AD578/AD579
CLOCK RATE CONTROL
J GRADE
K,T GRADES
The internal clock is preset to a nominal conversion time of 5.6 µs
(AD578) or 4.8 µs (AD579). It can be adjusted for either faster
or slower conversion rates. For faster conversions, connect the
appropriate 1% resistor between Pins 17 and 18 and short Pin 18
to Pin 19 (see Figures 6, 7, and 8).
2.2s
CONVERSION
RATE
1.8s
CONVERSION
RATE
649⍀
422⍀
1%
1%
For slower conversions (AD578 only), connect a capacitor
between Pins 15 and 17.
Figure 8. AD579 Clock Rate Control Connection
Note that the No Missing Code operation is not guaranteed
when operating in this mode if a particular grade’s conversion
speed specification is exceeded.
Short Cycle Input—A short cycle input, Pin 14, permits the
timing cycle to be terminated after any number of desired bits
has been converted, allowing shorter conversion times in appli-
cations not requiring the full 10-bit (AD579) or 12-bit (AD578)
resolution. Short cycle pin connections and associated conver-
sion times are summarized in Tables III and IV.
3s
CONVERSION
RATE
6s
CONVERSION
RATE
4.5s
CONVERSION
RATE
L GRADE
K,T GRADES
J, S GRADES
TO SLOW
CONVERSION,
USE C
FROM
FIGURE 7
Table III. AD578 Short Cycle Connections
825⍀
؎1%
3.32⍀
؎1%
Connect
Pin 14 to
Conversion Speed
(s)
For Resolution Bit
Figure 6. AD578 Clock Rate Control Connection
12
10
8
Pin 16
Pin 2
Pin 4
3
2.5
2
CAPACITANCE – pF
1000
680
330
13.0
10.0
8.0
Table IV. AD579 Short Cycle Connections
CAPACITOR
RESISTOR
Connect
Pin 14 to
Conversion Speed
(s)
For Resolution Bit
10
8
Pin 2
Pin 4
1.8
1.5
6.0
4.0
2.0
External Clock—An external clock may be connected directly
to the clock input, Pin 19. When operating in this mode, the
convert start should be held high for a minimum of one clock
period in order to reset the SAR and synchronize the conversion
cycle. A positive-going pulse width of 100 ns to 200 ns will provide
a continuous string of conversions that start on the first rising
edge of the external clock after the EOC output has gone low.
1.5
10k
200
2k
RESISTANCE – ⍀
Figure 7. AD578 Conversion Times vs. R or C Values
External Buffer Amplifier—In applications where the AD578 or
AD579 is to be driven from high impedance sources or directly
from an analog multiplexer, a fast slewing, wideband op amp like
the AD711 should be used (see Figure 9).
V
V
GND
SS
DD
+5V +15V –15V
+V
AD711
–V
OUT
ANALOG INPUT
0V TO 10V
S1
AD7506
S16
DATA BITS
1 TO 12
A0 A1 A2 A3
EN
AD578
OR
AD579
Figure 9. Input Buffer
REV. C
–9–
ADI [ ADI ]
