DT80 Range Analog Inputs
From dataTaker Wiki (FAQ)
What does 'Universal' or 'Versatile' mean in regard to the dataTaker analog inputs?
A basic concept of the dataTaker data logger is that analog inputs can be used with more than one type of sensor, without needing additional input modules. The dataTaker analog channel is a group of 4 terminals that can be used in many combinations.
On the DT80 we have 5 (DT82=2, DT85=16), 4 wire (4 terminal) channels that allow sensors and signals based on 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.
Each of these channels can be connected to between 1 and 3 sensors or signals, depending on the requirements of that sensor or signal type. Refer to the user manual for detailed connection information.
What is the fastest sample speed?
You can get up to 25 Hz on a single channel by holding the relay closed (P62=1) and turn off the auto-calibration (/k). Remember if you turn off the auto-cal you will get temperature drift, you might only want to do this somewhere the ambient temperature isn't going to vary.
The 25Hz is the total number of samples rather than per channel. You could get five sensors, five times a second for example.
You will get a little bit more speed out of the logger if you increase the mains rejection (P11). The mains rejection reduces the sample time so don't go too far, say to a maximum of 200, and remember we have mains rejection for a reason, without it you might get some noise into your system.
How many analog inputs can I use on the DT80 range?
DT80: From 5 to 15 depending on the input or sensor type (early build DT80 is 4 to 12).
DT81: From 2 to 6 depending on the input or sensor type.
DT85: From 16 to 48 depending on the input or sensor type.
The universal analog inputs used on the DT80 range allow from 1 to 3 inputs per 4 wire ‘channel’. Refer to the user manual for detailed connection information.
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Example: 1 only RTD input per 4 wire dataTaker universal channel. |
Example: Up to 3 Voltage based inputs per 4 wire dataTaker universal channel. |
Example: 1 full bridge strain gauge input per 4 wire dataTaker universal channel. |
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Can I expand the DT80 range loggers?
Yes, the Series 2 & 3 DT80 & DT85 can be expanded using the CEM20 expansion module. Product Details A maximum of 5 modules can be connected to the DT80 Series 2 and a maximum of 16 modules can be connected to the DT85 Series 2. CEM20 FAQ page
For the earlier Series 1 DT80 & DT85 no dedicated expansion unit is available, however it is possible to use 3rd party Mux units connected to the Analog inputs or I/O modules connected to the Serial Sensor Channel.
How many voltage inputs can I use?
DT80: Up to 15 voltage based inputs can be used. These will be arranged in 5 groups of three. Each group of three must share a common 0V. Alternatively, unshared inputs can be used for a reduced number of inputs.
DT82: Up to 6 voltage based inputs can be used.
DT85: Up to 48 voltage based inputs can be used. Refer to the user manual for detailed connection information
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Example: Up to 3 inputs per 4 wire dataTaker universal channel. |
Example: 2 inputs per 4 wire dataTaker universal channel (common mode voltage range applies). |
Example: 1 input per 4 wire dataTaker universal channel. |
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How many thermocouple inputs can I use?
DT80: Up to 15 thermocouple inputs can be used.
DT82: Up to 6 thermocouple inputs can be used.
DT85: Up to 48 thermocouple inputs can be used. The DT85 also supports up to 32 fully isolated thermocouple inputs using its dual isolated channel capability.
The connections used are the same as other voltage based inputs. Refer to the user manual for detailed connection information
How many RTD inputs can I use?
DT80: Up to 5 RTD inputs can be used.
DT82: 2 RTD input only can be used.
DT85 Up to 16 RTD inputs can be used.
Connection is 1 RTD per 4 wire dataTaker universal channel. Refer to the user manual for detailed connection information
How many 4-20mA inputs can I use?
DT80: Up to 15 current loop or 4-20mA inputs can be used. A maximum of 5 inputs using the internal current shunt, or 15 using external current shunts.
DT82: Up to 6 current loop or 4-20mA inputs can be used. A maximum of 1 input using the internal current shunt, or 3 using external current shunts.
DT85: Up to 48 current loop or 4-20mA inputs can be used. A maximum of 16 inputs using the internal current shunt, or 48 using external current shunts.
See question on 2 wire transducers.
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Example: 4-20mA connection using internal shunt resistor. |
Example: 4-20mA connection using external shunt resistor. |
Example: 3 x 4-20mA connection using external shunt resistor. |
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Refer to the user manual for detailed connection information.
Can I use 2 wire 4-20mA transducer inputs?
Yes, the DT8x includes support for current loop inputs using the internal current shunt and an external power supply. Refer to the user manual for detailed connection information.
Example: 2 wire 4-20mA connection using switched external excitation.
NOTE: Switched external excitation is only provided during measurement, use MD channel option to allow transducer warm up time. Not suitable for current loops that must be continuously powered or require a complete loop at all times.
Can I use 4-20mA inputs that include HART?
Although not specifically tested by Datataker, feedback from several users of HART based transducers is positive. We advise that the internal 100ohm shunt not be used and external shunts of at least 250ohms are used. This is the only specific issue we are aware of when using HART on circuits connected to the DT80 range logger products.
What is meant by ‘switchable current shunt’?
The internal current shunt is not permanently in circuit. It is only connected during an actual current measurement. For this reason it should not be used if the external circuit requires a complete loop at all times.
Switching the shunt out of circuit is part of the design for interchannel isolation.
What is meant by ‘interchannel isolation’?
Because we have used relay multiplexors rather than solid state versions, there is complete isolation between each dataTaker 4 wire input channel.
DT80 example: sensors connected to channel 1 are isolated from sensors connected to channel 2. Note that external wiring practices can affect the practical level of isolation obtained. e.g. Shared wire connections with connection of AGND to an external reference.
NOTE: The DT85 and the DT80 Series 2 & 3 loggers have different characteristics related to isolation. They use "dual channel" isolation technology to allow two isolated two wire inputs on the same channel as well as disconnecting AGND (EXT#) when not in use. This allows up to 32 fully isolated 2 wire sensors to be connected on a DT85(G) and 10 for a DT80(G) Series 2 & 3.
Are the analog inputs isolated?
Yes, the analog inputs are isolated. Relays provide interchannel isolation (for example between channels 1 & 2) and the analog system is isolated from the Communications/Digital/Power section. Because the analog section is isolated from the rest of the DT80/81, sensor-to-equipment ground current loops are unlikely to occur.
NOTE: Refer to the user manual for information on the use of the AGND terminal and effects on isolation.
NOTE: The DT85 has different characteristics related to isolation. It can isolated two wire inputs on the same channel as well as disconnecting AGND (EXT#) when not in use.
What is the input voltage range?
Overall range is ±30Vdc, however the default range for most measurement types is <±3Vdc.
The higher ±30Vdc range is only available if the internal attenuation is on. By default all channel types except HV (high voltage) and AS (analog state) channel types have attenuation off.
Note: Firmware versions prior to Version 6.18 also turned on attenuators for the L (current loop) channel type as well.
See question on attenuation for additional information.
What is the maximum input over Voltage?
The analog inputs are protected to +/- 35 VDC
How does the auto ranging work?
The DT8x analog inputs can operate with 3 different levels of amplifier gain. By default the most appropriate amplifier gain is selected automatically.
This allows small signals to be amplified for measurement, improving the effective resolution of that measurement. The auto ranging operates over 3 ranges that are dependant on the attenuation mode;
Attenuation off: 30mV – 300mV – 3V
Attenuation on: 300mV – 3V – 30V
See question on attenuation for additional information. Auto-ranging can be disabled by using Gain Lock channel options.
Do I still need external attenuators?
If your voltage input is over 30V then you will need external attenuation.
What are the internal attenuators?
The DT80 range loggers includes internal 10:1 voltage attenuators. These are switched in or out of circuit as appropriate for the channel type, or by channel option. Available voltage input ranges are;
Attenuation off: 30mV – 300mV – 3V
Attenuation on: 300mV – 3V – 30V
By default all channel types except HV (high voltage) and AS (analog state) channel types have attenuation off.
Note: Firmware versions prior to Version 6.18 also turned on attenuators for the L (current loop) channel type as well.
Can I turn the internal attenuators on or off?
Yes you can. To switch the attenuators in use the A channel option To switch the attenuators out use the NA channel option
What is the DT80 range Series Input Impedance ?
When making a HV - High Voltage measurement the attenuators are switched into the input circuit path. The channel input impedance is 110k ohms (ie. 100k, 10k) voltage divider.
When making a V - measurement the input circuit path goes directly to the inputs of an instrumentation amplifier. The channel input impedance is >100M ohms
Can I use a common power supply for the DT80 range logger and Switched External Excitation?
Yes, as long as the rating is sufficient to cater for both logger and sensor requirements. Note that only 1 sensor will be excited at any one time through this method.
Use of a common power supply may impact on isolation of the Analog sub section. Care must be taken to avoid ground loops or unintended current loops.
How do I allow for sensor warm up before measurement?
If using the Switched External Excitation, by setting the MD channel option to the delay required for each individual sensor input.
This is very useful with 2 wire 4-20mA transducers using Switched External Excitation.
If using external power switched via the Latching Relay, this will depend on your program but may require a DELAY channel or PAUSE command within the schedule in place of the MD (Measurement Delay) channel option.
Where has the P10 setting for ADC settling time gone in the programming code?
There is no global parameter setting for ADC settling time in the DT80 range. This has been replaced by the MD channel option, allowing different settings for each channel.
Default measurement delay (settling time) is 10mS.
What is the setting range for the MD channel option?
Measurement Delay of 0-30,000mS can be set using the MD channel option.
What is the rating of the Switched External Excitation circuit?
Nominally maximum of 30VDC, 50mA.
How do the DT80G and DT85G deal with long lead lengths and noise when measuring vibrating wire sensors?
The DT80G and DT85G use three main methods for dealing with long leads and noise on vibrating wire based sensors.
- Long lead length are supported by two main attributes of the loggers circuitry: excitation strength and signal amplification. The strength of the excitation signal determines how far it can travel before being ineffective. Once excited, the output of the sensor must travel back along the same wires. If the output signal is not large enough then the signal will be lost. The DT80G/DT85G data loggers include a stonger excitation signal to allow it to travel longer distances as well as a higher gain amplifier to allow it to pick up weaker output signals from sensors. The actual distances that can be achieved depends mainly on the cable resistance, lower resistance cable allows longer distances. Distances of over 1km have been achieved.
- Noise filtering improves the ability to measure the correct signal, by removing unwanted noise influences. The DT80G/85G include noise filtering circuits to remove noise outside of the measurement range.
- A phase lock loop (PLL) circuit is used in the DT80G/DT85G to allow the logger to lock-on to the sensors output frequency, which then excludes other unwanted noise and harmonics of the output frequency. The PLL works by locking onto the largest amplitute frequency, which is generally the sensors output frequency.
The combination of the above techniques makes for a very reliable VW sensor measurement circuit that can measure VW sensors on long leads
What is the difference between calibration and characterisation?
- Characterisation happens during product manufacturing. Its purpose is to remove variations in component values or characteristics due to manufacturing tolerances.
- Calibration happens during normal use of the product (runtime). Its purpose is to remove variations in component values or characteristics when the ambient temperature changes. Its common for most components (resistors, capacitors, amplifiers, etc.) to drift when the ambient temperature changes. This is known as the components Temperature Co-efficient (TC)
The principal components of the DT80 analog section are:
- input amplifier
- A to D converter, comprising a voltage controlled oscillator (VCO)
- precision 2.5V voltage reference
- various precision resistors for gain, attenuation and current measurement
- precision current sources for excitation
What is Characterised on the DT80 range
- precision 2.5V reference voltage
- precision 100R reference resistance
- amplifier gain (nominally x1, x10 and x100)
- high input voltage attentuators
The actual values for the above characteristics are recorded using NATA certified test equipment and then stored permanently into the DT80 range logger memory and allows the firmware to correct for these variations.
What is Calibrated on the DT80 range loggers
- amplifier gain & offset voltage (using Analog Ground and 2.5V reference)
- precision current sources for excitation
- shunt resistor for current measurement
If the DT80 range logger is to return accurate readings over the full temperature range then these characteristics must be corrected. This cannot be done in the factory; it must be done during operation, as the temperature goes up and down. This process is known as "calibration" (not to be confused with characterisation).
When & Why of Calibration
On power-up and at regular intervals during operation, the DT80 range logger measures the amplifier offset voltage. If it is found to have drifted by a specified amount (3uV by default) then a calibration cycle is performed, as follows:
- The offset (zero) voltages on various amplifier gain settings are measured, as is the voltage reference. From these measurements, the overall slope and offset for the amplifer+VCO subsystem is calculated, and these form the basis of all subsequent voltage measurements.
- A number of other offset (zero) voltages are then measured, for the various possible input configurations.
- The actual current source output is then determined using the characterised value of a precision resistor.
- Various other measurements are taken (eg. relating to attenuators or current shunt) and correction offsets and factors are calculated.
The above set of calibration parameters are then used for all subsequent analog measurements, until the next calibration cycle occurs.
To summarise, the DT80 range loggers have certain temperature-stable characteristics, whose absolute values are measured and recorded during manufacture. This process corrects for any differences between individual units. Using these characterised values, further measurements are taken during operation to correct for differences between the current temperature and the temperature at which the factory characterisation was performed.
