AD7703

REV. D

–13–

Therefore, the two analog supplies should be individually

decoupled to AGND using 100 nF ceramic capacitors to

provide power supply noise rejection at these frequencies. The

two digital supplies should similarly be decoupled to DGND.

supply sequencing is therefore important. If separate analog and

digital supplies are used, care must be taken to ensure that the

analog supply is powered up first.

It is also important that power is applied to the AD7703 before

possibility of latch-up. If separate supplies are used for the

AD7703 and the system digital circuitry, then the AD7703

should be powered up first.

A typical scheme for powering the AD7703 from a single set of

±5 V rails is shown Figure 7. In this circuit AV

brought along separate tracks from the same +5 V supply.

Thus, there is no possibility of the digital supply coming up

before the analog supply.

SLEEP MODE

The low power standby mode is initiated by taking the SLEEP

input low, which shuts down all analog and digital circuits and

reduces power consumption to 10

µW. When coming out of

SLEEP mode it is sometimes possible (when using a crystal to

generate CLKIN, for example) to lose the calibration coeffi-

cients. Therefore, it is advisable as a safeguard to always do a

calibration cycle after coming out of SLEEP mode.

DIGITAL INTERFACE

The AD7703’s serial communications port allows easy

interfacing to industry standard microprocessors. Two different

modes of operation are available, optimized for different types

of interface.

VOLTAGE REFERENCE CONNECTIONS

range. The specified reference voltage is 2.5 V, but the AD7703

will operate with reference voltages from 1 V to 3 V with little

degradation in performance.

The reference input presents exactly the same dynamic load as

the analog input, but in the case of the reference input, source

resistance and long settling time introduce gain errors rather

than offset errors. Fortunately, most precision references have

sufficiently low output impedance and wide enough bandwidth

to settle to the required accuracy within 62 clock cycles.

The digital filter of the AD7703 removes noise from the refer-

ence input, just as it does with noise at the analog input, and

the same limitations apply regarding lack of noise rejection at

integer multiples of the sampling frequency. Note that the refer-

ence should be chosen to minimize noise below 10 Hz. The

AD7703 typically exhibits 1.6 LSB rms noise in its measure-

ments. This specification assumes a clean reference. Many

monolithic bandgap references are available which can supply

the 2.5 V needed for the AD7703. However, some of these are

not specified for noise especially in the 0.1 Hz to 10 Hz band-

width. If the reference noise in this bandwidth is excessive, it

can degrade the performance of the AD7703. Recommended

references are the AD580 and the LT1019. Both of these 2.5 V

references typically have less than 10 V p-p noise in the 0.1 Hz

to 10 Hz band.

POWER SUPPLIES AND GROUNDING

AGND is the ground reference voltage for the AD7703, and is

completely independent of DGND. Any noise riding on the

AGND input with respect to the system analog ground will

cause conversion errors. AGND should therefore be used as the

system ground and also as the ground for the analog input and

the reference voltage.

The analog and digital power supplies to the AD7703 are

independent and separately pinned out, to minimize coupling

between analog and digital sections of the device. The digital

filter will provide rejection of broadband noise on the power

supplies, except at integer multiples of the sampling frequency.