AD7888 Ver la hoja de datos (PDF) - Analog Devices

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AD7888

2.7 V to 5.25 V, Micropower, 8-Channel, 125 kSPS, 12-Bit ADC in 16-Lead TSSOP

Analog Devices 

AD7888

CONTROL REGISTER

The Control Register on the AD7888 is an 8-bit, write-only register. Data is loaded from the DIN pin of the AD7888 on the rising

edge of SCLK. The data is transferred on the DIN line at the same time as the conversion result is read from the part. This requires

16 serial clocks for every data transfer. Only the information provided on the first 8 rising clock edges (after CS falling edge) is loaded

to the Control Register. MSB denotes the first bit in the data stream. The bit functions are outlined in Table I. The default contents

of the Control Register on power-up is all zeros.

MSB

Table I. Control Register Bit Function Description

DONTC ZERO ADD2 ADD1 ADD0

REF

PM1

PM0

Bit Mnemonic

7 DONTC

6 ZERO

5 ADD2

4 ADD1

3 ADD0

2 REF

1, 0 PM1, PM0

Comment

Don’t Care. The value written to this bit of the Control Register is a don’t care, i.e., it doesn’t matter if the bit is

0 or 1.

A zero must be written to this bit to ensure correct operation of the AD7888.

These three address bits are loaded at the end of the present conversion sequence and select which analog input

channel is converted for the next conversion. The selected input channel is decoded as shown in Table II.

Reference Bit. With a 0 in this bit, the on-chip reference is enabled. With a 1 in this bit, the on-chip reference

is disabled. To obtain best performance from the AD7888, the internal reference should be disabled when

using an externally applied reference source. (See On-Chip Reference section.)

Power Management Bits. These two bits decode the mode of operation of the AD7888 as shown in Table III.

PERFORMANCE CURVES

Figure 2 shows a typical FFT plot for the AD7888 at 100 kHz

sample rate and 10 kHz input frequency.

–10

4096 POINT FFT

SAMPLING

100kSPS

fIN = 10kHz

–30

SNR = 70dB

–50

–70

–90

–110

0

12.21

24.41

36.62

FREQUENCY – kHz

Figure 2. Dynamic Performance

48.83

Figure 3 shows a typical plot for the SNR vs. frequency for a

5 V supply and with a 5 V external reference.

73.0

72.5

VDD = 5V

5V EXT REFERENCE

72.0

71.5

71.0

0

10.89

21.14

31.59

INPUT FREQUENCY – kHz

Figure 3. SNR vs. Input Frequency

42.14

REV. C

–7–

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