AD9864-EBZ datasheet(24/48 Pages) AD

24 / 48 page

Data Sheet

AD9864

Rev. A | Page 23 of 47

I15

I14

tCLK

tHI

tDV

tLOW

CLKOUT

DOUT

Figure 34. SSI Timing Parameters for SSI Timing

In Figure 34, the timing parameters also apply to inverted

CLKOUT or FS modes, with tDV relative to the falling edge of

the CLK and/or FS.

The AD9864 also provides the means for controlling the

switching characteristics of the digital output signals via the drive

strength (DS) field of the SSICRB. This feature is useful in

limiting switching transients and noise from the digital output

that may ultimately couple back into the analog signal path,

potentially degrading the sensitivity performance of the AD9864.

Figure 35 and Figure 36 show how the NF can vary as a function

of the SSI setting for an IF frequency of 109.65 MHz. The

following two observations can be made from these figures:

1. The NF becomes more sensitive to the SSI output drive

strength level at higher signal bandwidth settings.

2. The NF is dependent on the number of bits within an SSI

frame that become more sensitive to the SSI output drive

strength level as the number of bits is increased. Therefore,

select the lowest possible SSI drive strength setting that still

meets the SSI timing requirements.

SSI OUTPUT DRIVE STRENGTH SETTING

10.0

9.6

8.0

24-BIT I/O DATA

9.8

9.4

9.2

9.0

8.6

8.8

8.4

8.2

16-BIT I/0 DATA

w/ DVGA ENABLED

16-BIT I/O DATA

Figure 35. NF vs. SSI Output Drive Strength

(VDDx = 3.0 V, fCLK = 18 MSPS, BW = 10 kHz)

SSI OUTPUT DRIVE STRENGTH SETTING

24-BIT I/O DATA

16-BIT I/O DATA

w/DVGA ENABLED

16-BIT I/O DATA

Figure 36. NF vs. SSI Output Drive Strength

(VDDx = 3.0 V, fCLK = 18 MSPS, BW = 75 kHz)

Table 15 lists the typical output rise/fall times as a function of

DS for a 10 pF load. Rise/fall times for other capacitor loads can

be determined by multiplying the typical values presented by a

scaling factor equal to the desired capacitive load divided by 10 pF.

Table 15. Typical Rise/Fall Times (±25%) with a 10 pF

Capacitive Load for Each DS Setting

Typ (ns)

13.5

7.2

3.7

3.2

2.8

2.3

2.0

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