ADC12D1800RFIUT Datasheet, PDF (53/78 Pages) Texas Instruments – ADC12D1800RF 12-Bit, Single 3.6 GSPS RF Sampling ADC

ADC12D1800RF
www.ti.com
SNAS518H – JULY 2011 – REVISED APRIL 2013
It is strongly recommended to set CalDly Pin (to either logic-high or logic-low) before powering the device
on since this pin affects the power-on calibration timing. This may be accomplished by setting CalDly via
an external 1kΩ resistor connected to GND or V_A. If the CalDly Pin is toggled while the device is powered-
on, it can execute a calibration even though the CAL Pin / Bit remains logic-low.
The power-on calibration will be not be performed if the CAL pin is logic-high at power-on. In this case, the
calibration cycle will not begin until the on-command calibration conditions are met. The ADC12D1800RF
will function with the CAL pin held high at power up, but no calibration will be done and performance will
be impaired.
If it is necessary to toggle the CalDly Pin during the system power up sequence, then the CAL Pin / Bit
must be set to logic-high before the toggling and afterwards for 10⁹ Sampling Clock cycles. This will
prevent the power-on calibration, so an on-command calibration must be executed or the performance will
be impaired.
6.3.3.4 On-command Calibration
In addition to the power-on calibration, it is recommended to execute an on-command calibration
whenever the settings or conditions to the device are altered significantly, in order to obtain optimal
parametric performance. Some examples include: changing the FSR via ECM, power-cycling either
channel, and switching into or out of DES Mode. For best performance, it is also recommended that an
on-command calibration be run 20 seconds or more after application of power and whenever the operating
temperature changes significantly, relative to the specific system performance requirements.
Due to the nature of the calibration feature, it is recommended to avoid unnecessary activities on the
device while the calibration is taking place. For example, do not read or write to the Serial Interface or use
the DCLK Reset feature while calibrating the ADC. Doing so will impair the performance of the device until
it is re-calibrated correctly. Also, it is recommended to not apply a strong narrow-band signal to the analog
inputs during calibration because this may impair the accuracy of the calibration; broad spectrum noise is
acceptable.
6.3.3.5 Calibration Adjust
The sequence of the calibration event itself may be adjusted. This feature can be used if a shorter
calibration time than the default is required; see t_CAL in Converter Electrical Characteristics Calibration.
However, the performance of the device, when using this feature is not ensured.
The calibration sequence may be adjusted via CSS (Addr: 4h, Bit 14). The default setting of CSS = 1b
executes both R_IN and R_{IN_CLK} Calibration (using Rtrim) and internal linearity Calibration (using Rext).
Executing a calibration with CSS = 0b executes only the internal linearity Calibration. The first time that
Calibration is executed, it must be with CSS = 1b to trim R_IN and R_{IN_CLK}. However, once the device is at
its operating temperature and R_IN has been trimmed at least one time, it will not drift significantly. To save
time in subsequent calibrations, trimming R_IN and R_{IN_CLK} may be skipped, i.e. by setting CSS = 0b.
6.3.3.6 Read / Write Calibration Settings
When the ADC performs a calibration, the calibration constants are stored in an array which is accessible
via the Calibration Values register (Addr: 5h). To save the time which it takes to execute a calibration, t_CAL,
or to allow for re-use of a previous calibration result, these values can be read from and written to the
register at a later time. For example, if an application requires the same input impedance, R_IN, this feature
can be used to load a previously determined set of values. For the calibration values to be valid, the ADC
must be operating under the same conditions, including temperature, at which the calibration values were
originally determined by the ADC.
To read calibration values from the SPI, do the following:
1. Set ADC to desired operating conditions.
2. Set SSC (Addr: 4h, Bit 7) to 1.
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Functional Description
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