Instructions

UM-0085-B09 DT80 Range User Manual Page 23

The exact function of each terminal varies depending on how the channel is programmed. In general terms:

• The * ("Excite") terminal can be a voltage input (relative to # terminal), or it can provide sensor excitation (for

example, for resistance measurement) See Sensor Excitation (P24)

• The + ("Plus") terminal is a voltage input (relative to – or # terminal)

• The – ("Minus") terminal is a voltage input (relative to # terminal)

• The # ("Return") terminal is normally used as a common or return terminal. It can also be used as a current

input, using the DT80's internal shunt resistor.

Figure 3: Analog input channel terminal labels

Multiplexers

The DT80's analog input channels are multiplexed. The required input terminals are first connected to the input of the

DT80's instrumentation amplifier and analog to digital converter, then a measurement is taken. The next channel to be

sampled is then switched through to the amplifier and ADC, and so on. Simultaneous sampling of analog channels is not

possible.

Channel definition commands in the DT80 program determine which terminals are used for a particular measurement.

For example, the channel definition

1+V measures the voltage between the + and # terminals on channel 1.

Gain Ranges and Attenuators

The DT80's instrumentation amplifier has three switchable gain settings. These give three basic voltage measurement

ranges (3V, 300mV and 30mV full scale)

The DT80’s default is for its instrumentation amplifier to automatically change gain range to suit the input signal applied

to it by the multiplexers.

If the amplitude of your input signals are known, then the gain can be set manually. Do this by applying the

GLx (gain

lock) channel option, which disables autoranging for that channel and sets the gain to a fixed range.

The analog inputs also include switchable 16:1 (10:1 for Series 3 or older) attenuators, which effectively provide a fourth

range.

Warning: Maximum input voltage on any analog input is ±50V dc (±30V for Series 3 or older), relative to the AGND/EXT# terminal. If

this is exceeded then permanent damage may occur.

Analog Input Configurations

The basic quantity that the DT80 measures is voltage. Voltages can be measured using two different input

configurations:

• shared-terminal analog inputs

• independent analog inputs

 Shared-Terminal Analog Inputs

Sometimes called "single-ended" inputs, a shared-terminal input is one that shares one or more of its terminals with

another input. In Figure 4, the three sensors share channel 1’s # terminal. Each of the three inputs is a shared-terminal

input.

Figure 4: Shared-terminal voltage inputs sharing a channel’s # terminal (voltage inputs used as example)