MCP3910 datasheet(62/90 Pages) MICROCHIP

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MCP3910

DS20005116D-page 62

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8.5

Differential Inputs Anti-Aliasing

Filter

Due to the nature of the ADCs used in the MCP3910

(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 the 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 8-6. If wires are involved, twisting them is also

recommended.

FIGURE 8-6:

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 8-7:

Second-Order Anti-Aliasing

Filter for Rogowski Coil-Based Designs.

The filter presented in Figure 8-7 is an anti-aliasing

filter. The di/dt integrator can be created in firmware as

a first-order low-pass filter with corner frequency much

lower than the input signal.

The MCP3910 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 cutoff 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 sensor.

Because of this filter, the SNR will be decreased, since

the signal will attenuate by a few orders of magnitude,

while the low-frequency noise will not be attenuated.

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 con-

sume a significant portion of the computation power of

the MCU. When using the MCP3910, 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.

8.6

Energy Measurement Error

Considerations

The measurement error is a typical representation of

the nonlinearity of a pair of ADCs (see Section 4.0

“Terminology and Formulas”

for the definition of

measurement error). The measurement error is depen-

dent on the THD and on the noise floor of the ADCs.

Improving the measurement error specification on the

MCP3910 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, such as in 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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