Instructions

UM-0085-B09 DT80 Range User Manual Page 303
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The terminal specifier can be either:
• nothing : measure between + and – terminals,
•
* : measure between * and # terminals,
•
+ : measure between + and # terminals,
•
- : measure between – and # terminals, or
•
# : measure between # and AGND/EXT# terminals
For example, the command 3+V causes the DT80 to measure the voltage between the + and # terminals on analog
input 3, while
1R means measure the voltage between + and – on analog input 1 (which will then be used to calculate
the unknown resistance, provided it is connected as shown in the wiring diagrams).
A particular channel type will not necessarily support all of the above terminal specifiers. For example, the
# specifier is
only available for current-based channel types.
Voltage
Voltage (difference in electrical potential) is the fundamental quantity that is measured during any analog measurement.
The DT80 measures a voltage by:
1. connecting the required input terminals to the instrumentation amplifier by closing the appropriate input relays
and selecting input attenuators if required;
2. converting the amplified signal to a frequency using a precision Voltage Controlled Oscillator (VCO);
3. digitally measuring the frequency and calculating the resultant voltage reading;
4. possibly repeating steps 2 and 3 using a different amplifier gain setting if required.
Note: Because all analog measurements (including current, resistance, thermocouple, etc.) are fundamentally voltage measurements,
much of the information in this section is applicable to them, too.
Channel Types
The following channel types will directly return a voltage value:
Channel Type
Description
Units
V
voltage, attenuators disabled by default (max input 3V)
mV
HV
voltage, attenuators enabled by default (max input 30V, max input 50V for S4)
V
For example, to measure the voltage between the * and # terminals on analog input 2 you would include the channel
definition 2*V in your DT80 program (or, to make a single "immediate" measurement, simply send 2*V to the DT80's
command interface, e.g. by typing it into the DeTransfer send window).
Channel Options
The following channel options are commonly used when measuring voltages:
•
GL30MV, GL300MV, GL3V, and GL50V (GL30V in Series 3) (gain lock) allow the DT80's input gain to be
locked on a particular range. See Gain Ranges and Attenuators below.
•
A (attenuator) and NA (no attenuator; default) allow the DT80's 10:1 (16:1 in Series 4) input attenuators to be
switched in or out. See Gain Ranges and Attenuators below.
• ESn (extra samples) specifies that n additional measurements should be taken and the result averaged. This
can help with very noisy signals.
• a channel factor (a floating point number) can be specified as a simple scaling factor. The measured voltage
will be multiplied by this factor before being returned. For more advanced scaling options, see Manipulating
Data (P65).
Gain Ranges and Attenuators
The DT80's instrumentation amplifier has three switchable gain settings. These give three basic voltage measurement
ranges (30mV, 300mV and 3V full scale).
The analog inputs also include switchable 16:1 (10:1 in Series 3) attenuator, which effectively provide a fourth range
(50V) (30V in Series 3).
By default the appropriate gain range is selected automatically. The first time a channel is measured, the DT80 will select
the highest input range (3V if attenuators are not enabled, 30V if they are). If the reading is close to zero then up to two
additional measurements will be made on progressively lower input ranges.
For subsequent measurements of the same channel, the DT80 will initially use the same input range as was used
previously. If the reading is over range or close to zero then the input range will be adjusted up or down respectively and
the measurement repeated.