ADS61B49_09 Datasheet, PDF (51/62 Pages) Texas Instruments – 14-/12-Bit, 250-MSPS ADCs With Integrated Analog Buffer

ADS61B29
ADS61B49
www.ti.com ...................................................................................................................................................... SLWS214B – OCTOBER 2008 – REVISED MAY 2009
Pins
OVR_SDOUT
CLKOUT
14-Bit ADC Data
D0
D1
D2
•
•
•
D11
D12
D13
ADS614x
Figure 75. CMOS Output Interface
Output Buffer Strength Programmability
Switching noise (caused by CMOS output data transitions) can couple into the analog inputs during the instant of
sampling and degrade the SNR. The coupling and SNR degradation increases as the output buffer drive is made
stronger. To minimize this, the CMOS output buffers are designed with a controlled drive strength to achieve the
best SNR. The default drive strength also ensures a wide data stable window for load capacitances up to 5 pF.
CMOS Interface Power Dissipation
With CMOS outputs, the DRVDD current scales with the sampling frequency and the load capacitance on every
output pin. The maximum DRVDD current occurs when each output bit toggles between 0 and 1 every clock
cycle. In an actual application, the DRVDD current would be determined by the average number of output bits
switching, which is a function of the sampling frequency and the nature of the analog input signal.
Digital current due to CMOS output switching = C_L × DRVDD × (N × F_AVG),
where
C_L = load capacitance,
N x F_AVG = average number of output bits switching.
Figure 54 shows the current across the sampling frequencies with a 3-MHz analog input frequency.
Output Data Format
Two output data formats are supported – 2s complement and offset binary. They can be selected using the serial
interface register bit <DATA FORMAT> or controlling the DFS pin in parallel configuration mode.
In the event of an input voltage overdrive, the digital outputs go to the appropriate full-scale level. For a positive
overdrive, the output code is 0x3FFF in offset binary output format, and 0x1FFF in 2s complement output format.
For a negative input overdrive, the output code is 0x0000 in offset binary output format and 0x2000 in 2s
complement output format.
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