MCP3912 datasheet(55/82 Pages) MICROCHIP

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 2014 Microchip Technology Inc.

DS20005348A-page 55

MCP3912

7.5

Differential Inputs

Anti-Aliasing Filter

Due to the nature of the ADCs used in the MCP3912

(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.

1024

2.44

512

3.91

512

2.93

256

3.91

TABLE 7-1:

SAMPLING SPEED VS.

MCLK AND OSR,

ADC PRESCALE 1:1

MCLK

(MHz)

Boost<1:0>

OSR

Sampling

Speed

(ksps)

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