Datasheet

LM12L458
SNAS085B –JULY 1999–REVISED MARCH 2013 www.ti.com
POWER SUPPLIES
Noise spikes on the V
A
+ and V
D
+ supply lines can cause 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 inductance 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
A
+ and
V
D
+ supplies and placed as close as possible to these pins.
GROUNDING
The LM12L458's nominal performance can be maximized through proper grounding techniques. These include
the use of a single ground plane and meticulously separating analog and digital areas of the board. The use of
separate analog and digital digital planes within the same board area generally provides best performance. All
components that handle digital signals should be placed within the digital area of the board, as defined by the
digital power plane, while all analog components should be placed in the analog area of the board. Such
placement and the routing of analog and digital signal lines within their own respective board areas greatly
reduces the occurrence of ground loops and noise. This will also minimize EMI/RFI radiation and susceptibility.
It is recommended that stray capacitance between the analog inputs (IN0–IN7, V
REF+
, and V
REF−
) be reduced by
increasing the clearance (+1/16th inch) between the analog signal and reference pins and the ground plane.
CLOCK SIGNAL CONSIDERATIONS
The LM12L458'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.
Avoid overshoot and undershoot on the clock line by treating this line as a transmission line (use proper
termination techniques). Failure to do so can result in erratic operation. Generally, a series 30Ω to 50Ω resistor in
the clock line, located as close to the clock source as possible, will prevent most problems. The clock source
should drive ONLY the LM12L458 clock pin.
Common Application Problems
Driving the analog inputs with op-amp(s) powered from supplies other than the supply used for the
LM12L458. This practice allows for the possibility of the amplifier output (LM12L458 input) to reach potentials
outside of the 0V to V
A
+ range. This could happen in normal operation if the amplifier use supply voltages
outside of the range of the LM12L458 supply rails. This could also happen upon power up if the amplifier supply
or supplies ramp up faster than the supply of the LM12L458. If any pin experiences a potential more than 100
mV below ground or above the supply voltage, even on a fast transient basis, the result could be erratic
operation, missing codes, one channel interacting with one or more of the others, skipping channels or a
complete malfunction, depending upon how far the input is driven beyond the supply rails.
Not performing a full calibration at power up. This can result in missing codes. The device needs to have a
full calibration run and completed after power up and BEFORE attempting to perform even a single conversion or
watchdog operation. The only way to recover if this is violated is to interrupt the power to the device.
Not waiting for the calibration process to complete before trying to write to the device. Once a calibration
is requested, the ONLY read of the LM12L458 should be if the Interrupt Status Register to check for a completed
calibration. Attempting a write or any other read during calibration would cause a corruption of the calibration
process, resulting in missing codes. The only way to recover would be to interrupt the power.
Improper termination of digital lines. Improper termination can result in energy reflections that build up to
cause overshoot that goes above the supply potential and undershoot that goes below ground. It is never good
to drive a device beyond the supply rails, unless the device is specifically designed to handle this situation, but
the LM12L458 is more sensitive to this condition that most devices. Again, if any pin experiences a potential
more than 100 mV below ground or above the supply voltage, even on a fast transient basis, the result could be
erratic operation, missing codes, or a complete malfunction, depending upon how far the input is driven beyond
the supply rails. The clock input is the most sensitive digital one. Generally, a 50Ω series resistor, located very
close to the signal source, will keep digital lines "clean".
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