MCP3910 datasheet(30/90 Pages) MICROCHIP

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MCP3910

DS20005116D-page 30

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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 prop-

agated 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 leak-

age currents at the analog inputs if the input voltage is

highly negative (typically, below -0.6V, referred to as

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 are 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 return 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

nonlinearity error of the energy power across the

current 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 the current channel and

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

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

ured 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 voltage 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

calculation 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 specifications. The

measurement error is a function of the linearity and THD

of the ADCs, while the standard deviation of the

measurement error is a function of the noise specifica-

tion of the ADCs. Overall, the low THD specification

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 measurement

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 cali-

bration. 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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100%

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