ADC10D1000QML Datasheet, PDF (46/64 Pages) National Semiconductor (TI) – Low Power, 10-Bit, Dual 1.0 GSPS or Single 2.0 GSPS A/DConverter

17.4.3 Calibration Feature
The ADC10D1000 calibration must be run to achieve speci-
fied performance. The calibration procedure is exactly the
same regardless of how it was initiated or when it is run. The
DCLK outputs will be present during all phases of the cali-
bration process. All data and over-range output bits are held
at logic low during calibration. Calibration should be per-
formed in the planned mode of operation. Calibration trims the
analog input differential termination resistor, the CLK input
resistor, and sets internal bias currents which affects the Lin-
earity of the converter. This minimizes full-scale error, offset
error, DNL and INL, resulting in maximizing the dynamic per-
formance as measured by: SNR, THD, SINAD (SNDR) and
ENOB.
17.4.3.1 Calibration Pins
Table 21 is a summary of the pins used for calibration. See
Section 8.0 Column Descriptions and Equivalent Circuits for
complete pin information and Figure 10 for the timing dia-
gram.
Pin
D6
B5
C1/D2
C3/D3
TABLE 21. Calibration Pins
Name
Function
CAL
(Calibration)
Initiate calibration event;
see Section 17.3.1.3
Calibration Pin (CAL)
CalRun
Indicates when
(Calibration Running) calibration is running
Rtrim+/-
External resistor used to
(Input termination trim calibrate analog and CLK
resistor)
inputs
Rext+/-
(External Reference
resistor)
External resistor used to
calibrate internal linearity
17.4.3.2 How to Initiate a Calibration Event
The calibration event must be initiated by holding the CAL pin
low for at least t_{CAL_L} clock cycles, and then holding it high for
at least another t_{CAL_H} clock cycles, as defined in Table 12.
The minimum t_{CAL_L} and t_{CAL_H} input clock cycle sequences
are required to ensure that random noise does not cause a
calibration to begin when it is not desired. The time taken by
the calibration procedure is specified as t_CAL. In ECM, either
the CAL bit (Addr: 0h; Bit: 15) or the CAL pin may be used to
initiate a calibration event.
17.4.3.3 On-command Calibration
An on-command calibration must be run after power up and
whenever the FSR is changed. It is recommended to execute
an on-command calibration whenever the settings or condi-
tions to the device are altered significantly, in order to obtain
optimal parametric performance. Some examples include:
changing the FSR via either ECM or Non-ECM, power-cycling
either channel, and switching into or out of DES Mode. For
best performance, it is also recommended that an on-com-
mand calibration be run 20 seconds or more after application
of power and whenever the operating temperature changes
significantly, relative to the specific system performance re-
quirements.
Due to the nature of the calibration feature, it is recommended
to avoid unnecessary activities on the device while the cali-
bration is taking place. For example, do not read or write to
the Serial Interface or use the DCLK Reset feature while cal-
ibrating the ADC. Doing so will impair the performance of the
device until it is re-calibrated correctly. Also, it is recommend-
ed to not apply a strong narrow-band signal to the analog
inputs during calibration because this may impair the accu-
racy of the calibration; broad spectrum noise is acceptable.
17.4.3.4 Calibration Adjust
The calibration event itself may be adjusted, for sequence and
mode. This feature can be used if a shorter calibration time
than the default is required; see t_CAL in Table 12. However,
the performance of the device, when using a shorter calibra-
tion time than the default setting, is not guaranteed.
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 Cali-
bration (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 calibra-
tions, trimming R_IN and R_IN_CLK may be skipped, i.e. by
setting CSS = 0b.
The mode may be changed, to save calibration execution time
for the internal linearity Calibration. See t_CAL in Table 12. Ad-
justing CMS(1:0) will select three different pre-defined cali-
bration times. A larger amount of time will calibrate each
channel more closely to the ideal values, but choosing shorter
times will not significantly impact the performance. The fourth
setting, CMS(1:0) = 11b, is not available.
17.4.3.5 Calibration and Power-Down
If PDI and PDQ are simultaneously asserted during a cali-
bration cycle, the ADC10D1000 will immediately power down.
The calibration cycle will continue when either or both chan-
nels are powered back up, but the calibration will be compro-
mised due to the incomplete settling of bias currents directly
after power up. Therefore, a new calibration should be exe-
cuted upon powering the ADC10D1000 back up. In general,
the ADC10D1000 should be re-calibrated when either or both
channels are powered back up, or after one channel is pow-
ered down. For best results, this should be done after the
device has stabilized to its operating temperature.
17.4.4 Power Down
On the ADC10D1000, the I- and Q-channels may be powered
down individually. This may be accomplished via the control
pins, PDI and PDQ, or via ECM. In ECM, the PDI and PDQ
pins are logically OR'd with the Control Register setting. See
Section 17.3.1.4 Power Down I-channel Pin (PDI) and Sec-
tion 17.3.1.5 Power Down Q-channel Pin (PDQ) for more
information.
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