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AD2S105

Three-Phase Current Conditioner

Analog Devices 

AD2S105

Output Analog Signals

There are two sets of analog output from the AD2S105.

Sin/Cos orthogonal output signals are derived from the Clark/

three-to-two-phase conversion before the Park angle rotation.

These signals are available on Pin 25 (Cos ␪) and Pin 26 (Sin

␪), and occur before Park angle rotation.

Two-Phase (Sin (␪ + ␾), Cos (␪ + ␾)) Signals

These represent the output of the coordinate transformation.

These signals are available on Pin 6 (PH/OP4, Sin (␪ + ␾)) and

Pin 7 (PH/OP1, Cos (␪ + ␾)).

HOMOPOLAR OUTPUT

HOMOPOLAR Reference

In a three-phase ac system, the sum of the three inputs to the

converter can be used to indicate whether or not the phases are

balanced.

If VSUM = PH/IP1 + PH/IP2 + PH/IP3 (or PH/IPH1 + PH/

IPH2 + PH/IPH3) this can be rewritten as VSUM = [Cos␪, +

Cos (␪ + 120°) + Cos (␪ + 240°)] = 0. Any imbalances in the

line will cause the sum VSUM ≠ 0. The AD2S105 homopolar

output (HPOP) goes high when VSUM > 3 × Vts. The voltage

level at which the HPOP indicates an imbalance is determined

by the HPREF threshold, Vts. This is set internally at ± 0.5 V dc

(± 0.1 V dc). The HPOP goes high when

V ts

<

(Cosθ

+

Cos(θ

+ 120° ) +

3

Cos(θ

+ 240° ))

×V

where V is the nominal input voltage.

With no external components VSUM must exceed ± 1.5 V dc in

order for HPOP to indicate an imbalance. The sensitivity of the

threshold can be reduced by connecting an external resistor be-

tween HPOP and ground in Figure 5 where

REXT is in Ω.

V ts

=

0. 5

REXT

REXT

+ 20,000

HOMOPOLAR

REFERENCE

25µA

TO TRIGGER

Example: From the equivalent circuit, it can be seen that the in-

clusion of a 20 kΩ resistor will reduce Vts to ± 0.25 V dc. This

corresponds to an imbalance of ± 0.75 V dc in the inputs.

Homopolar Filtering

The equation VSUM = Cos␪ + Cos (␪ + 120°) + Cos (␪ + 240°)

= 0 denotes an imbalance when VSUM ≠ 0. There are conditions,

however, when an actual imbalance will occur and the condi-

tions as defined by VSUM will be valid. For example, if the first

phase was open circuit when ␪ = 90° or 270°, the first phase is

valid at 0 V dc. VSUM is valid, therefore, when Cos␪ is close to 0.

In order to detect an imbalance ␪ has to move away from 90° or

270°, i.e., when on a balanced line Cos ␪ ≠ 0.

Line imbalance is detected as a function of HPREF, either set

by the user or internally set at ± 0.5 V dc. This corresponds to a

dead zone when ␾ = 90° or 270° ± 30°, i.e., VSUM = 0, and,

therefore, no indicated imbalance. If an external 20 kΩ resistor

is added, this halves Vts and reduces the zone to ± 15°. Note this

example only applies if the first phase is detached.

In order to prevent this false triggering an external capacitor

needs to be placed from HPFILT to ground, as shown in Figure

5. This averages out the perceived imbalance over a complete

cycle and will prevent the HPOP from alternatively indicating

balance and imbalance over ␪ = 0° to 360°.

For

dθ = 1000 rpm

dt

CEXT

= 220 nF

dθ = 100 rpm

dt

CEXT

= 2.2 µF

Note: The slower the input rotational speed, the larger the time

constant required over which to average the HPOP output. Use

of the homopolar output at slow rotational speeds becomes im-

practical with respect to the increased value for CEXT.

34

DGND

1

AD2S105

TOP VIEW

23

HPFILT

HPOP

HPREF

CEXT

220nF

HPOP

HPREF

EXTERNAL

RESISTOR

20kΩ

Figure 5. The Equivalent Homopolar Reference Input

Circuitry

12

REXT

GND

Figure 6. AD2S105 Homopolar Output Connections

REV. 0

–7–

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