MCP3913 datasheet(54/82 Pages) MICROCHIP

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MCP3913

DS20005227A-page 54

 2013 Microchip Technology Inc.

7.5

Differential Inputs

Anti-Aliasing Filter

Due to the nature of the ADCs used in the MCP3913

(oversampling converters), each differential input of the

ADC channels requires an anti-aliasing filter so that the

oversampling frequency (DMCLK) is largely attenuated

and does not generate any disturbances on the ADC

accuracy. This anti-aliasing filter also needs to have a

gain close to one in the signal bandwidth of interest.

Typically for 50/60 Hz measurement and default set-

tings (DMCLK = 1 MHz), a simple RC filter with 1 k



and 100 nF can be used. The anti-aliasing filter used

for the measurement graphs is a first-order RC filter

with 1 k

 and 15 nF. The typical schematic for connect-

ing a current transformer to the ADC is shown in

Figure 7-6. If wires are involved, twisting them is also

recommended.

FIGURE 7-6:

First-Order Anti-Aliasing

Filter for CT-Based Designs.

The di/dt current sensors, such as Rogowski coils, can

be an alternative to current transformers. Since these

sensing elements are highly sensitive to high-

frequency electromagnetic fields, using a second order

anti-aliasing filter is recommended to increase the

attenuation of potential perturbing RF signals.

FIGURE 7-7:

Second-Order Anti-Aliasing

Filter for Rogowski Coil-Based Designs.

The MCP3913 is highly recommended in applications

using di/dt as current sensors because of the extremely

low noise floor at low frequencies. In such applications,

a low-pass filter (LPF) with a cut-off frequency much

lower than the signal frequency (50-60 Hz for metering)

is used to compensate for the 90 degree shift and for

the 20 db/decade attenuation induced by the di/dt sen-

sor. Because of this filter, the SNR will be decreased,

since the signal will attenuate by a few orders of mag-

nitude, while the low-frequency noise will not be atten-

uated. Usually, a high-order high-pass filter (HPF) is

used to attenuate the low-frequency noise in order to

prevent a dramatic degradation of the SNR, which can

be very important in other parts. A high-order filter will

also consume a significant portion of the computation

power of the MCU. When using the MCP3913, such a

high-order HPF is not required, since this part has a low

noise floor at low frequencies. A first-order HPF is

enough to achieve very good accuracy.

7.6

Energy Measurement

Error Considerations

The measurement error is a typical representation of

the non-linearity of a pair of ADCs (see Section 4.0

“Terminology And Formulas”

for the definition of

measurement error). The measurement error is

dependent on the THD and on the noise floor of the

ADCs.

Improving the measurement error specification on the

MCP3913 can be realized by increasing the OSR (to

get a better SINAD and THD performance) and, to

some extent, the BOOST settings (if the bandwidth of

the measurements is too limited by the bandwidth of

the amplifiers in the sigma-delta ADCs). In most of the

energy metering AC applications, high-pass filters are

used to cancel the offset on each ADC channel (current

and voltage channels), and therefore a single-point

calibration is necessary to calibrate the system for

active energy measurement. This calibration is a

system gain calibration, and the user can utilize the

EN_GAINCAL bit and the GAINCAL_CHn registers to

perform this digital calibration. After such calibration,

typical measurement error curves like Figure 2-7 can

be generated by sweeping the current channel

amplitude and measuring the energy at the outputs (the

energy calculations here are being realized off-chip).

The error is measured using a gain of 1x, as it is

commonly used in most CT-based applications.

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