MCP3906AT-ISLASHSS datasheet(17/32 Pages) MICROCHIP

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DS22011B-page 17

MCP3905A/05L/06A

4.3

Ultra-Low Drift VREF

The MCP3905A/05L/06A devices contain an internal

voltage reference source specially designed to mini-

mize drift over temperature. This internal VREF supplies

reference voltage to both current and voltage channel

ADCs. The typical value of this voltage reference is

2.4V ±100 mV. The internal reference has a very low

typical temperature coefficient of ±15 ppm/°C, allowing

the output frequencies to have minimal variation with

respect to temperature since they are proportional to

(1/VREF)².

The output pin for the voltage reference is REFIN/OUT.

Appropriate bypass capacitors must be connected to

the

REFIN/OUT

for

proper

operation

(see

Section 5.0

“Applications

Information”).

The

voltage reference source impedance is typically 4 k

Ω,

which enables this voltage reference to be overdriven

by an external voltage reference source.

If an external voltage reference source is connected to

the REFIN/OUT pin, the external voltage will be used

as the reference for both current and voltage channel

ADCs. The voltage across the source resistor will then

be the difference between the internal and external

voltage. The allowed input range for the external

voltage source goes from 2.2V to 2.6V for accurate

measurement error. A VREF value outside of this range

will cause additional heating and power consumption

due to the source resistor, which might affect measure-

ment error.

4.4

Power-On Reset (POR)

The MCP3905A/05L/06A devices contain an internal

POR circuit that monitors analog supply voltage AVDD

during operation. This circuit ensures correct device

startup at system power-up and system power-down

events. The POR circuit has built-in hysteresis and a

timer to give a high degree of immunity to potential

ripple and noise on the power supplies, allowing proper

settling of the power supply during power-up. A 0.1 µF

decoupling capacitor should be mounted as close as

possible to the AVDD pin, providing additional transient

immunity

(see

Section 5.0

“Applications

Information”).

The threshold voltage is typically set at 4V, with a

tolerance of about ±5%. If the supply voltage falls below

this threshold, the MCP3905A/05L/06A devices will be

held in a Reset condition (equivalent to applying logic

‘0’ on the MCLR pin). The typical hysteresis value is

approximately 200 mV in order to prevent glitches on

the power supply.

Once a power-up event has occurred, an internal timer

prevents the part from outputting any pulse for

approximately 1s (with MCLK = 3.58 MHz), thereby

preventing potential metastability due to intermittent

resets caused by an unsettled regulated power supply.

Figure 4-3 illustrates the different conditions for a

power-up and a power-down event in the typical

conditions.

FIGURE 4-3:

Power-on Reset Operation.

4.5

High-Pass Filters and Multiplier

The active real-power value is extracted from the DC

instantaneous

power.

Therefore,

any

offset

component present on Channel 0 and Channel 1

affects the DC component of the instantaneous power

and will cause the real-power calculation to be

erroneous. In order to remove DC offset components

from the instantaneous power signal, a high-pass filter

has been introduced on each channel. Since the high-

pass filtering introduces phase delay, identical high-

pass filters are implemented on both channels. The

filters are clocked by the same digital signal, ensuring

a phase difference between the two channels of less

than one MCLK period. Under typical conditions

(MCLK = 3.58 MHz), this phase difference is less than

0.005°, with a line frequency of 50 Hz. The cut-off

frequency of the filter (4.45 Hz) has been chosen to

induce minimal gain error at typical line frequencies,

allowing sufficient settling

time

for

the desired

applications. The two high-pass filters can be disabled

by applying logic ‘0’ to the HPF pin.

FIGURE 4-4:

HPF Magnitude Response

(MCLK = 3.58 MHz).

AVDD

4.2V

DEVICE

MODE

RESET

PROPER

OPERATION

RESET

PULSE

OUT

Time

-40

-35

-30

-25

-20

-15

-10

-5

0.1

100

1000

Frequency (Hz)

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