MCP3551-ESLASHMS datasheet(13/30 Pages) MICROCHIP

13 / 30 page

DS21950B-page 13

MCP3551/3

4.1

MCP3551/3 Delta-Sigma Modulator

with Internal Offset and Gain

Calibration

The converter core of the MCP3551/3 devices is a

third-order delta-sigma modulator with automatic gain

and offset error calibrations. The modulator uses a 1-bit

DAC structure. The delta-sigma modulator processes

the sampled charges through switched capacitor

structures controlled by a very low drift oscillator for

reduced clock jitter.

During the conversion process, the modulator outputs

a bit stream with the bit frequency equivalent to the

fOSC/4 (see Table 4-1). The high oversampling

implemented in the modulator ensures very high

resolution and high averaging factor to achieve

low-noise specifications. The bit stream output of the

modulator is then processed by the digital decimation

filter in order to provide a 22-bit output code at a data

rate of 13.75 Hz for the MCP3551 and 60 Hz for the

MCP3553. Since the oversampling ratio is lower with

the MCP3553 device, a much higher output data rate is

achieved while still achieving 20 bits No Missing Codes

(NMC) and 20.6 ENOB.

A self-calibration of offset and gain occurs at the onset

of every conversion. The conversion data available at

the output of the device is always calibrated for offset

and gain through this process. This offset and gain

auto-calibration is performed internally and has no

impact on the speed of the converter since the offset

and gain errors are calibrated in real-time during the

conversion. The real-time offset and gain calibration

schemes do not affect the conversion process.

4.2

Digital Filter

The MCP3551/3 devices include a digital decimation

filter, which is a fourth-order modified SINC filter. This

filter averages the incoming bitstream from the modu-

lator and outputs a 22-bit conversion word in binary

two's complement. When all bits have been processed

by the filter, the output code is ready for SPI communi-

cation, the RDY flag is set on the SDO/RDY pin and all

the internal registers are reset in order to process the

next conversion.

Like the commonly used SINC filter, the modified SINC

filter in the MCP3551/3 family has the main notch

frequency located at fS/(OSR*L), where fS is the bit-

stream sample frequency. OSR is the Oversampling

Ratio and L is the order of the filter.

For the MCP3551 device, this notch is located at

55 Hz. For the MCP3553 device, the main notch is

located at 240 Hz, with an OSR of 128. (see Table 4-1).

The digital decimation SINC filter has been modified in

order to offer staggered zeros in its transfer function.

This modification is intended to widen the main notch in

order to be less sensitive to oscillator deviation or

line-frequency drift. The MCP3551 filter has staggered

zeros spread in order to reject both 50 Hz and 60 Hz

line frequencies simulaneously (see Figure 4-2).

TABLE 4-1:

DATA RATE, OUTPUT NOISE AND DIGITAL FILTER SPECIFICATIONS BY DEVICE

Device

Output Data

Rate (tCONV)

(Note)

Output

Noise

(µVRMS)

Primary

Notch

(Hz)

Sample

Frequency

(fS)

Internal

Clock

fOSC

50/60 Hz Rejection

MCP3551

72.73 ms

2.5

28160 Hz

112.64 kHz

-82 dB min. from

48 Hz to 63 Hz.

-82 dB at 50 Hz and

-88 dB at 60 Hz

MCP3553

16.67 ms

240

30720 Hz

122.88 kHz

Not Applicable

Note:

For the first conversion after exiting Shutdown, tCONV must include an additional 144 fOSC periods before

the conversion is complete and the RDY (Ready) flag appears on SDO/RDY.

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