IM CA71-E
4-7
Source
4
4.3
Sourcing Resistance or RTD Signal
• The calibrator sources a resistance signal by 1) receiving the resis-
tance-measuring current I supplied from the device being calibrated,
such as a resistance meter or RTD thermometer, and 2) delivering the
voltage V = R
×
I proportional to the preset resistance R between the
output terminals, and 3) thus producing the equivalent resistance R =
V/I. Consequently, the calibrator sources the signal correctly only for
such devices that employ this method of measurement.
• The allowable range of the resistance measuring current I that the cali-
brator receives from a resistance measuring device under calibration is
rated as 0.1 to 5 mA. Note, however, that accuracy lowers for resis-
tance measuring currents smaller than 0.5 mA. For further details, see
Chapter 12, “Specifications.”
• Any resistance signal being sourced does not include the resistance
component of the lead cables for source. The calibrator is adjusted so
that the signal has a resistance value as viewed from the output termi-
nals. The whole resistance, when measured at the ends of the lead
cables for source, is given by adding the resistance of the lead cables
themselves (approximately 0.1
Ω
on a round-trip basis) to the sourced
resistance signal. For source of precise resistance signals, use three-
wire connection.
• If capacitance between the terminals of a device under calibration is
greater than 0.1
µ
F, the calibrator may fail to source correct resistance
signals.
◆
Output Method Based on Three-wire Connection
Attach another lead cable to the L output terminal, as shown in the
following figure. The output is provided through the three wires, H, L
and L’. Connect these three wires to the device being calibrated.
CA71
SOURCE
Three-wire measuring equipment
H
H
L
L'
L
Three-wire Connection for Resistance Signal Source
4.3 Sourcing Resistance or RTD Signal
