AD7555_15 Datasheet, PDF (9/12 Pages) Analog Devices – Digit ADC Subsystem

LOGIC AND DISPLAY CIRCUITRY
MODIFYING THE FULL SCALE DISPLAY
8
The AD7555 possesses 41/2 digit accuracy with potential for
5 1/2 digit resolution. Figure 6b shows the logic and display
Availability of the SCO and SCI terminals on the AD7555
provides flexibility for range-switching and modified data-
circuitry when operating the AD7555 with this high resolution. ^{format applications.}
For example, in the 5 1/2 digit mode, inserting a + 5 counter
between SCO and SCI provides a full scale count at SCI of
39,999 counts (199,999+5).
VCC' +5V
I _{VCC = +5V}
O _{+5V SUPPLY}
BS^{DGND RETURN}
NOTE 4
8 ^{OL AD7555}
ETE ^IC'
Vss
-5V
'2
11
IC2 10
7447 9
15
8 ¹⁴
TO
DGND
R7
W'o
R8
'"
R9
RIO
R11
R12
R13
R7-R13
200"
I
VCC = +5V
11
NOTES.
NOTE 4
, FOR 4112 DIGIT MODE HREN IPIN 81 = lOGIC LOW
FOR 5 1/2 DIGIT MODE HREN (PIN 81 = lOGIC HIGH.
2. SEE ClK PIN DESCRIPTION (PAGE 4) FOR LINE
REJECTION INFORMATION.
3.06 AND DSO ARE NOT REOUIRED FOR 4 1/2 DIGIT
OPERATION.
sec
4. CALIBRATION CIRCUITRY SHOWN IN FIGURE 7.
fOLK = '.O24MH,
TO +15V
02
IN914
Figure 6b. Logic and Display Circuitry (for 5 1/2 Digit Resolution)
CALIBRATING THE AD7555
When the AD7555 is placed in the calibrate mode, any re-
sulting error voltage in Vs (summing junction voltage), due
to drift, etc., will be contained in the resulting display. To
display the error SC1 and seo must be taken HIGH (only
allowable when DAVis HIGH). In the calibrate mode the
display indicates +16.0480::!:n(+16.04800 ::!:nin 5 1/2 digit
mode) where 16indicates a blanked digit and n is a number
represen ting the reference inpu t errors. This gives the change
required in VREF2 (::!:1:1VREFf2o)r proper calibration (ie.,
n "" 0). The exact relationship between nand 1:1VREF2 can
be shown to be equal to:
(VREFl )n
1:1VREF2 = 40,960 + n ^{(4 1/2 digit operation)}
8
1:1VREF2 = 4⁽0^V,^R9^E6^F0¹⁾+ ^II^OOⁿn ^{(5 1/2 digit operation)}
Figure 7 shows the hardware connections for manual cali-
bration. With the switch in the calibrate mode, adjust VREF2
(potentiometer R5 as shown in Figure 6a) until the display
reads +16.0480(+160.04800 in 5 1/2 digit mode). The AD7555
is now calibrated to the center of its error correcting range.
Return the switch to normal to resume normal conversion.
+5V
AD7555
SCO
sct
DAV
AIN
R3
CAL
,3 DAV
DAV
AIN
II 'NORM _+5V
AIN
: L- VRm
I CAl
For this capability to operate, IVREF21 must be 1/2 VREFl ::!:2%.
Figure 7. Hardware Requirements for Manual
Calibration of n = 0
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