AD7911AUJZ-R2 datasheet(8/28 Pages) AD

8 / 28 page

AD7911/AD7921

Rev. A | Page 8 of 28

TIMING EXAMPLES

Figure 6 and Figure 7 show some of the timing parameters from

the Timing Specifications section.

Timing Example 1

As shown in Figure 7, when fSCLK = 5 MHz and the throughput is

250 kSPS, the cycle time is

t2 + 12.5(1/fSCLK) + tACQ = 4 μs

With t2 = 10 ns minimum, then tACQ is 1.49 μs, which satisfies

the requirement of 290 ns for tACQ.

In Figure 7, tACQ is comprised of 2.5(1/fSCLK) + t10 + tQUIET, where

t10 = 30 ns maximum. This allows a value of 960 ns for tQUIET,

satisfying the minimum requirement of 30 ns.

Timing Example 2

The AD7921 can also operate with slower clock frequencies. As

shown in Figure 7, when fSCLK = 2 MHz and the throughput rate

is 100 KSPS, the cycle time is

t2 + 12.5(1/fSCLK) + tACQ = 10 μs

With t2 = 10 ns minimum, then tACQ is 3.74 μs, which satisfies

the requirement of 290 ns for tACQ.

In Figure 7, tACQ is comprised of 2.5(1/fSCLK) + t10 + tQUIET, where

t10 = 30 ns maximum. This allows a value of 2.46 μs for tQUIET,

satisfying the minimum requirement of 30 ns.

In this example, as with other slower clock values, the signal

might already be acquired before the conversion is complete,

but it is still necessary to leave 30 ns minimum tQUIET between

conversions. In this example, the signal should be fully acquired

at approximately point C in Figure 7.

ZERO

CHN

DB11

DB10

DB2

DB1

DB0

t10

tQUIET

tCONVERT

SCLK

DOUT

THREE-STATE

DIN

Figure 6. AD7921 Serial Interface Timing Diagram

tQUIET

tACQUISITION

1/THROUGHPUT

12.5(1/fSCLK)

tCONVERT

SCLK

t10

Figure 7. Serial Interface Timing Example

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