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LM12454 Datasheet(PDF) 39 Page - Texas Instruments |
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LM12454 Datasheet(HTML) 39 Page - Texas Instruments |
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39 / 45 page  LM12454, LM12458, LM12H458 www.ti.com SNAS079A – MAY 2004 – REVISED FEBRUARY 2006 6.6 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 errors. Input filtering can be used to reduce the effects of the noise sources. 6.7 POWER SUPPLIES Noise spikes on the VA+ and VD+ 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 VA+ and VD+ supplies and placed as close as possible to these pins. 6.8 GROUNDING The LM12(H)454/8'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 or outputs, including the reference pins, be kept to a minimum by increasing the clearance (+1/16th inch) between the analog signal and reference pins and the ground plane. 6.9 CLOCK SIGNAL CONSIDERATIONS The LM12(H)458'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 30 Ω 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 LM12(H)458 clock pin. 7.0 Common Application Problems Driving the analog inputs with op-amp(s) powered from supplies other than the supply used for the LM12(H)458. This practice allows for the possibility of the amplifier output (LM12(H)458 input) to reach potentials outside of the 0V to VA+ range. This could happen in normal operation if the amplifier use supply voltages outside of the range of the LM12(H)458 supply rails. This could also happen upon power up if the amplifier supply or supplies ramp up faster than the supply of the LM12(H)458. 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 LM12(H)458 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. Copyright © 2004–2006, Texas Instruments Incorporated Submit Documentation Feedback 39 Product Folder Links: LM12454 LM12458 LM12H458 |
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