+
Sense
-
+
Therm
-
VW-108
Thermistor
Sensor Connection Diagram
Connection to an analogue data acquisition system

The following details show how to configure and optimise the VibWire-108 analogue outputs to operate with a analogue input data acquisition system or logger unit.


Technical Specifications - Analogue Output Ports

8  x  0 - 2.5V DC single analogue output ports - 16 bit DAC
8 x   thermistor outputs - 3.3 KOhm completion resistors


Theory of Operation

The VW-108 can be connected to an external data acquisition system or data logger using the analogue output port fitted onto the instrument. In order that the correct values can be interpreted by the logger/acquisition system they are first scaled into a suitable analogue signal by the VW-108 before being passed on for measurement. Each output channel can be uniquely configured to support any manufactures sensor.

When defining the operation of the analogue output each channel has to have the sensor operating characteristics defined. For the VW-108 this means that the minimum operating frequency and span are set into the instrument.

Once the operating frequencies for the sensor are assigned the instrument scales the measured sensor frequency
over the range 0V = minimum frequency and 2.5V = maximum frequency.  

Low Frequency := 500 - 10000Hz  defined in 100 Hz intervals
Range                 := 100 Hz steps.
 0V
- Vin
+Vin
+2.5V
 0V
- Vin
+Vin
+2.5V
 0V
- Vin
+Vin
+2.5V
       +
Sense
        -
       +
Therm
        -
       +
Sense
        -
       +
Therm
        -
The VibWire-108 contains 8 independently configurable analogue output ports and they are used to represent the output signal from the sensor.

Each analogue output is of the range   0 - 2.5V DC and any analogue output must scale a result to within this range Care should be taken to ensure that the output signal is scaled as close as possible to sensor range with the minimum overlap in order to obtain the highest resolution..  

For  example, Channel 0 is used to output a signal from a sensor with operating range of 1452 - 3176 Hz

0V = 1400 Hz   and 2.5V = 3200 Hz            

 so  

CH0 LF = 1400   and    CH0 RA = 3200 - 1400 = 1800 Hz             

will give the highest resolution for this example

DAC Resolution output port = 16 Bit      

so Frequency Resolution = 1800 / 65536 = 0.03 Hz

in practice accuracy of around 0.5 Hz can be achieved when connecting the VW-108 to an analogue data acquisition system  after allowing for the losses due to the Digital-analogue and Analogue-digital conversion process. The 0.5 Hz measurement accuracy is achieved using the Keynes Controls NDACS loggers.


Only when operating the VibWire-108 with an active analogue output port need the operating characteristics for the vibrating wire sensor be defined.

For general purpose operations the analogue output should be set to represent the full operating range of the sensor.