Datasheet

11
®
ADS574
tion Mode, system throughput can be speeded up, since the
input to the ADS574 can start slewing before the end of a
conversion (after the acquisition time), which is not possible
with existing ADC574s.
INSTALLATION
LAYOUT PRECAUTIONS
Analog (pin 9) and digital (pin 15) commons are not con-
nected together internally in the ADS574, but should be
connected together as close to the unit as possible and to an
analog common ground plane beneath the converter on the
component side of the board. In addition, a wide conductor
pattern should run directly from pin 9 to the analog supply
common, and a separate wide conductor pattern from pin 15
to the digital supply common.
If the single-point system common cannot be established
directly at the converter, pin 9 and pin 15 should still be
connected together at the converter. A single wide conductor
pattern then connects these two pins to the system common.
In either case, the common return of the analog input signal
should be referenced to pin 9 of the ADC. This prevents any
voltage drops that might occur in the power supply common
returns from appearing in series with the input signal.
systems (due to multiplexers, sample/holds, etc. in front of
the converter) does not affect the accuracy of the ADS574
conversion in the Emulation Mode.
In both modes, as soon as the conversion is completed the
internal sample/hold circuit immediately begins slewing to
track the input signal.
Basically, the Control Mode is provided to allow full use of
the internal sample/hold, eliminating the need for an exter-
nal sample/hold in most applications. As compared with
systems using separate sample/hold and A/D, the ADS574
in the Control Mode also eliminates the need for one of the
control signals, usually the convert command. The com-
mand that puts the internal sample/hold in the hold state also
initiates a conversion, reducing timing constraints in many
systems.
The Emulation Mode allows the ADS574 to be dropped into
almost all existing ADC574 sockets without changes to any
other existing system hardware or software. The input to the
ADS574 in the Emulation Mode does not need to be stable
before a convert command is received, so that multiplexers,
programmable gain amplifiers, etc., can be slewing quickly
any time before a convert command is given as long as the
analog input to the ADS574 is stable after the convert
command is received, as it needs to be in existing ADC574
systems for accurate operation. In fact, even in the Emula-
S/H CONTROL MODE ADC574 EMULATION MODE
(Pin 11 Connected to +5V) (Pin 11 Connected to 0V to –15V)
SYMBOL PARAMETER
MIN TYP MAX MIN TYP MAX UNITS
t
AQ
+ t
C
Throughput Time:
12-bit Conversions 22 25 22 25 µs
8-bit Conversions 16 18 16 18 µs
t
C
Conversion Time:
12-bit Conversions 18 18 µs
8-bit Conversions 12 12 µs
t
AQ
Acquisition Time 4 4 µs
t
AP
Aperture Delay 20 4000 ns
t
J
Aperture Uncertainty 0.3 30 ns
TABLE VI. Conversion Timing, T
MIN
to T
MAX
.
FIGURE 9. Signal Acquisition and Conversion Timing.
Signal
Acquisition
Conversion
Signal
Acquisition
t
AP
Signal
Acquisition
Conversion
Signal
Acquisition
t
C
t
AQ
t
C
t
AQ
t
AP
R/C
S/H Control Mode
Pin 11 connected to +5V.
ADC574 Emulation Mode*
Pin 11 connected to V
EE
or ground.
*In the ADC574 Emulation Mode, a convert command triggers a delay that
allows the ADS574 enough time to acquire the input signal before converting.