sampling rate, a tone will appear as the clock fre-
quency aliases into the baseband. The tone
frequency can be calculated using the equation
below and could be selectively filtered in soft-
ware using DSP techniques.
f
tone
= (N f
s
- f
clk
)
where N = f
clk
/f
s
rounded to the nearest integer
The magnitude of clock feedthrough depends on
the master clock conditions and the source im-
pedance applied to the analog input. When
operating with the CS5014/16’s internally gener-
ated clock, the CLKIN input is grounded and the
dominant source of coupling is through the de-
vice’s substrate. As shown in Figure 35, a typical
CS5014/16 operating with their internal oscillator
at 2 MHz and 50 Ω of analog input source im-
pedance will exhibit only 15 µV rms of clock
feedthrough. However, if a 2 MHz external clock
is applied to CLKIN under the same conditions,
feedthrough increases to 25 µV rms. Feedthrough
also increases with clock frequency; a 4 MHz
clock yields 40 µV rms.
Clock feedthrough can be reduced by limiting the
source impedance applied at the analog input. As
shown in Figure 35, reducing source impedance
from 50 Ω to 25 Ω yields a 15 µV rms reduction
in feedthrough. Therefore, when operating the
CS5014/16 with high-frequency external master
clocks, it is important to minimize source imped-
ance applied to the CS5014/16’s input.
Also, the overall effect of clock feedthrough can
be minimized by maximizing the input range and
LSB size. The reference voltage applied to VREF
can be maximized, and the CS5014/16 can be op-
erated in bipolar mode which inherently doubles
the LSB size over the unipolar mode.
Differences between the CS5012A and the
CS5012
The differences between the CS5012A and the
CS5012 are tabulated in Table 3. The CS5012 is
a short-cycled version of the CS5016 A/D con-
verter and includes the same 18-bit calibration
circuitry. This calibration circuitry sets the cali-
bration resolution of the CS5012 at 1/64th of an
LSB and achieves the near perfect differential
linearity performance illustrated by the CS5012
DNL plot in Figure 15. The CS5012A calibration
circuitry was modified to provide calibration to
15-bit resolution therefore achieving calibration
to 1/8 of an LSB. This reduction in calibration
resolution for the CS5012A reduces the time re-
quired to calibrate the device (see Table 3) and
reduces the size of the total array capacitance.
The reduced array capacitance improves the high
frequency performance by allowing higher slew
rate in the input circuitry.
Table 3 documents some other improvements in-
cluded in the CS5012A. The burst mode
calibration was made functional, although it
should not be used. The device was also modified
so the EOC signal goes low at the end of a reset
calibration in either microprocessor or microproc-
essor-independent mode. The CS5012A was
modified to maintain a throughput rate of 64
CLKIN cycles in loopback mode for all frequen-
cies of CLKIN.
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CS5012A, CS5014, CS5016
DS14F6 2-37
