A Short Note on What
Happens in a GPIB Voltmeter.
When you convert data in a
GPIB voltmeter, there are some interesting things that happen. In the process
the data takes on many forms, and the way the data takes on those forms and is
processed needs to be understood so that you can understand meter accuracy.
Step 1 - Data
Conversion
The first step in
using a GPIB voltmeter is to do the conversion. The input from a voltage source
being measured first goes to an A/D converter. That converter will produce an
integer that is
representative of the voltage being converted. In the figure below, the analog
input voltage (shown as a narrow line) produces an integer (and since that could
be 8 bits, or 12, bits, etc., we show that with a thick line).
if you want to see a simulator for an A/D converter.
Step 2 - Compute the
Voltage from the Count Value
The integer produced
by the A/D depends upon the input voltage, but it is not usually the case that
the integer is equal to the voltage input to the A/D. You need to compute the
actual voltage - and that is going to be a floating
point number. The voltmeter will need to have some
sort of computational capability to perform the computation. That's going to be
something like this:
Vcalc =
Lower Limit + Count*(Higher Limit - Lower Limit)/(2#Bits)
Example
If the voltmeter is really
an oscilloscope with 8 bits, and the lower limit is -4v with an upper limit of
+4 volts, you would get
Vcalc = -4
+ Count*8/(256)
That is the computation that would
be done on an oscilloscope with an 8 bit A/D, set on 1v/Div, when there are 8
divisions (4 up and 4 down from the center of the scope).
Step 3 - Generate an
ASCII String
In many cases, GPIB
instruments send data to a computer as an ASCII string, so there has to be some
further data manipulation to complete the process and get the data to the
computer. The entire operation would look something like that shown in the
figure below.
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