MCP3910 datasheet(29/86 Pages) MICROCHIP

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DS20005116B-page 29

MCP3910

4.22

Full Shutdown Mode

The lowest power consumption can be achieved when

SHUTDOWN<1:0> = 11, VREFEXT = CLKEXT = 1. This

mode is called Full Shutdown mode, and no analog

circuitry is enabled. In this mode, both AVDD and DVDD

POR monitoring are also disabled, and no clock is

propagated throughout the chip. All ADCs are in

Shutdown mode, and the internal voltage reference is

disabled. This mode can only be entered during SPI

mode.

The clock is no longer distributed to the input structure

either. This can potentially cause high analog input

leakage currents at the analog inputs, if the input volt-

age is highly negative (typically below -0.6V referred to

AGND).

The only circuit that remains active is the SPI interface,

but this circuit does not induce any static power

consumption. If SCK is idle, the only current

consumption comes from the leakage currents induced

by the transistors and is less than 5 µA on each power

supply.

This mode can be used to power down the chip

completely and to avoid power consumption when

there is no data to convert at the analog inputs. Any

SCK or MCLK edge occurring while in this mode will

induce dynamic power consumption.

Once any of the SHUTDOWN, CLKEXT and VREFEXT

bits returns to ‘0’, the two POR monitoring blocks are

operational, and AVDD and DVDD monitoring can take

place.

4.23

Measurement Error

The measurement error specification is typically used

in power metering applications. This specification is a

measurement of the linearity of the active energy of a

given power meter across its dynamic range.

For this measurement, the goal is to measure the

active energy of one phase when the voltage Root

Mean Square (RMS) value is fixed, and the current

RMS value is sweeping across the dynamic range

specified by the meter. The measurement error is the

non-linearity error of the energy power across the cur-

rent dynamic range. It is expressed in percent (%).

Equation 4-13 shows the formula that calculates the

measurement error:

EQUATION 4-13:

In the present device, the calculation of the active

energy is done externally, as a post-processing step

that typically happens in the microcontroller, consider-

ing, for example, channel 0 as current channel and

channel 1 as voltage channel. Channel 1 is fed with a

full-scale sine wave at 600 mV peak, and is configured

with GAIN = 1 and DITHER = Maximum. To obtain the

active energy measurement error graphs, Channel 0 is

fed with sine waves with amplitudes that vary from

600 mV peak to 60 µV peak, representing a 10,000:1

dynamic range. The offset is removed on both current

and voltage channels, and the channels are multiplied

together to give instantaneous power. The active

energy is calculated by multiplying the current and volt-

age channel, and averaging the results of this power

during 20 seconds, to extract the active energy. The

sampling frequency is chosen as a multiple integer of

line frequency (coherent sampling). Therefore, the cal-

culation does not take into account any residue coming

from bad synchronization.

The measurement error is a function of IRMS, varies

with the OSR, averaging time, MCLK frequency and is

tightly coupled with the noise and linearity specifica-

tions. The measurement error is a function of the linear-

ity and THD of the ADCs, while the standard deviation

of the measurement error is a function of the noise

specification of the ADCs. Overall, the low THD speci-

fication enables low measurement error on a very large

dynamic range (e.g. 10,000:1). A low noise and high

SNR specification enables the decrease of the mea-

surement time and, therefore, of the calibration time, to

obtain a reliable measurement error specification.

Figure 2-5 shows the typical measurement error

curves obtained with the samples acquired by the

MCP3910, using the default settings with 1-point and

2-point calibration. These calibrations are detailed in

Section 8.6

“Energy

Measurement

Error

Considerations”

Measurement Error I

RMS

 Measured Active Energy Active Energy present at inputs

–

Active Energy present at inputs

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