Valhalla 2300, Operation Manual

Page: 107 / 115

APPLICATION NOTE AN107
MINIMIZING ERROR SOURCES IN THREE-WIRE CONNECTED
DIGITAL POWER ANALYZERS
107-1. General
This application note describes the sources of
measurement error in three-wire type
digital
power analyzers. Methods for reducing these
error sources are also discussed.
107-2. 3-Wire Digital Power Analyzers
The term "3-wire digital power analyzer"
refers to the number of terminals the analyzer
uses to connect to the load under test.
Referring to Figure 1, the power analyzer
measures the current flowing through the
current shunt and the voltage between the
current shunt and voltage common terminal.
The voltage measurement and current
measurement sections of the analyzer share a
common terminal.
107-3. Error Sources
Measurement errors arise in three-wire type
digital power analyzers due to connecting lead
resistances. A power analyzer hook-up
showing error sources is shown in Figure 2.
The contact and lead resistances are combined
and shown as "R LEAD x", with the x
denoting the individual lead. Lead resistances
1 and 2 do not contribute any error other than
lowering the effective source voltage. Lead
resistances 3 and 4 are sources of error. The
power analyzer measures the voltage drop
across the load plus the lead resistances. The
power measurement will be in error by the
sum of the lead (I
2
R) powers.
107-4. Error Reduction
The errors caused by the lead resistances can
be reduced by a factor of 2 by simply
reconnecting the voltage common terminal of
the power analyzer as shown in Figure 3. The
effect of "R LEAD 3" is eliminated. The
effect of "R LEAD 4" can be reduced by
using a short heavy gauge wire and ensuring
tight connections. In applications where it is
impractical to shorten lead 4, an alternate
technique is discussed below.
A measurement is first made using the
connections of Figure 3. The connections are
then changed to those of Figure 4. This
connection measures the power in lead 4.
This power is then subtracted from the first
measurement yielding an accurate
representation of the power in the load only.