LM12H458MEL/883 NSC [National Semiconductor], LM12H458MEL/883 Datasheet - Page 32

• Manufacturer Part Number: LM12H458MEL/883
• Description: 12-Bit Sign Data Acquisition System with Self-Calibration
• Manufacturer: NSC [National Semiconductor]

www.national.com
5.0 Analog Considerations
5.1 REFERENCE VOLTAGE
The difference in the voltages applied to the V
V
between the voltages applied between two multiplexer inputs
or the voltage applied to one of the multiplexer inputs and
analog ground), over which 4095 positive and 4096 negative
codes exist. The voltage sources driving V
must have very low output impedance and noise.
The ADC can be used in either ratiometric or absolute refer-
ence applications. In ratiometric systems, the analog input
voltage is proportional to the voltage used for the ADC’s ref-
erence voltage. When this voltage is the system power sup-
ply, the V
nected to GND. This technique relaxes the system reference
stability requirements because the analog input voltage and
the ADC reference voltage move together. This maintains
the same output code for given input conditions.
For absolute accuracy, where the analog input voltage varies
between very specific voltage limits, a time and temperature
stable voltage source can be connected to the reference in-
puts. Typically, the reference voltage’s magnitude will require
an initial adjustment to null reference voltage induced
full-scale errors.
When using the LM12(H)454/8’s internal 2.5V bandgap ref-
erence, a parallel combination of a 100 µF capacitor and a
0.1 µF capacitor connected to the V
mended for low noise operation. When left unconnected, the
reference remains stable without a bypass capacitor. How-
ever, ensure that stray capacitance at the V
mains below 50 pF.
5.2 INPUT RANGE
The LM12(H)454/8’s fully differential ADC and reference
voltage inputs generate a two’s-complement output that is
found by using the equation below.
Round up to the next integer value between −4096 to 4095
for 12-bit resolution and between −256 to 255 for 8-bit reso-
lution if the result of the above equation is not a whole num-
ber. As an example, V
and V
full-scale, or 0,1111,1111,1111. If V
V
0,1100,0000,0000.
5.3 INPUT CURRENT
A charging current flows into or out of (depending on the in-
put voltage polarity) the analog input pins, IN0–IN7 at the
start of the analog input acquisition time (t
rent’s peak value will depend on the actual input voltage ap-
plied.
5.4 INPUT SOURCE RESISTANCE
For low impedance voltage sources (
eration and
current will decay, before the end of the S/H’s acquisition
time, to a value that will not introduce any conversion errors.
For higher source impedances, the S/H’s acquisition time
REF−
IN+
= 3V, and V
IN−
defines the analog input voltage span (the difference
= GND. The 12-bit + sign output code is positive
REF+
<
60
pin is connected to V
IN−
for 8 MHz operation), the input charging
= GND, the 12-bit + sign output code is
REF+
= 2.5V, V
REF+
REF−
A
REFOUT
<
+ and V
100
= 5V, V
= 1V, V
ACQ
REF+
REFOUT
for 5 MHz op-
pin is recom-
REF−
). This cur-
REF−
IN+
REF+
or V
is con-
= 1.5V
pin re-
= 1V,
REF−
and
32
can be increased. As an example, operating with a 5 MHz
clock frequency and maximum acquisition time, the
LM12(H)454/8’s analog inputs can handle source imped-
ance as high as 6.67 k . When operating at 8 MHz and
maximum acquisition time, the LM12H454/8’s analog inputs
can handle source impedance as high as 4.17 k . Refer to
Section 2.1, Instruction RAM “00”, Bits 12–15 for further in-
formation.
5.5 INPUT BYPASS CAPACITANCE
External capacitors (0.01 µF–0.1 µF) can be connected be-
tween the analog input pins, IN0–IN7, and analog ground to
filter any noise caused by inductive pickup associated with
long input leads. It will not degrade the conversion accuracy.
5.6 NOISE
The leads to each of the analog multiplexer input pins should
be kept as short as possible. This will minimize input noise
and clock frequency coupling that can cause conversion er-
rors. Input filtering can be used to reduce the effects of the
noise sources.
5.7 POWER SUPPLIES
Noise spikes on the V
conversion errors; the comparator will respond to the noise.
The ADC is especially sensitive to any power supply spikes
that occur during the auto-zero or linearity correction. Low in-
ductance tantalum capacitors of 10 µF or greater paralleled
with 0.1 µF monolithic ceramic capacitors are recommended
for supply bypassing. Separate bypass capacitors should be
used for the V
possible to these pins.
5.8 GROUNDING
The LM12(H)454/8’s nominal high resolution performance
can be maximized through proper grounding techniques.
These include the use of separate analog and digital ground
planes. The digital ground plane is placed under all compo-
nents that handle digital signals, while the analog ground
plane is placed under all analog signal handling circuitry. The
digital and analog ground planes are connected at only one
point, the power supply ground. This greatly reduces the oc-
currence of ground loops and noise.
It is recommended that stray capacitance between the ana-
log inputs or outputs (LM12(H)454: IN0–IN3, MUXOUT+,
MUXOUT−, S/H IN+, S/H IN−; LM12(H)458: IN0–IN7,
V
(+1/16th inch) between the analog signal and reference pins
and the ground plane.
5.9 CLOCK SIGNAL LINE ISOLATION
The LM12(H)454/8’s performance is optimized by routing the
analog input/output and reference signal conductors (pins
34–44) as far as possible from the conductor that carries the
clock signal to pin 23. Ground traces parallel to the clock sig-
nal trace can be used on printed circuit boards to reduce
clock signal interference on the analog input/output pins.
6.0 Application Circuits
PC EVALUATION/INTERFACE BOARD
Figure 16 is the schematic of an evaluation/interface board
designed to interface the LM12(H)454 or LM12(H)458 with
an XT or AT
REF+
, and V
®
REF−
style computer. The board can be used to de-
A
+ and V
) be reduced by increasing the clearance
A
D
+ and V
+ supplies and placed as close as
D
+ supply lines can cause