MAX71020(2012) 查看數據表（PDF） - Maxim Integrated

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MAX71020
(Rev.:2012)

Single-Chip Electricity Meter AFE

Maxim Integrated 

MAX71020

Single-Chip Electricity Meter AFE

For a standard single-phase application with tamper sen-

sor in the neutral path, two current inputs are configured for

differential mode, using the pin pairs IAP-IAN and IBP-IBN.

In the MAX71020, the system uses two locally connected

current sensors via IAP-IAN and IBP-IBN and configured

as differential inputs. The VA pin is typically connected to

the phase voltage via resistor-dividers.

The MAX71020 adds the ability to sample a second

phase voltage (applied at the VB pin), which makes it

suitable for meters with two voltage and two current sen-

sors, such as meters implementing Equation 2 for dual-

phase operation (P = VA x IA + VB x IB).

Table 1 summarizes the AFE input configuration.

Delay Compensation

When measuring the energy of a phase (i.e., Wh and

VARh) in a service, the voltage and current for that phase

must be sampled at the same instant. Otherwise, the

phase difference, φ, introduces errors.

=φ

tdelay × 3= 60 

T

tdelay × f × 360 

where f is the frequency of the input signal, T = 1/f and

tDELAY is the sampling delay between current and voltage.

Table 1. ADC Input Configuration

PIN

COMMENT

IAP

The ADC results are stored in register IA.

IAN

IBP

The ADC results are stored in register IB.

IBN

VA

The ADC result is stored in register VA.

VB

The ADC result is stored in register VB.

Traditionally, sampling is accomplished by using two

ADCs per phase (one for voltage and the other one for

current) controlled to sample simultaneously. Maxim’s

Teridian™ Single-Converter Technology®, however,

exploits the 32-bit signal processing capability of its CE

to implement “constant delay” allpass filters. The allpass

filter corrects for the conversion time difference between

the voltage and the corresponding current samples that

are obtained with a single multiplexed ADC.

The constant delay allpass filter provides a broadband

delay 360N - θ, which is precisely matched to the differ-

ence in sample time between the voltage and the current

of a given phase. This digital filter does not affect the

amplitude of the signal, but provides a precisely con-

trolled phase response.

The ADC multiplexer samples the current first, immedi-

ately followed by sampling of the corresponding phase

voltage, thus the voltage is delayed by a phase angle

φ relative to the current. The delay compensation imple-

mented in the CE aligns the voltage samples with their

corresponding current samples by first delaying the cur-

rent samples by one full sample interval (i.e., 360N), then

routing the voltage samples through the allpass filter, thus

delaying the voltage samples by 360N - θ, resulting in the

residual phase error between the current and its corre-

sponding voltage of θ - φ. The residual phase error is neg-

ligible, and is typically less than ±0.0015N at 100Hz, thus it

does not contribute to errors in the energy measurements.

ADC Preamplifier

The ADC preamplifier is a low-noise differential amplifier

with a fixed gain of 9 available on the IAP and IAN current-

sensor input pins. It is provided only in versions of the

MAX71020 AFE configured for use with current shunts.

CK32

MUX STATE S

0

IA

CROSS

MUX_SYNC

MULTIPLEXER FRAME

MUX DIV = 4 CONVERSIONS

1

2

VA

IB

SETTLE

3

S

0

VB

Figure 2. States in a Multiplexer Frame

Teridian is a trademark and Single Converter Technology is a registered trademark of Maxim Integrated Products, Inc.

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