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98640A Analog Input Interface
Dealing with Data
NOTE
Any reading that returns a I as the busy bit is invalid;
the address provided in such a reading is not latched into
the address buffers on the A-to-O card. You can ensure
valid readings by checking the busy bit (bit 15 of the
data word) or the not busy bit (bit 6 of the status word)
and making your analog read only when the card is not
busy.
The value returned in the data word is a raw reading produced by the AOC chip and the absolute
value circuit on the A -to-O card. The data word's 13 least significant bits (sign and magnitude) are
not adjusted for gain or input offset voltage. The data in the 13 bits is in signed binary form; the most
signific~nt
of the 13 bits reperesents the sign (polarity) of the voltage the A-to-O card measured,
while the remaining 12 least significant bits (lsbs) are a magnitude, not a directly readable voltage.
We call these 1 3 bits the raw reading.
To change a raw reading into a voltage you could multiply the number the 12 lsbs represent by the
least significant bit value for the gain you used. Polarity is determined from the sign bit. That will
work, but the accuracy of the voltage obtained may not be suitable for your needs. You must decide,
based on experiments, if an uncorrected voltage reading is accurate enough for your needs.
One of the major factors affecting accuracy is noise. Later in this section we will devote several para-
graphs to dealing with noise. Another major factor affecting accuracy is input offset voltages. We
will deal with input offset voltages here.
CALIBRAT][ON
Most of the A-to-O card's input offset voltage is caused by operational amplifiers (op amps) on the
card. You ca.n correct for most of these offsets by "calibrating" the card.
In order to calibrate the A -to-O card you will need to use one channel as a reference channel. Any
channel can serve as your reference; just short the channel's two inputs together and tie them to
ground.
Below we explain two algorithms you might use to calibrate the card. However, you can save con-
siderable development time if you use the calibration subroutine contained in the HP 98645A
Measurement Library.
Calibrating the A -to-O card requires measuring the input offset voltage. Simply reading the
reference channel provides a gross offset value which may be acceptable in many applications. Tables
4-1 and 4-2, and the explanation of the quick calibration scheme may help you design your own
quick calibration subroutine. If you need to write a subroutine offering greater accuracy, see the
requirements for a detailed calibration. In either case, also see the discussion under the heading
IINoise" in this section.
In our explanations of calibration we use names for variables, such as Named_Variable. Since these
variable namles are to clarify procedures, no attempt was made to limit a name's length.
4-3
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