STPMS1

Theory of operation

Doc ID 16524 Rev 2

15/23

8

Theory of operation

8.1

General operation description

The STPMS1 performs first-order analog modulation of signals which have frequencies

varying from DC to 2 kHz on two independent channels in parallel. There is a current

channel for measuring line current and a voltage channel for measuring line voltage. The

outputs of the converters provide two streams of digital ones and zeros which are therefore

multiplexed in time to reduce the number of external connections.

The sampling and the data multiplexing are driven by an external clock signal, as it is used

to strobe the analog inputs. The combination of one or more STPMS1s and an STPMC1

(which implements the digital filtering) constitutes a conversion system for energy metering

applications.

The STPMS1 can also be used along with a DSP programmed to demultiplex the output

bitstream and to implement the digital filtering as a medium resolution ADC system.

When used in energy metering applications, the voltage channel is connected externally and

differentially to a line voltage divider which provides an analog signal proportional to the

voltage u. The current channel is connected to a Rogowski coil, or to a current transformer

(CT) or a shunt, which are used to interface the line current. The Rogowski coil provides an

analog signal proportional to di/dt, while the shunt or CT provides an analog signal

proportional to the current i. A CT differs from a shunt in sensitivity and phase error. There

should be an anti-aliasing LP filter inserted between the sensors and the inputs of both

channels of the STPMS1.

Internally, the differential voltage input related to the voltage channel is connected directly to

the A/D converter, which implies an amplification of x4. On the other side, the differential

voltage input related to the current channel is connected first to a configurable x2 or x8 pre-

amplifier and the output of this pre-amplifier to the similar A/D converter (x4 gain), which

implies selectable pre-amplification of x8 or x32 and uses the same reference voltage.

A pair of digital inputs (MS0 and MS1) is used to configure the device.

8.2

Function description of the analog part

band-gap, and bias generators.

The analog part consists of several modules:

●

Band-gap reference and bias generators

●

+3 V low-drop regulator

●

two DC buffer amplifiers

●

two

ΣΔ AD converters

●

control signal module