HI1-574ATD/883 INTERSIL [Intersil Corporation], HI1-574ATD/883 Datasheet - Page 9

• Manufacturer Part Number: HI1-574ATD/883
• Description: Complete, 12-Bit A/D Converters with Microprocessor Interface
• Manufacturer: INTERSIL [Intersil Corporation]

change from the initial (25
T
Power Supply Rejection
The standard specifications for the HI-X74A assume use of
+5.00V and 15.00V or 12.00V supplies. The only effect of
power supply error on the performance of the device will be
a small change in the full scale calibration. This will result in
a linear change in all lower order codes. The specifications
show the maximum change in calibration from the initial
value with the supplies at the various limits.
Code Width
A fundamental quantity for A/D converter specifications is
the code width. This is defined as the range of analog input
values for which a given digital output code will occur. The
nominal value of a code width is equivalent to 1 least
significant bit (LSB) of the full scale range or 2.44mV out of
10V for a 12-bit ADC.
Quantization Uncertainty
Analog-to-digital converters exhibit an inherent quantization
uncertainty of
characteristic of the quantization process and cannot be
reduced for a converter of given resolution.
Left-Justified Data
The data format used in the HI-X74A is left-justified. This
means that the data represents the analog input as a
fraction of full-scale, ranging from 0 to
binary point to the left of the MSB.
Applying the HI-X74A
For each application of this converter, the ground
connections, power supply bypassing, analog signal source,
digital timing and signal routing on the circuit board must be
optimized to assure maximum performance. These areas
are reviewed in the following sections, along with basic
operating modes and calibration requirements.
Physical Mounting and Layout Considerations
LAYOUT
Unwanted, parasitic circuit components, (L, R, and C) can
make 12-bit accuracy impossible, even with a perfect A/D
converter. The best policy is to eliminate or minimize these
parasitics through proper circuit layout, rather than try to
quantify their effects.
The recommended construction is a double-sided printed
circuit board with a ground plane on the component side.
Other techniques, such as wire-wrapping or point-to-point
wiring on vector board, will have an unpredictable effect on
accuracy.
In general, sensitive analog signals should be routed between
ground traces and kept well away from digital lines. If analog
and digital lines must cross, they should do so at right angles.
MAX
.
1
/
2
LSB. This uncertainty is a fundamental
o
C) value to the value at T
9
4095
4096
. This implies a
MIN
or
Power Supplies
Supply voltages to the HI-X74A (+15V, -15V and +5V) must be
“quiet” and well regulated. Voltage spikes on these lines can
affect the converter’s accuracy, causing several LSBs to flicker
when a constant input is applied. Digital noise and spikes from
a switching power supply are especially troublesome. If
switching supplies must be used, outputs should be carefully
filtered to assure “quiet” DC voltage at the converter terminals.
Further, a bypass capacitor pair on each supply voltage
terminal is necessary to counter the effect of variations in
supply current. Connect one pair from pin 1 to 15 (V
supply), one from pin 7 to 9 (V
one from pin 11 to 9 (V
capacitor pair, a 10 F tantalum type in parallel with a 0.1 F
ceramic type is recommended.
Ground Connections
Pins 9 and 15 should be tied together at the package to
guarantee specified performance for the converter. In
addition, a wide PC trace should run directly from pin 9 to
(usually) +15V common, and from pin 15 to (usually) the +5V
Logic Common. If the converter is located some distance from
the system’s “single point” ground, make only these
connections to pins 9 and 15: Tie them together at the
package, and back to the system ground with a single path.
This path should have low resistance. (Code dependent
currents flow in the V
through the HI-X74A’s Analog Common or Digital Common).
Analog Signal Source
HI-574A and HI-674A
The device chosen to drive the HI-X74A analog input will see a
nominal load of 5k (10V range) or 10k (20V range).
However, the other end of these input resistors may change
current at the analog input. Thus, the signal source must
maintain its output voltage while furnishing these step changes
in load current, which occur at 1.6 s and 950ns intervals for the
HI-574A and HI-674A, respectively. This requires low output
impedance and fast settling by the signal source.
The output impedance of an op amp, for example, has an
open loop value which, in a closed loop, is divided by the
loop gain available at a frequency of interest. The amplifier
should have acceptable loop gain at 600kHz for use with the
HI-X74A. To check whether the output properties of a signal
source are suitable, monitor the HI-X74A’s input (pin 13 or
14) with an oscilloscope while a conversion is in progress.
Each of the twelve disturbances should subside in 1 s or
less for the HI-574A and 500ns or less for the HI-674A. (The
comparator decision is made about 1.5 s and 850ns after
each code change from the SAR for the HI-574A and
HI-674A, respectively.)
If the application calls for a Sample/Hold to precede the
converter, it should be noted that not all Sample/Holds are
compatible with the HI-574A in the manner described above.
400mV with each bit decision, creating abrupt changes in
CC
EE
, V
to Analog Common). For each
EE
and V
CC
to Analog Common) and
LOGIC
terminals, but not
LOGIC