LM97600CIUTSLASHNOPB datasheet(39/58 Pages) TI1

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LM97600

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SNAS600A – JULY 2012 – REVISED MARCH 2013

Hertz, at the ADC analog input.

(3)

Note that the maximum jitter described above is the Root Sum Square, (RSS), of the jitter from all sources,

including that in the ADC input clock, that added by the system to the ADC input clock and input signals and that

added by the ADC itself. Since the effective jitter added by the ADC is beyond user control, the best the user can

do is to keep the sum of the externally added input clock jitter and the jitter added by the analog circuitry to the

analog signal to a minimum.

Input clock amplitudes above those specified in the Electrical Characteristics table may result in increased input

offset voltage. This would cause the converter to produce an output code other than the expected 128 when both

input pins are at the same potential.

Aperture Delay

The sample clock aperture delay can be manually increased to accommodate subtle layout differences when

synchronizing multiple ADCs. Adjustments are made via the Aperture Delay register (0Ch, Bits 15:0).

It should be noted that by just enabling the aperture delay capability (register 01h; Bit 0), degradation of dynamic

performance is expected, specifically SFDR. It is intended that very small adjustments are used. Larger

increases in phase adjustments will begin to affect SNR and ultimately ENOB. Therefore, the use of coarse

phase adjustment should be minimized in favor of better system design.

CONTROL PINS

Three control pins (without the use of the serial interface) provide control over the operation of the LM97600 and

facilitate its use. These control pins support Calibration, Synchronization of serial data and multiple ADCs, and a

Power Down feature.

Calibration

The LM97600 calibration must be run to achieve specified performance. The calibration procedure is run upon

power-up and can be run any time on command. The calibration procedure is exactly the same whether there is

an input clock present upon power up or if the clock begins some time after application of power. The CalRun

output indicator is high while a calibration is in progress. The calibration duration is nominally 1.16ms (with a 2.5

GHz input clock).

Power-On Calibration

Power-on calibration begins after a time delay following the application of power. This time delay is fixed at

8,388,608 input CLK cycles. With a 2.5 GHz input CLK, this gives an approximate delay of 3.36 ms.

The calibration process will be not be performed if the CAL pin is concurrently high with the application of power.

In this case, the calibration cycle will not begin until the on-command calibration conditions are met. The

LM97600 will function with the CAL pin held high at power up, but no calibration will be done and performance

will be impaired. A manual calibration, however, may be performed after powering up with the CAL pin high. See

the On-Command Calibration section.

On-Command Calibration

To initiate an on-command calibration, bring the CAL pin high for a minimum of 80 input clock cycles after it has

been low for a minimum of 80 input clock cycles. Holding the CAL pin high upon power up will prevent execution

of power-on calibration until the CAL pin is low for a minimum of 80 input clock cycles, then brought high for a

minimum of another 80 input clock cycles. The calibration cycle will begin 80 input clock cycles after the CAL pin

is thus brought high. The CalRun signal should be monitored to determine when the calibration cycle has

completed.

The minimum 80 input clock cycle sequences are required to ensure that random noise does not cause a

calibration to begin when it is not desired. As mentioned in the OVERVIEW section for best performance, a

calibration should be performed 20 seconds or more after power up and repeated when the operating

temperature changes significantly relative to the specific system design performance requirements. ENOB

changes slightly with increasing junction temperature and can be easily corrected by performing an on-command

calibration.

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