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GENERAL INFORMATION
IF' ! ' H ' ,
OVERVIEW
The HP 98640A 7-channel Analog Input Interface is an analog-to-digital converter (ADC) for
Hewlett~Packard
9000 Series 200 computers. On each HP 98640A 8 analog channels are available
for measuring small differential voltages with 13-bits resolution (including sign-bit). We call the HP
98640A a 117 -channel" interface because in applications where an input offset voltage is less tolerable
the two leads of one channel may be shorted together and grounded. The reading from the shorted
channel serves as a reference you can use to correct readings made on the other channels.
The HP 98640A can sample its inputs in any order. You can control the amount of time between
readings by programmatically setting the interface's pace timer. The interface will then do the pacing
automatically. It can send an interrupt to the computer when a sample h'as been taken. Even the
voltage range for a channel can be changed between samples. The maximum voltage for inputs is plus
or minus 10 volts. The minimum full-scale voltage range is plus or minus 19.5 millivolts. The maxi-
mum differential voltage measurable is plus or minus 9.99756 volts (best case) at a gain of 1 (full scale
voltage minus one least significant bit (Isb) value).
The interface's four input voltage ranges will accommodate signal sources ranging from control cir-
cuits to thermocouples. The voltage ranges are determined by the amplification of the input voltage
by the card. Y--ou can select any of 4 gains for any channel. The four gain factors are 1, 8, 64, and
512. This allows the ADC integrated circuit (IC) on the card to always see a voltage between 0 and 10
volts.
The rest of this section describes the interface and includes the operational specifications.
DESCRIPTION
The HP 98640A is actually two printed circuit assemblies (PCA). The main PCA is the A-to-D card
which includes the analog-to-digital converter, programmable gain amplifier (PGA), pace timer, and
backplane interface circuitry. Unlike most interface cards for 9000 Series 200 computers, the cover
plate is not attached to the main card. Instead, the plate is is attached to the second PCA.
The second peA plus the attached cover plate is called the wire termination assembly (WTA). On this
PCA are the wire termination blocks, input protection circuitry, and the control inputs for external
pacing. The wire termination blocks accept individual solid hook -up wires up to 18 gauge. Each wire
is clamped in a termination block's receptacle by turning a setscrew. The cover plate attached to the
PCA has two captive thumbscrews (dog bolts) to secure the entire assembly to the back of the com-
puter. The main PCA should never be operated unless a WTA or test assembly is attached.
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