Specifications

D1500 Data Sheet D-5
This value is within the ±30mV offset necessary to provide enough head-
room for the strain gage bridge.
Trim out the remaining offset with the Trim Zero (TZ) command:
Command: $1WE
Response: *
Command: $1TZ+00000.00
Response: *
The bridge is now trimmed to zero. Verify:
Command: $1RD
Response: * +00000.00
The Trim Zero (TZ) command may be used at any time to balance out offsets
due to temperature, residual stress, tare, etc.
Excitation
D1500 modules may be ordered with either 5V or 10V excitation. Maximum
excitation current available is 60mA. Modules with 10V excitation may be
used with bridges that have input impedances of 166 ohms or greater. Half-
bridges of 120 Ω strain gages may be used with 10V excitation if the bridge
is completed with 350 Ω resistors. Modules with 5V excitation will source
bridges of 85 Ω and up.
The actual excitation voltage may vary ±0.5V from the nominal values of
+10V and +5V. However, the module’s internal microprocessor constantly
monitors the actual excitation voltage and provides compensation for any
deviation from the nominal value. This results in a constant data output for
a constant bridge load even if the excitation changes. From a user’s point
of view, the excitation voltage will appear to be exactly +10V or +5V.
Calibration
Since the D1500 modules use a ratiometric technique to compensate for
variances in the excitation voltage, special consideration is required to
properly calibrate the unit. Figure 3 shows the calibration setup. The Digital
Voltmeter (DVM) must be capable of measuring the excitation voltage to 4
digit accuracy. The voltage source must be able to provide millivolt signals
accurate to ±5 microvolts. The resistive divider may be constructed from 1%
resistors of equal value from 100 to 1000 Ω. The resistor divider places the
voltage source in the center of the common-mode range of the input
amplifier for best accuracy.