lm12l458civf National Semiconductor Corporation, lm12l458civf Datasheet - Page 28

• Manufacturer Part Number: lm12l458civf
• Description: 12-bit Sign Data Acquisition System With Self-calibration
• Manufacturer: National Semiconductor Corporation

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7.0 Common Application Problems
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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 volt-
age, 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 be-
fore 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 calibra-
tion. 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 over-
shoot that goes above the supply potential and undershoot
that goes below ground. It is never good to drive a device
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A
+ range. This could happen in normal operation if the
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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 com-
plete 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".
Excessive output capacitance on the digital lines. The
current required to charge the capacitance on the digital
outputs can cause noise on the supply bus within the
LM12L458, causing internal supply "bounce" even when the
external supply pin is pretty stable. The current required to
discharge the output capacitance can cause die ground
"bounce". Either of these can cause noise to be induced at
the analog inputs, resulting in conversion errors.
Output capacitance should be limited as much as possible. A
series 100Ω resistor in each digital output line, located very
close to the output pin, will limit the charge and discharge
current, minimizing the extent of the conversion errors.
Improper CS decoding. If address decoder is used, care
must be exercised to ensure that no "runt" (very narrow)
pulse is produced on the CS line when trying to address
another device or memory. Even sub-nanosecond spikes on
the CS line can cause the chip to be reprogrammed in
accordance with what happens to be on the data lines at the
time. The result is unexpected operation. The worst case
result is that the device is put into the "Test" mode and the
on-board EEPROM that corrects linearity is corrupted. If this
happens, the only recourse is to replace the device.