MCP3918 datasheet(62/88 Pages) MICROCHIP

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MCP3918

DS20005287A-page 62

 2014 Microchip Technology Inc.

8.4

Sampling Speed and Bandwidth

If ADC power consumption is not a concern in the

design, the boost settings can be increased for best

performance so that the OSR is always kept at the

maximum settings to improve the SINAD performance

(see Table 7-1). If the MCU cannot generate a clock

fast enough, it is possible to tap the OSC1/OSC2 pins

of the MCP3918 crystal oscillator directly to the crystal

of the microcontroller. When the sampling frequency is

enlarged, the phase resolution is improved, and with

the OSR increased, the phase compensation range

can be kept in the same range as the default settings.

8.5

Differential Inputs Anti-Aliasing

Filter

Due to the nature of the ADC used in the MCP3918

(oversampling converter), 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

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

 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

connecting 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

high-frequency

electromagnetic

fields,

using

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 MCP3918 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

sensor. Because of this filter, the SNR will be

decreased, since the signal will be attenuated 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 consume a significant portion of the

computation power of the MCU. When using the

MCP3918, 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.

TABLE 8-1:

SAMPLING SPEED VS. MCLK

AND OSR, ADC PRESCALE

1:1

MCLK

(MHz)

Boost

OSR

Sampling

Speed

(ksps)

0b11

1024

3.91

0b11

1024

3.42

0b11

1024

2.93

0b10

1024

2.44

0b10

512

3.91

0b01

512

2.93

0b01

256

3.91

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