AD1385 datasheet(11/20 Pages) AD

11 / 20 page

AD1385

REV. 0

–11–

mode as long either Hold Command Out or Start Convert is

high. Care is needed in defining system timing to ensure that

the T/H has a minimum of 700 ns for signal acquisition before

another conversion begins. The minimum width of Start Con-

vert is 20 ns, the sum of tSCS and tSCH, the minimum setup and

hold times.

Transmission line effects at the Start Convert and Hold Com-

mand In inputs should be considered when designing circuit

boards for the AD1385. A series termination resistor of 50

Ω to

100

Ω is recommended when the source of either of these sig-

nals is more than a few inches away from the AD1385. This will

control reflections and transients which could otherwise degrade

the part’s performance.

Output Data

The output data are multiplexed in two bytes onto an 8-bit data

bus. Data are guaranteed to be stable at the time of the edges of

Data Strobe (Pin 15). Hi/Lo Byte Select (Pin 16) controls

which byte is presented first. If Hi/Lo Byte Select is high, then

BYTE0 is B9–B16 and BYTE1 is B1–B8. The order of the data

bytes is interchanged when Hi/Lo Byte Select is low. BYTE 0

and BYTE 1 are defined in the timing diagram Figure 17. B1 is

the most significant bit of the reconstructed 16-bit data.

B1 SELECT (Pin 44) determines whether data is presented in

complementary twos complement or complementary offset bi-

nary form. Complementary twos complement data is provided

when B1 Select is LOW. OE may be used to place the data bus

into a high impedance state.

The arithmetic unit in the AD1385 saturates at all 0s or all 1s if

the input range is exceeded.

Table I.

B1 Select

Complementary Twos

Complementary Offset

Data Format

Complement

Binary

–Full-Scale Data

7FFFH

FFFFH

0 V Data

FFFFH

8000H

+Full-Scale Data

8000H

0000H

CALIBRATION (Pins 28 and 41)

Calibration corrects for linearity errors in the Reference DAC

arising from internal component mismatches or temperature

changes. It has a negligible effect on gain and offset errors, and

these should be corrected by other means. The AD1385 must

be calibrated after power-up, and recalibration is recommended

whenever the part’s temperature has changed by more than

°C. Performance degrades gracefully with temperature

changes, resulting in small but gradual decreases in SNR and

increases in distortion which may be eliminated by recalibra-

tion. Calibration codes are stored in internal RAM and are lost

when power is removed. Figures 20–22 show the effects of

uncalibrated versus calibrated operation.

Figure 20. Full-Scale Power Spectral Density after Power-

up at TCASE = +25°C Without Calibration, ±5 V Range,

16384-Point FFT, 500 kHz Sample Rate. Compare with

Figure 4.

Figure 21. Full-Scale Power Spectral Density at TCASE =

+125

°C, Calibration Performed at T

CASE = +25°C, ±5 V

Range, 16384-Point FFT, 500 kHz Sample Rate

Figure 22. Same as Figure 21 Following Recalibration at

TCASE = +125°C

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