MCP3551-ESLASHMS datasheet(14/30 Pages) MICROCHIP

14 / 30 page

MCP3551/3

DS21950B-page 14

FIGURE 4-2:

SINC filter response,

MCP3551 device, simultaneous 50/60 Hz

Rejection.

FIGURE 4-3:

SINC Filter Response at

Integer Multiples of the Sampling Frequency (fs)

4.3

Internal Oscillator

The MCP3551/3 devices include a highly stable and

accurate internal oscillator that provides clock signals

to the delta-sigma ADC with minimum jitter. The oscil-

lator is a specialized structure with a low temperature

coefficient across the full range of specified operation,

see Table 4-1 for oscillator frequencies.

The conversion time is an integer multiple of the inter-

nal clock period and, therefore, has the same accuracy

the

internal

clock

frequency.

The

oscillator

frequency is 112.64 kHz ±0.5% for the MCP3551 and

122.88 kHz ±0.5% for the MCP3553 devices across

the full power supply voltage and specified temperature

ranges.

The notch of the digital filter is proportional to the

internal oscillator frequency, with the exact notch

frequency equivalent to the oscillator accuracy (< 0.5%

deviation). This high accuracy, combined with wide

notches, will ensure that the MCP3551 includes both

50 Hz and 60 Hz line frequency rejection by the digital

filtering, even when jitter is present.

The internal oscillator is held in the Reset condition

when the part is in Shutdown mode to ensure very low

power consumption (< 1 µA in Shutdown mode). The

internal oscillator is independent of all serial digital

interface edges (i.e., state machine processing the

digital SPI interface is asynchronous with respect to the

internal clock edges).

4.4

Differential Analog Inputs

The MCP3551/3 devices accept a fully differential ana-

log input voltage to be connected to the VIN+ and VIN-

input pins. The differential voltage that is converted is

defined by VIN = VIN+ - VIN-. The differential voltage

range specified for ensured accuracy is from -VREF to

+VREF.

The converter will output valid and usable codes from

-112% to 112% of output range (see Section 5.0

“Serial Interface”) at room temperature. The ±12%

overrange is clearly specified by two overload bits in

the output code: OVH and OVL. This feature allows for

system calibration of a positive gain error.

The absolute voltage range on these input pins extends

from VSS - 0.3V to VDD + 0.3V. If the input voltages are

above or below this range, the leakage currents of the

ESD diodes will increase exponentially, degrading the

accuracy and noise performance of the converter. The

common mode of the analog inputs should be chosen

such that both the differential analog input range and

absolute voltage range on each pin are within the

specified operating range defined in the Section 1.0

“Electrical Characteristics”.

Both the analog differential inputs and the reference

input have switched-capacitor input structures. The

input capacitors are charged and discharged alterna-

tively with the input and the reference in order to

process a conversion. The charge and discharge of the

input capacitors create dynamic input currents at the

VIN+ and VIN- input pins inversely proportional to the

sampling capacitor. This current is a function of the

differential input voltages and their respective common

modes. The typical value of the differential input imped-

ance is 2.4 M

Ω, with VCM = 2.5V, VDD = VREF = 5V. The

DC leakage current caused by the ESD input diodes,

even though on the order of 1 nA, can cause additional

offset errors proportional to the source resistance at the

VIN+ and VIN- input pins.

From a transient response standpoint and as a

first-order approximation, these input structures form a

simple RC filtering circuit with the source impedance in

series with the RON (switched resistance when closed)

of the input switch and the sampling capacitor. In order

to ensure the accuracy of the sampled charge, proper

settling time of the input circuit has to be considered.

Slow settling of the input circuit will create additional

gain error. As a rule of thumb, in order to obtain 1 ppm

absolute measurement accuracy, the sampling period

must be 14 times greater than the input circuit RC time

constant.

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

102030

60708090 100 110

Frequency (Hz)

-140

-120

-100

-80

-60

-40

-20

28160

56320

84480 112640 140800 168960 197120 225280 253440

Frequency (Hz)

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