Analog Data Capture
Firstly what is the difference between an analog and a digital signal?
Clearly a digital signal consists of zero's (say 0 Volt) and one's (say 5 Volt).On one wire then, this is the type of signal that feeds a modem on the modulate side,before it's fed out to the telephone network.
Still on one wire ,an analog signal between zero and five volts, can have any value between zero and five volts and every value counts.
Sounds are analog signals,they are signals that have different frequencies for the different sounds. A sin wave is an analog signal.
Analog and Digital signals can be converted into Digital and Analog signals respectively.
To build a computer controlled instrument you have to be able to read and write digital and analog data to and from the instrument , before you can display the result of the experiment on the screen.The way Analog data is read is to use an Analog Digital Converter or ADC.The digital result can then be processed and stored for later recall.This usually uses 2 pins on a device input ,namely earth and the analog signal.Low frequency digital signals are read also on 2 pins ,again earth and the digital signal this last pin is also called a port.High frequency digital signals up to 5 MHz can be resolved by a counter,and read.
Analog data between 0 and 5Volt can also be written to the instrument on a single port.
Through Visual Basic(VB) software one is able to control the dialogue and thus control the instrument with the PC.
To enable us to create dialogue we us the USB6008. TheUSB6008 is a "Data Aquisition Terminal" for the USB Port
manufactured by National Instruments here in Midrand and you have to buy your own one.!
It may be that your case isn't necessarily to build an entire instrument like a Voltammeter or Direct Pulse Potentiometer or Chromatography but instead just a case of logging data,X-Y data or just 2 channels of data.
In the good old days your first choice would have been a mechanical X-Y Plotter.With the relatively recent advent of good 8 pin ADC chips it became possible to produce small portable Automatic Intelligent Motionless(AIM) X-Y or X-t Plotters.These would be computer or micro-controller based and the user has to reconstruct the graph mechanically from more or less points captured in RAM by the computer or micro-controller driven Plotter.We have built and will supply such an AIM Plotter.Using an "Atmel" Japanese Micro-controller plus a keyboard plus a 4 line LCD and a few switches,potentiometers and Input jacks, we have all the hardware required to put together an effective AIM Plotter.
This is a 2 (simultaneous) channel (capturing 2X500 points of 12 bit data) menu driven device.There are 4 menu items:
1)Set the timing.A range of alternatives can be scrolled through from 500 samples per second(sps) .(this limit can be increased with some loss in accuracy to 10,000sps .At the moment we are programming the Atmel chip for a max of approximately 500sps.) to 1 every 12 hours.
2)Scroll through the 2X500 points in RAM.From a couple of critical points you can reconstruct the graph.
3)Scroll through the Peak Detector.There is a peak detector algorithm in software.If Guassian peaks are produced on a smooth background the peak detector will report on their occurence.
4)Direct Input metering-allows for live monitoring of the signal which makes it possible to get eveything on scale in a pre-run etc.
There are divider potentiometers on the inputs which allow for a signal of up to 100V to be divided till it makes the scale of 4.1V f.s.d. of ADC.
Calibration is possible with a switch on the panel which provides a a choice of the sample or a 4.00V calibrate signal .You can measure the calibrate signal directly ,bypassing some internal error in chip (MAX187 (ADC)) and other variable in electronics manufacture for an accurate signal.Of course this measurement should be made early on since the divisors must be set full on i.e. zero division.Then you can divide your signal and measure the calibrate signal again on menu option 4) Direct Input Metering.Surprise or not, not both Channels are the same,slight differences exist.
On the back the AIM Plotter has inputs for X and Y and in addition it has a SYNC. input. The SYNC. signal is T.T.L. that is it has an active high level of 5V.This signal ,provided externally, offers an alternative to the key press and can be used to electronically synchronize a run ,by setting it off @SYNC.=5V.Faster work generally requires electronic synchronisation.
The AIM Plotter is small and can be made to power by 2X9V batteries or Mains or both.
Using the USB6008 8 simultaneous channels of data can be brought but now you need a PC. The PC has Video and Printer available to compile reports in software and completes the USB6008 in a special way.The USB6008 has several other features which make for good computer control.Components are the key. You need to have components of the machine you want to build. You can even count a fifth of a microsecond with the USB6008.Fast experiments outstripping the power of a mechanical plotter are possible. Top conversion speeds of 10000 sps are available. The USB6009 is faster.
If your requirement is to produce a PC computer controlled instrument we can design and supply both the electronic hardware and VB software. The AIM Plotter is useful and cost effective if you want to replace a mechanical X-Y or X-t Plotter. Also it may be a more simple case when conditioning of the input signal is unnecessary and direct connection to the USB6008 is possible ,then all you need for a working system is VB software, we do all this!