Measurements on the DT80 range

Measurements

What can the DT80 Measure?

Analog

Using its analog inputs, the DT80 can directly measure the following:

  • DC voltage (30mV, 300mV, 3V and 30V ranges)
  • DC current (0.3mA, 3mA and 30mA ranges)
  • frequency (0.1 to 10,000 Hz)

Many other quantities can be measured by connecting appropriate sensors which convert a physical quantity into something that the DT80 can measure. The DT80 directly supports:

  • 4-20mA current loop sensors (0 to 100%)
  • temperature sensors (thermocouples, RTDs, thermistors, IC sensors)
  • bridges and strain gauges
  • vibrating wire strain gauges (DT80G/85G only)

This list can be extended by means of user specified scaling calculations.

Digital

The DT80’s digital and counter channels allow the measurement of:

  • digital input state (contact closure or TTL logic)
  • pulse count (32 bit)
  • phase encoder position (32 bit)

Serial

Various “smart sensors” can also be read:

  • SDI-12 (Serial Data Interface – 1200 baud) based sensor networks
  • other serial sensor devices with an RS232/422/485 interface, such as weighing machines, barcode scanners
  • CAN (Controller Area Network) sensors, using the optional dataTaker CANgate CAN to ASCII gateway device. The use of this product is outside the scope of this manual.

Analog Channels

    Analog Terminal Strip

    Figure 2: DT80 analog terminals

Input Terminals

The DT80 provides five analog input channels, numbered 1 to 5. Depending on the wiring configuration used, these allow between 5 and 15 separate voltages to be measured. The DT81 has one analog input channel, allowing 1-3 separate voltages to be measured, and the DT82E has two (2-6 separate voltage measurements). Finally, the DT85 has 16 analog input channels, allowing 16-48 separate voltage measurements.

Each analog input channel on a DT80 is a 4-wire connection (see Figure 3) that allows voltage, current, resistance and frequency to be measured. These are the fundamental signals output by most sensors. It is not necessary to use all four terminals on each channel— two are often adequate.

    DT80 terminal block

Figure 3: Analog input channel terminal labels

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 in the User Manual
  • 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.

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.

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.

The analog inputs also include switchable 10:1 attenuators, which effectively provide a fourth range (30V).

Warning Maximum input voltage on any analog input is ±35V dc, 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.

    DT80 Shared-terminal voltage inputs

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

In a shared-terminal configuration, a sensor’s "return" or "negative" wire is usually connected to the channel's # terminal. The remaining sensor wire (the "positive" or "signal") is connected to any of the channel’s other three terminals. The common terminal need not be at ground potential – all voltage measurements (shared or unshared) are differential, i.e. only the difference in voltage between the two terminals is reported.

For shared-terminal inputs, the channel number is given a suffix indicating the terminal to which the positive wire is connected. For example, a shared-terminal voltage input applied to channel 1 between the + and # terminals is recognized by the channel definition 1+V.

Independent Analog Inputs

An independent input (also known as an "unshared" input) is one that connects to its own terminals and does not share any of those terminals with any other inputs. For example, in Figure 5, sensor A is connected to channel 1’s + and – terminals, and sensor B is connected to the other two terminals of the channel. In other words, each sensor’s terminals are independent of the other’s — no terminal is used by both sensors.

    DT80 independent voltage inputs

Figure 5 Wiring one or two independent inputs to a single channel (voltage inputs used as example)

Note that each analog input channel can support two independent voltage inputs. In the above example, the channel definition 1V will read sensor A while 1*V will read sensor B. The channel definition syntax is fully described in the Channels section of the DT8x Users Manual

Sensor Excitation

Many sensors require excitation (electrical energy) so that they can provide an output signal. For example, to read the temperature of a thermistor, excitation current is passed through the thermistor to generate a voltage drop that can be measured.

The DT80 can provide

  • Voltage source of approx. 4.5V via 1kO. Useful for powering some sensors however the supply is not regulated and consequently liable to drift with temperature
  • 200µA (approx.) current source. Default excitation for resistance measurement. Very stable over environmental temperature range.
  • 2.5mA (approx.) current source. Default excitation for RTD and bridge measurement. Very stable over environmental temperature range.
  • User supplied external excitation EXT* terminal. The user can provide an external excitation which is appropriate to the sensor being used. (The DT80 Series 2 and DT85 provide two general purpose DC power outputs which may be connected to the EXT* terminal to provide external excitation if required.)

Further information is available in the Excitation category in the DT80 Channel Options table of the DT8x Users Manual

More Information

For full details on how to connect sensors and make measurements using the DT80's analog inputs, see Analog Channels section of the DT8x Users Manual.

Digital Channels

    Digital Terminal Strip

Figure 6: DT80 digital terminals

The DT80 provides:

  • 4 bidirectional digital I/O channels (1D-4D) with open drain output driver and pull-up resistor (DT81/82E: 3 channels, 1D-3D)
  • 4 bidirectional digital I/O channels (5D-8D) with tri-stateable output driver and weak pull-down resistor. These channels may also be used for controlling intelligent sensors using the SDI-12 protocol (DT81/82E: 1 channel, 4D)
  • 1 voltage free latching relay contact output (RELAY)
  • 1 LED output (Attn)
  • 4 hardware counter inputs (1C-4C) which can be used as independent counter channels or as two quadrature (phase encoder) inputs (DT81: one phase encoder input, shared with inputs 3C and 4C. There are no phase encoder inputs on the DT82E.)

As with analog channels, channel definition commands are used to specify which digital inputs are to be measured and/or what digital output states are to be set. For example, the command 1DS will read the digital state (0 or 1) on channel 1D, while 3DSO=0 will set channel 3D low.

A transition on a digital channel can be used to trigger a schedule. This allows a series of measurements to be made (or commands executed) in response to a change in digital state.

The DT80 can count the number of pulses received on any digital input. The four dedicated counter inputs provide additional capabilities:

  • a higher maximum count rate
  • the ability to keep counting even if the logger is in low-power "sleep" mode
  • optional low-level (5mV) input threshold levels
  • optional decoding of phase-encoded input signals

Further information is available in the Digital Channels section of the DT8x Users Manual

Serial Channels

The DT80 supports two main classes of "smart sensor":

  • A wide range of sensors, particularly in the environmental monitoring field, use the SDI-12 protocol. The DT80 fully supports this protocol making it a simple process to read measured values. See the SDI-12 Channel section of the DT8x User Manual
  • The DT80 also provides a generic serial channel.

The serial channel allows a wide variety of sensors and devices to be controlled and polled. The serial channel:

  • can use the dedicated serial sensor port (not DT81/82E), the host RS232 port, and/or the USB port
  • supports USB, RS232, RS422 and RS485 signal levels (depending on the port)
  • supports point-to-point or multi-drop operation (point-to-point only for the host/USB port)
  • features programmable output (poll) strings and a variety of options for parsing returned data
  • can trigger execution of a schedule in response to received data

For more details, see the Generic Serial Channel section of the DT8x Users Manual