ST STPM10 User Manual

Programmable single-phase energy metering IC

Features

■ Measures active, reactive, and apparent

energies

■ Current, voltage RMS and instantaneous

measurement

■ Frequency measurement

■ Ripple-free active energy pulsed output

■ Live and neutral monitoring for tamper

detection

■ Fast and simple one-point digital calibration

over the whole current range

■ Integrated linear voltage regulators for digital

and analog supply

■ Selectable RC or crystal oscillator

■ Supports 50 - 60 Hz - IEC62052-11, IEC62053-

2x specifications

■ Less than 0.1% error in the 1000:1 range

■ Precision voltage reference: 1.23 V with 30

ppm/°C max

Description

The STPM10 is designed for effective
measurement of active, reactive and apparent
energy in a power line system using current
transformer and shunt sensors. The device can
be implemented for peripheral measurement in a
microcontroller-based single-phase or poly-phase
energy meter. The STPM10 consists of two main
sections: analog and digital. The analog part is
composed of preamplifier and first-order sigma­delta A/D converter blocks, a band-gap voltage
reference and low-drop voltage regulator. The
digital part is composed of system control,
oscillator, hard-wired DSP and SPI interface.
There is also an internal volatile memory, which is

Table 1. Device summary

STPM10

with tamper detection

TSSOP20

controlled through the SPI by means of a
dedicated command set. The configured bits are
used for configuration and calibration purposes.
From a pair of sigma-delta output signals
produced by the analog section, the DSP unit
computes the amount of active, reactive and
apparent energy consumed, as well as the RMS
and instantaneous voltage and current values.
The results of the computation are available as
pulse frequencies and states on the digital
outputs of the device, or as data bits in a data
stream, which can be read from the device by
means of the SPI interface. The system bus
interface is also used for temporary programming
of bits of internal volatile memory. The STPM10
generates an output signal with a pulse frequency
proportional to the energy, and this signal is used
in the calibration phase of the energy metering
application.

Order code Temperature range Package Packaging

STPM10BTR - 40 to 85 °C TSSOP20 (tape and reel) 2500 parts per reel

June 2011 Doc ID 17728 Rev 3 1/51

www.st.com

51

Contents STPM10

Contents

1 Schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.1 Measurement error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.2 ADC offset error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.3 Gain error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.4 Power supply DC and AC rejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.5 Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6 Typical performance characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7 Theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7.1 General operation description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7.2 Analog inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7.3 ΣΔ A/D converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7.4 Zero-crossing detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7.5 Period and line voltage measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7.6 Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

7.7 Load monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

7.8 Error detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

7.9 Tamper detection module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

7.9.1 Detailed operational description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

7.10 Phase compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

7.11 Clock generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

7.11.1 RC startup procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

7.12 Resetting the STPM10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

7.13 Using the STPM10 in microcontroller-based meters . . . . . . . . . . . . . . . . 24

2/51 Doc ID 17728 Rev 3

STPM10 Contents

7.14 Energy to frequency conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

7.15 Status bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

7.16 Programming the STPM10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

7.16.1 Data records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

7.17 Configuration bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

7.18 Mode signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

7.19 SPI interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

7.20 Remote reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

7.21 Reading data records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

7.22 Writing procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

7.22.1 Interfacing the standard 3-wire SPI with the STPM10 SPI . . . . . . . . . . 36

7.23 Energy calculation algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

7.23.1 Active power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

7.23.2 Reactive power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.23.3 Apparent power and RMS values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7.24 STPM10 calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

8 Application design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

9 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Doc ID 17728 Rev 3 3/51

List of tables STPM10

List of tables

Table 1. Device summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Table 2. Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 3. Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Table 4. Thermal data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Table 5. Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Table 6. Gain of voltage and current channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table 7. Configuration of current sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table 8. No-load detection thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Table 9. LED pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Table 10. Status bit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 11. Configuration bit map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Table 12. Mode signal description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Table 13. Working point settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Table 14. Device constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Table 15. Resistor divider ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Table 16. Current channel typical components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Table 17. Footprint data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Table 18. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4/51 Doc ID 17728 Rev 3

STPM10 List of figures

List of figures

Figure 1. Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 2. Pin connections (top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 3. Supply current vs. supply voltage, T
Figure 4. RC oscillator frequency vs. V

CC

Figure 5. RC oscillator: frequency jitter vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 6. Analog voltage regulator: line - load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 7. Digital voltage regulator: line - load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 8. Voltage channel linearity at different V
Figure 9. Power supply AC rejection vs. V
Figure 10. Power supply DC rejection vs. V

Figure 11. Error over dynamic range gain dependence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 12. Primary current channel linearity at different V

Figure 13. Gain response of ΔΣ A/D converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 14. First-order ΣΔ A/D converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 15. ZCR signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 16. LIN and BFR signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 17. Band-gap temperature variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 18. Timings of tamper module - Primary channel selected. . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 19. Timings of tamper module - Secondary channel selected . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 20. Different oscillator circuits with (a) quartz, (b) internal oscillator, (c) external source . . . . . 23

Figure 21. STPM10 data records map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 22. Timing for providing remote reset request . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Figure 23. Data record reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 24. Timing for data record reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 25. Timing for writing configuration and mode bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 26. Active energy computation diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 27. STPM10 reference schematic with one current transformer and one shunt. . . . . . . . . . . . 45

Figure 28. TSSOP20 footprint recommended data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

= 25 °C (f = 4.194 MHz, 8.192 MHz). . . . . . . . . . . . 13

A

, R = 12 kΩ, TA = 25 °C . . . . . . . . . . . . . . . . . . . . . . . . . . 13

voltages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

CC

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

CC

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

CC

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

CC

Doc ID 17728 Rev 3 5/51

Schematic diagram STPM10

1 Schematic diagram

Figure 1. Block diagram

VCC

VIP

VIN

IIN1

IIP2

IIN2

VSS

VDDA VDDD

Voltage Ref

+

4

-

Gain: 8-32

IIPI

+

-

Gain: 8-32

ΔΣ A/D

Tamper

Linear Vregs

ΔΣ A/D

φ

VO

56-bit

Configurators

DSP

Energies

and

RMS Values

Energy to Freq

Converters

LED

WDG

+

-

Registers

Oscillator

and

SPI Interface

SDA/TDSYNSCSSCL/NCCLKOUTCLKIN

ZCR

AM00176v1

6/51 Doc ID 17728 Rev 3

STPM10 Pin configuration

2 Pin configuration

Figure 2. Pin connections (top view)

Table 2. Pin description

Pin Symbol Type

(1)

1 WDG D O Watchdog

2 ZCR D O Zero-crossing signal

3 SCS D IN SPI interface Enable pin

4V

5V

6V

7V

8V

9I

10 I

11 I

12 I

13 V

14 V

DDD

SS

CC

O

DDA

IP1

IN1

IP2

IN2

IP

IN

A OUT 1.8 V output of internal low drop regulator which supplies the digital core

GND Ground

P IN Supply voltage

P OUT Output of internal low drop regulator

A OUT 3 V output of internal low drop regulator which supplies the analog part

A IN Positive input of primary current channel

A IN Negative input of primary current channel

A IN Positive input of secondary current channel

A IN Negative input of secondary current channel

A IN Positive input of voltage channel

A IN Negative input of voltage channel

15 SYN D I/O SPI interface pin

Description

16 CLKIN A IN Crystal oscillator input

17 CLKOUT A OUT Crystal oscillator output

18 SCL D I/O SPI interface clock pin

19 SDA D I/O SPI interface data pin

20 LED D O Active energy pulsed output

1. A: analog, D: digital, P: power

Doc ID 17728 Rev 3 7/51

Maximum ratings STPM10

3 Maximum ratings

Table 3. Absolute maximum ratings

Symbol Parameter Value Unit

V

I

V

V

CC

PIN

ID

IA

DC input voltage -0.3 to 6 V

Current on any pin (sink/source) ± 150 mA

Input voltage at digital pins (SCS, ZCR, WDG, SYN, SDA,
SCL, LED)

Input voltage at analog pins (I

IP1

, I

, I

, I

IN1

, VIP, VIN) -0.7 to 0.7 V

IP2

IN2

-0.3 to V

+ 0.3 V

CC

ESD Human body model (all pins) ± 3.5 kV

T

T

OP

T

J

STG

Operating ambient temperature - 40 to 85 °C

Junction temperature - 40 to 150 °C

Storage temperature range - 55 to 150 °C

Note: Absolute maximum ratings are those values beyond which damage to the device may occur.

Functional operation under these condition is not implied.

Table 4. Thermal data

Symbol Parameter Value Unit

R

thJA

1. This value is based on a single-layer PCB, JEDEC standard test board.

Thermal resistance junction-to-ambient 114.5

(1)

°C/W

8/51 Doc ID 17728 Rev 3

STPM10 Electrical characteristics

4 Electrical characteristics

VCC = 5 V, TA = 25 °C,100 nF to 1 µF between V
and V

Table 5. Electrical characteristics

, 100 nF to 1 µF between VCC and VSS unless otherwise specified.

SS

and VSS, 100 nF to 1 µF between V

DDA

Symbol Parameter Test conditions Min. Typ. Max. Unit

Energy measurement accuracy

f

e

e

e

Effective bandwidth Limited by digital filtering (-3 dB) 4 800 Hz

BW

Accuracy of active power Over 1 to 1000 of dynamic range 0.1 %

AW

Accuracy of reactive power Over 1 to 1000 of dynamic range 0.1 %

RW

Accuracy of apparent power Over 1 to 500 of dynamic range 0.1 %

SW

SNR Signal-to-noise ratio Over the entire bandwidth 52 db

PSRR

Power supply DC rejection

DC

Voltage signal: 200 mV
Current signal: 10 mV
f

= 4.194 MHz

CLK

rms

rms

/50Hz

/50Hz

0.2 %

VCC=3.3V±10%, 5V±10%

PSRR

Power supply AC rejection

AC

Voltage signal: 200 mV
Current signal: 10 mV
f

= 4.194 MHz

CLK

=3.3 V+0.2 V

V

CC

VCC=5.0 V+0.2 V

rms

rms

/50 Hz

rms

/50 Hz

rms

1@100 Hz
1@100 Hz

0.1 %

General section

DDD

V

I

Operating supply voltage 3.165 5.5 V

CC

Supply current. Configuration

CC

registers cleared

POR Power on reset on V

V

V

f

CLK

f

LINE

I

LATCH

Analog inputs (I

V

f

ADC

f

SPL

Analog supply voltage 2.85 3.00 3.15 V

DDA

Digital supply Voltage 1.725 1.80 1.875 V

DDD

Oscillator clock frequency

Nominal line frequency 45 65 Hz

Current injection latch-up
immunity

, I

IP1

IN1

Maximum input signal levels

MAX

A/D converter bandwidth 10 kHz

A/D sampling frequency F

4 MHz; V

8 MHz; V

CC

=5 V 3 4 mA

CC

=5 V 5 6 mA

CC

2.5 V

MDIV bit=0 4.000 4.194 MHz

MDIV bit=1 8.000 8.192 MHz

300 mA

, I

, I

, VIP, VIN)

IP2

IN2

Voltage channel -0.3 +0.3 V

Gain 8X -0.15 +0.15 V

Current channels:

Gain 32X -0.035 +0.035

/4 Hz

CLK

Doc ID 17728 Rev 3 9/51

Electrical characteristics STPM10

Table 5. Electrical characteristics (continued)

Symbol Parameter Test conditions Min. Typ. Max. Unit

V

Z

Z

G

I

Amplifier offset ±20 mV

OFF

VIP, VIN impedance

IP

I

, I

, I

, I

IN

IP1

IN1

Current channels gain error ±10 %

ERR

Voltage channel leakage

VL

current

IP2

impedance

IN2

Over the total operating voltage
range

Over the total operating voltage
range

Channel disabled (PST=0 to 1

Current channel leakage

I

IL

current

CH2 disabled if CSEL=0; CH1
disabled if CSEL=1) or device off

Input enabled -10 10 µA

Digital I/O Characteristics (SDA, CLKIN, CLKOUT, SCS, SYN, LED)

SDA, SCS, SYN, LED 0.75V

V

Input high voltage

IH

CLKIN 1.5

SDA, SCL, SYN, LED 0.25V

V

V

V

I

t

Input low voltage

IL

Output high voltage IO=-2 mA VCC-0.4 V

OH

Output low voltage IO=+2 mA 0.4 V

OL

Pull up current 15 µA

UP

Transition time C

TR

CLKIN 0.8

=50 pF 10 ns

LOAD

Crystal oscillator (see circuit Figure 19)

100 400 kΩ

100 kΩ

-1 1 µA

-1 1 µA

CC

CC

V

V

I

Input current on CLKIN 1 µA

I

R

External resistor 1 4 MΩ

P

Cp External capacitors 22 pF

4.00 4.194

f

CLK

I

CLKIN

R

t

Nominal output frequency

8.00 8.192

Settling current f

Settling resistor 12 kΩ

SET

Frequency jitter 1 ns

JIT

= 4 MHz 40 60 µA

CLK

On chip reference voltage

Reference voltage 1.23 V

V

REF

Reference accuracy ±1 %

Temperature coefficient After calibration 30 50

T

C

SPI interface timing

F

SCLKr

Data read speed After calibration 32 MHz

10/51 Doc ID 17728 Rev 3

MHz

ppm/

°C

STPM10 Electrical characteristics

Table 5. Electrical characteristics (continued)

Symbol Parameter Test conditions Min. Typ. Max. Unit

F

SCLKw

t

t

DH

t

ON

t

OFF

t

SYN

Data write speed 100 kHz

Data setup time 20 ns

DS

Data hold time 0 ns

Data driver on time 20 ns

Data driver off time 20 ns

SYN active width 2/f

CLK

s

Doc ID 17728 Rev 3 11/51

Terminology STPM10

5 Terminology

5.1 Measurement error

The error associated with the energy measurement made by the STPM10 is defined as:
Percentage error = [STPM10 (reading) - true energy] / true energy

5.2 ADC offset error

This is the error due to the DC component associated with the analog inputs of the A/D
converters. Due to the internal automatic DC offset cancellation, the STPM10 measurement
is not affected by DC components in the voltage and current channel. The DC offset
cancellation is implemented in the DSP.

5.3 Gain error

The gain error is gain due to the signal channel gain amplifiers. This is the difference
between the measured ADC code and the ideal output code. The difference is expressed as
percentage of the ideal code.

5.4 Power supply DC and AC rejection

This parameter quantifies the STPM10 measurement error as a percentage of the reading
when the power supplies are varied. For the PSRRAC measurement, a reading at two
nominal supply voltages (3.3 and 5 V) is taken. A second reading is obtained with the same
input signal levels when an AC (200 mV
Any error introduced by this AC signal is expressed as a percentage of the reading. For the
PSRRDC measurement, a reading at two nominal supply voltages (3.3 and 5 V) is taken. A
second reading is obtained with the same input signal levels when the supplies are varied ±
10%. Any error introduced is again expressed as a percentage of the reading.

/100 Hz) signal is introduced on the supplies.

RMS

5.5 Conventions

The lowest analog and digital power supply voltage is called VSS, which represents system
ground (GND). All voltage specifications for digital input/output pins are referred to GND.

Positive currents flow into a pin. Sinking current refers to the current flowing into the pin, and
thus it is positive. Sourcing current means that the current is flowing out of the pin, so it is
negative.

Timing specifications of signals treated by the digital control part are relative to CLKOUT.
This signal is provided by the 4.194 MHz nominal-frequency crystal oscillator or from the
internal RC oscillator. An external source of 4.194 MHz or 8.192 MHz can also be used.

Timing specifications of signals from the SPI interface are relative to the SCL, and there is
no direct relationship between the clock (SCL) of the SPI interface and the clock of the DSP
block. A positive logic convention is used in all equations.

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STPM10 Typical performance characteristics

6 Typical performance characteristics

Figure 3. Supply current vs. supply voltage,

T

= 25 °C (f = 4.194MHz, 8.192MHz)

A

Figure 5. RC oscillator: frequency jitter vs.

temperature

Figure 4. RC oscillator frequency vs. V

R = 12 kΩ, T

= 25 °C

A

CC

,

Figure 6. Analog voltage regulator: line - load

regulation

Figure 7. Digital voltage regulator: line - load

regulation

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Figure 8. Voltage channel linearity at

different V

voltages

CC

Typical performance characteristics STPM10

Figure 9. Power supply AC rejection vs. V

CC

Figure 11. Error over dynamic range gain

dependence

Figure 10. Power supply DC rejection vs. V

CC

Figure 12. Primary current channel linearity at

different V

CC

Figure 13. Gain response of ΔΣ A/D converters

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STPM10 Theory of operation

7 Theory of operation

7.1 General operation description

The STPM10 is capable of performing measurements of active, reactive and apparent
energy, RMS and instantaneous voltage and current values, and line frequency information.

Most of the functions are fully programmable using internal configuration bits accessible
through the SPI interface. The STPM10 works as a peripheral in microcontroller-based
metering systems. The ZCR and WDG pins are used to provide zero-crossing and watch­dog information, and the SPI pins are used to communicate with the microcontroller.

The STPM10 includes volatile internal registers that hold the useful information for the
metering system. Two kinds of active energy are available: wide-band active energy (AW)
which includes all harmonic content (also called type 0) and fundamental active energy
(AF), limited to the 1st harmonic (also called type 1). This latter energy value is obtained by
filtering type 0 active energy. Both the two active energies are stored in up-down counting
accumulator registers with a 20-bit length. Reactive and apparent energies are also
available with a 20-bit accumulation.

The STPM10 also provides the RMS values for voltage and current. Due to the modest
dynamic variation of the voltage, the RMS value is stored with a resolution of 11 bits, while
the RMS current value has a resolution of 16 bits. The instantaneous (momentary) sampled
value of voltage and current are also available with a resolution of 11 and 16 bits,
respectively. The line frequency value is stored with a resolution of 14 bits.

Due to the proprietary energy computation algorithm, the STPM10 calibration is quick and
simple, allowing calibration at only one point over the entire current range.

The configuration and calibration parameters must be downloaded in the internal non­volatile memory of STPM10 at power-up.

7.2 Analog inputs

Input amplifiers

The STPM10 has one fully differential voltage input channel and two fully differential current
input channels.

The voltage channel consists of a differential amplifier with a gain of 4. The maximum
differential input voltage for the voltage channel is ± 0.3 V.

The two current channels are multiplexed (see Chapter 7.9 for details) to provide a single
input to a preamplifier with a gain of 4. The output of this preamplifier is connected to the
input of a programmable gain amplifier (PGA) with possible gain selections of 2 and 8. The
total gain of the current channels are then 8 and 32. The gain selections are made by writing
to the gain register, and they can be different for the two current channels. If the tamper
function is not used, the secondary current can be disabled.

The maximum differential input voltage is dependent on the selected gain, in accordance
with Ta bl e 6 .

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Theory of operation STPM10

Table 6. Gain of voltage and current channels

Voltage channels Current channels

Gain Max input voltage (V) Gain Max input voltage (V)

4 ±0.30

8X ±0.15

32X ±0.035

The gain register is included in the device configuration register with the address name
PST. The table below shows the gain configuration according to the register values:

Table 7. Configuration of current sensors

Primary Secondary

Gain Sensor Gain Sensor PST TMP

8 CT Disabled Disabled 0 0

32 Shunt Disabled Disabled 1 0

8

CT

8 32 Shunt 1 1

8CT0 1

Configuration

Bits

Configuration

Bits

Note: If the device is used in configuration PST = 1, TMP = 1 (primary channel with CT, secondary

channel with Shunt), the shunt Ks must always be equal to one fourth of the current
transformer Ks.

Both the voltage and current channels implement an active offset correction architecture
which provides the benefit of avoiding any offset compensation.The analog voltage and
current signals are processed by the Σ Δ analog-to-digital converters, which feed the hard­wired DSP. The DSP implements an automatic digital offset cancellation that makes it
possible to avoid any manual offset calibration on the analog inputs.

7.3 ΣΔ A/D converters

Analog-to-digital conversion in the STPM10 is carried out using two first-order Σ Δ
converters. The device performs A/D conversions of analog signals on two independent
channels in parallel. The current channel is multiplexed as a primary or secondary current
channel in order to perform the tamper function, if enabled. The converted Σ Δ signals are
supplied to the internal hard-wired DSP unit, which filters and integrates these signals in
order to boost the resolution and to yield all the necessary signals for the computations.

A Σ Δ modulator converts the input signal into a continuous serial stream of 1’s and 0’s at a
rate determined by the sampling clock. In the STPM10, the sampling clock is equal to
f

/4.

CLK

The 1-bit DAC in the feedback loop is driven by the serial data stream. The DAC output is
subtracted from the input signal. If the loop gain is high enough, the average value of the
DAC output (and therefore the bit stream) can approach that of the input signal level. When
a large number of samples are averaged, a very precise value for the analog signal is
obtained. This averaging is carried out in the DSP section, which implements decimation,

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