AD677

REV. A

–13–

The standard deviation of this distribution is approximately

0.5 LSBs. If less uncertainty is desired, averaging multiple con-

versions will narrow this distribution by the inverse of the square

root of the number of samples; i.e., the average of 4 conversions

would have a standard deviation of 0.25 LSBs.

DSP INTERFACE

Figure 10 illustrates the use of the Analog Devices ADSP-2101

digital signal processor with the AD677. The ADSP-2101 FO

(flag out) pin of Serial Port 1 (SPORT 1) is connected to the

SAMPLE line and is used to control acquisition of data. The

ADSP-2101 timer is used to provide precise timing of the FO

pin.

ADSP-2101

FO

AD677

SAMPLE

SERIAL

PORT 0

CLK

BUSY

SCLK0

DR0

RFS0

DT0

TFS0

SDATA

Figure 10. ADSP-2101 Interface

The SCLK pin of the ADSP-2101 SPORT0 provides the CLK

input for the AD677. The clock should be programmed to be

approximately 2 MHz to comply with AD677 specifications. To

minimize digital feedthrough, the clock should be disabled (by

setting Bit 14 in SPORT0 control register to 0) during data ac-

quisition. Since the clock floats when disabled, a pulldown resis-

tor of 12 k

Ω–15 kΩ should be connected to SCLK to ensure it

will be LOW at the falling edge of SAMPLE. To maximize the

conversion rate, the serial clock should be enabled immediately

after SAMPLE is brought LOW (hold mode).

The AD677 BUSY signal is connected to RF0 to notify

SPORT0 when a new data word is coming. SPORT0 should be

configured in normal, external, noninverting framing mode and

can be programmed to generate an interrupt after the last data

bit is received. To maximize the conversion rate, SAMPLE

should be brought HIGH immediately after the last data bit is

received.

102

9.5

86

82

2.5

90

94

98

8.5

4.5

3.5

V

106

5.5

6.5

7.5

THD

S/(N+D)

10.0

INPUT LEVEL – dB

105

10

0

40

20

–70

30

–80

70

50

60

80

90

100

–10

–20

–30

–40

–50

–60

THD

S/(N+D)

Figure 12. S/(N+D) and THD vs. Input Amplitude,

0

–20

–40

–60

–80

–100

–120

–140

0

51015

20

25

30

35

40

45

50

FREQUENCY – kHz

0

–20

–40

–60

–80

–100

–120

–140

0

5

10

15

20

25

30

35

40

45

48

FREQUENCY – kHz