Analog Input Modules

	Isolated DAQ Module WTX4 Up to 16 differential input channels programmable to read voltages and convert them to direct engineering units of choice. 20-bit A/D resolution and input range that spans -8V to +10V with protection to ±40 volts. Can also include a combination of other I/O types. read more...
	Analog Input Card WTAIN-M 4 differential input channels programmable to read voltages and convert them to direct engineering units of choice. 20-bit A/D resolution and input range that spans -8V to +10V with protection to ±40 volts. read more...

 

Analog Input Definition

The acquiring and processing of data using a computer is a vital requirement of any industrial process or control operation. However, a computer only recognizes binary digital numbers and most sensors that acquire data by converting physical characteristics such as temperature, pressure, load, weight, etc. into an electrical signal, do so in an analog nature. In other words, the output of the sensor is some form of voltage or current which is not suitable for a computer to input or process. An analog input circuit, or A to D converter, is needed to change these analog voltages to a digital stream which can then be transferred into a computer via the communications port such as USB or RS232. The Analog Input modules shown here have all the necessary A/D circuitry for signal conversions including many advanced features not seen in most DAQ products on the market. For instance, the calibration of the voltage gain and offset is user programmable so that the output can be scaled to match the sensor and the type of information that the sensor is meant to represent.

Stackable Capability

Stackable refers to the ability to connect multiple units together and share a single communications port of a host PC, laptop, or Single Board Computer (SBC). The DAQ modules are offered in two different footprints as shown above, enclosed in a case with full electrical isolation, or a non-isolated bare OEM circuit board. Both of these versions are stackable and can be mixed together if desired. The communications bus uses a stratagem based on the Carrier Sense Multiple Access (CSMA/CD) protocol. Carrier Sense (CS) is the monitoring of the data bus for a period of inactivity before a DAQ module is allowed to begin its own transmission. Multiple Access (MA) means that once the bus is free, every DAQ module in the network has an equal opportunity to transmit a frame. And Collision Detection (CD) uses non destructive bit wise arbitration to preserve the integrity of a data frame when two or more DAQ modules try to transmit at the exact same time. And since the data frame that wins arbitration remains intact during a collision, there is no additional communications delay when a collision occurs no matter how often it happens.

Host Communications

To communicate with the DAQ modules, commands are sent to the individual units by including a header character at the beginning (the address) so that it can be routed to the appropriate unit. If using multiple units connected together, the DIP switch for each unit should be set to a different position so that it will be assigned a different header character. Each DAQ module has its own command set used to configure it, operate the functions, and to read data from it. These commands are listed in the product data sheets. A typical command string looks like this:

HCNV{cr}

H = Header Character
C = Command Character
N = Channel Number (if applicable)
V = Value (if applicable)
{cr} = Carriage Return

If using our ModCom HMI software, the carriage return is not necessary because ModCom automatically inserts this at the end of each command string. ModCom has a communications dialog box which can be used to transmit individual commands to the DAQ modules and see the data coming back. This is helpful in learning the style of the command-set protocol and testing the hardware that's attached to the DAQ module. It is highly recommended that this testing be done prior to setting up any complex control programs.

Calibration Mode & Range

Each of the analog input channels of the DAQ modules shown above can be set up to use 1 of 5 different calibration modes and input voltage ranges. For each input channel there are three factory calibrated modes which will return the results listed in voltage, and two user programmable custom modes which will return the results listed in any direct linear engineering units of choice. To calibrate a custom mode, first set the zero point, then either apply a known stimulus and tell the module what number you want to see on the computer screen, or tell the module what exact voltage you want to represent one unit of measurement. The gain and offset for this particular input channel will be automatically re-calibrated to meet this criteria. For more information, see "User Programmable Modes" shown below.

Input Channel Scanning

During normal operation, the Analog Input module continuously scans all channels in the background regardless of whether or not they are being polled by the host. The full 20-bit A/D results of each channel is stored in its own circular buffer which will always hold the most recent 8 samples. At any time a READ command is issued, the 8 samples associated with that particular channel are average together and the results will be the data which is returned to the host. This 8 times averaging smoothes out the readings and reduces the jitter shown on the computer screen.

User Programmable Modes

The Analog Input modules incorporate a 32-bit floating point math routine which provides data conversions using calibration coefficients stored in non volatile memory. Each of the modes selected with the MODE command has its own set of calibration coefficients. There are two modes which can be programmed by the user to display the results of each READ command in the linear engineering units preferred. The span of the internal A/D converter is extended across the voltage range of each mode and determines the maximum resolution available. Mode 4 has a range of -8V to +10V and a resolution of 22µV. Mode 5 has a range of -0.6V to +0.6V and a resolution of 1.4µV. Each mode can be individually programmed using one of two methods, SPAN or FACTOR.

SPAN can be used if a known output reading can be generated by the sensor. Sensor manufacturers often include a shunt resistor for this purpose and when attached to the sensor's bridge, will cause it to output a voltage which represents a specific reading. Applying a known load or stimulus to a sensor also works well. After setting up the sensor so that it outputs the known reading, transmit the SPAN command with the value field containing the reading that should be displayed on the computer screen. Multiplying the value by multiples of 10 will increase the resolution. For example, if applying a 50-lbs load to a pressure transducer, transmitting a value of 5000 in the SPAN command will set up the channel to display its readings in 0.01 lbs increments. The DECIMAL command can then be used to set up the channel to automatically insert a decimal point in the correct position of each reading if desirable.

FACTOR can be used if the actual voltage that equals one unit of measurement can be determined. To calculate this voltage in mV, use the factory listed output of the bridge sensor in the following equation:

value = Out * E / FS where:

Out = Output of the sensor in mV/V.
E = Excitation voltage applied to bridge.
FS = Full scale capacity of the sensor.

For best results, the excitation voltage should be measured directly at the bridge using the same analog input channel and lead wires that will be attached to the bridge output itself. Transmit the FACTOR command with the value field containing the results of this equation. Dividing the value by multiples of 10 will increase the resolution displayed on the computer screen in the same way as multiplying the value used in the SPAN command, as discussed earlier. The FACTOR command can also be used to set the display resolution so that it equals 1-bit of the internal A/D converter. This can be accomplished by using a FACTOR value of 0.022mV for Mode 4 and 0.0014mV for Mode 5.

Input Voltage Range

Each of the input channels of the Analog Input module has a common mode range of ±10V regardless of the MODE it is set to or the magnitude of the differential voltage it is measuring. Both positive and negative inputs must remain inside this range at all times or conversion errors will result. If connecting to a bridge sensor for instance, the excitation voltage applied to the bridge must be less than 20V in order to keep the inputs within this common mode range. An excitation of 5V to 15V is recommended for proper operation.

4-20 mA Current Transmitters

To read the data from a 4-20 mA current transmitter, place a 100-ohm resistor in the current loop and use an input channel to read the voltage across the resistor as shown in the product data sheet. The results returned to the host will be from 400mV to 2000mV representing the 4mA to 20mA current range. A user programmable mode can be used to automatically convert this voltage reading to any linear engineering units of choice and include a decimal point if desirable.