Datasheet

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DIGITAL OUTPUT INTERFACE
Parallel CMOS Interface
ADS6145, ADS6144
ADS6143, ADS6142
SLWS198B – JULY 2007 – REVISED MARCH 2008
The ADS614X output 14 data bits together with an output clock. The output interface is either parallel CMOS or
DDR LVDS voltage levels and can be selected using the serial register bit <LVDS CMOS> or parallel pin SEN.
In CMOS mode, the output buffer supply (DRVDD) can be operated over a wide range from 1.8 V to 3.3 V
(typical). Each data bit is output on a separate pin as a CMOS voltage level, every clock cycle.
For DRVDD ≥ 2.2 V, it is recommended to use the CMOS output clock (CLKOUT) to latch data in the receiving
chip. The rising edge of CLKOUT can be used to latch data in the receiver, even at the highest sampling speed
(125 MSPS). It is recommended to minimize the load capacitance seen by the data and clock output pins by
using short traces to the receiver. Also, match the output data and clock traces to minimize the skew between
them.
For DRVDD < 2.2 V, it is recommended to use an external clock (for example, input clock delayed to get desired
setup/hold times).
Output Clock Position Programmability
There is an option to shift (delay) the output clock position so that the setup time increases by 400 ps (typical,
with respect to the default timings specified). This may be useful if the receiver needs more setup time,
especially at high sampling frequencies. This can be programmed using the serial interface register bit
<CLKOUT_POSN> (Table 6 ).
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 ADS614X CMOS output buffers are designed with a controlled drive strength for
the best SNR. The default drive strength also ensures a wide data stable window for load capacitances up to 5
pF and a DRVDD supply voltage ≥ 2.2 V.
To ensure a wide data stable window for load capacitances > 5 pF, there is an option to increase the drive
strength using the serial interface ( <DRIVE STRENGTH>, see Table 12 ). Note that for a DRVDD supply voltage
< 2.2 V, it is recommended to use the maximum drive strength (for any value of load capacitance).
CMOS Mode 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 actual applications, this condition is unlikely to occur. The actual 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 x (N x F
AVG
)
where C
L
= load capacitance, N × F
AVG
= average number of output bits switching
Figure 87 shows the current with various load capacitances across sampling frequencies with a 2-MHz analog
input frequency.
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