metering 20mA to 100A with Metering IC - ADC

[hm_fa_da]

Hi,

in STPM10 datasheet it is mentioned dynamic range is 1000 and for current sensing inputs which are differential inputs, it says max voltage is +-0.3 V.
but it doesn't say anything about minimum voltage, is the minimum input 0.3/1000=3mV ? it means it won't measure lower inputs ?


if i want to design a 5(100)A meter, which should be able to measure current from 20mA to 100A, so i can't use this chip ?
if i adjust the higher side on (100A) then the lowest side will be 0.1 A.
if i adjust the lower side on 0.02A then the highest side will be 20A !
are my calculations true ?!

if yes, i just saw a meter from a company which claims 5(100)A in specifications but uses STPM10 chip for ADC ... !
or Actaris ACE5000 which uses Cirrus CS5451 with 1000 dynamic range for 5(120) A.
according to IEEE62053-21(8.3.3) standard, the meter should be able to start metering from 0.004*Ib => 0.004*5A= 20mA . and maximum 120A.

can i manage this problem by changing gain by software ?
for example i choose a shunt to produce 3mV for 20mA, then the max would be 300mV for 20A, but when i get near 20A, i decrease gain, so i have room for higher currents ...
is this possible ?!

about the minimum differential input voltage, it is written in ATM90E26 datasheet only, it is written 5uV to 25mV ( dynamic range is 5000 )


other Metering ICs just say maximum input voltage ! ( like STPM10 )
i myself calculated the minimum input voltage according to dynamic range of each device.

Is there something wrong with my calculations ? i am confused !

 

[Warpspeed]

can i manage this problem by changing gain by software ?
for example i choose a shunt to produce 3mV for 20mA, then the max would be 300mV for 20A, but when i get near 20A, i decrease gain, so i have room for higher currents ...
is this possible ?!

Yes its possible.
Autoranging is now a very common feature in many instruments.

For example in a multimeter you might switch to the dc volts function, and the meter steps up or down in range to find a range that gives highest resolution without exceeding the maximum for that range.

Its a bit more complicated with current measurement, but not impossible.

 

[hm_fa_da]

Yes its possible.
Autoranging is now a very common feature in many instruments.

For example in a multimeter you might switch to the dc volts function, and the meter steps up or down in range to find a range that gives highest resolution without exceeding the maximum for that range.

Its a bit more complicated with current measurement, but not impossible.

but i don't think other Electricity kwh meters do so ! because if i want to do it, i should change the calibration configurations too, because the chip measure Active, reactive and ... together, so if i change gain for current, i should change calibration data too, then current is measured true !
the problem is that some metering ICs have OTP (one time programmable) for calibration data ! so auto metering is not possible ... and it can't be a solution.

i in fact still doubt about my calculation about minimum input voltage on differential pins of metering IC !
why only atmel chip says about minimum ? ( 5uV to 25mV i.e )
but ST chip says only maximum.
a Cirrus (CS5451 which is used in Actaris meter) also says only about maximum differential input.
they don't say anything about minimum input voltage, so it is i.e 0 to 0.3V for STPM10 ? so i can set 100A equal to 0.3V and then it can measure down to 0mA current ? or because of 1000 dynamic range, i can only measure down to 0.01A (100mA) ?

 

[KlausST]

Hi,

in STPM10 datasheet it is mentioned dynamic range is 1000

I can´t find this information in the datasheet.

--> I doubt this information is correct because with that low dynamic range it is not useful for energy meters.

I find an ADC resolution for current to be 16 bits. And it seems there are internal selectable gain stages.

Klaus

 

[FvM]

but it doesn't say anything about minimum voltage, is the minimum input 0.3/1000=3mV ? it means it won't measure lower inputs ?

The lowest current range (gain x32) has an input range of +/- 35 mV, resolution of the 16 bit current ADC is about 1 µV in x32 range.

The datasheet actually specifies

Less than 0.1% error in the 1000:1 range

This 1000:1 refers to the selected range and it's the minimal input for the specified accuracy, minimal detectable current is much smaller.

There's however a threshold stopping energy integration below a certain VA value.

 

[hm_fa_da]

The lowest current range (gain x32) has an input range of +/- 35 mV, resolution of the 16 bit current ADC is about 1 µV in x32 range.

The datasheet actually specifies

This 1000:1 refers to the selected range and it's the minimal input for the specified accuracy, minimal detectable current is much smaller.

There's however a threshold stopping energy integration below a certain VA value.

what you said seems to be true completely, but how about atmel (ATM90E26) specifications ?
it says exactly input range ( for gain 24 ) : 5uV to 25mV in electrical specification table.

(this chip has 0.1% accuracy in the 5000:1 range.)
apart from accuracy, i want to know the minimum detectable current for this chip.
if i should calculate it according to ADC resolution, so why it says range 5uV to 25mV ?
according to ADC resolution it should be less than 1uV detectable on input ... so range will be 1uV to 25mV

according to your reply which seems most logical, i think atmel just said the range which accuracy is less than 0.1% and it doesn't refer to minimum detectable input, now it seems more logical but i think it's atmel's fault to mention 5uV as minimum input voltage in Electrical specifications table ( and not saying in condition = 0.1% accuracy ). so i thought it is the minimum electrical detectable input voltage for this chip :/

do you confirm this conclusion ?!

 

[FvM]

I didn't look at the Atmel datasheet, but a general consideration can probably help. For ADC specification you have at least two important specifications, resolution (based on number of bits N) and effective resolution respectively effective number of bits (ENOB).

The effective resolution describes the minimal input signal that can be distinguished from noise. This lower "detection limit" is always linked to an uncertainty number. A popular standard is to take twice the standard deviation (or noise magnitude) as detection limit.