AD7798 datasheet(18/28 Pages) AD

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AD7798/AD7799

Data Sheet

Rev. B | Page 18 of 28

ADC CIRCUIT INFORMATION

DOUT/RDY

DIN

SCLK

DVDD

SERIAL

INTERFACE

AND

CONTROL

LOGIC

Σ-Δ

ADC

AD7798/AD7799

AIN2(+)

AIN2(–)

AIN1(+)

AIN1(–)

AVDD

GND

MUX

INTERNAL

CLOCK

GND

AVDD

REFIN(–)

REFIN(+)

PSW

IN-AMP

REFERENCE

DETECT

IN+

IN–

OUT–

OUT+

AVDD

Figure 11. Basic Connection Diagram

OVERVIEW

The AD7798/AD7799 are low power ADCs that each incorporate

a ∑-∆ modulator, a buffer, an in-amp, and on-chip digital filtering

intended for the measurement of wide dynamic range, low

frequency signals, such as those in pressure transducers and

weigh scales.

Each part has three differential inputs that can be buffered or

unbuffered. The reference is provided by an external reference

source. Figure 11 shows the basic connections required to

operate the parts.

The output rate of the AD7798/AD7799 (fADC) is user-program-

mable. The allowable update rates, along with the corresponding

settling times, are listed in Table 14. Normal mode rejection is

the major function of the digital filter. Simultaneous 50 Hz and

60 Hz rejection is optimized when the update rate equals 16.7 Hz

or less, because notches are placed at both 50 Hz and 60 Hz with

these update rates (see Figure 13).

The AD7798/AD7799 use slightly different filter types,

depending on the output update rate, so that the rejection of

quantization noise and device noise is optimized. When the

update rate ranges from 4.17 Hz to 12.5 Hz, a sinc3 filter, along

with an averaging filter, is used. When the update rate ranges

from 16.7 Hz to 39 Hz, a modified sinc3 filter is used. This filter

gives simultaneous 50 Hz and 60 Hz rejection when the update

rate equals 16.7 Hz. A sinc4 filter is used when the update rate

ranges from 50 Hz to 242 Hz. Finally, an integrate-only filter is

used when the update rate equals 470 Hz. Figure 12 through

Figure 15 show the frequency responses of the different filter

types for a few of the update rates.

–20

–40

–60

–80

–100

120

100

FREQUENCY (Hz)

Figure 12. Filter Profile with Update Rate = 4.17 Hz

–20

–40

–60

–80

–100

200

180

160

140

120

100

FREQUENCY (Hz)

Figure 13. Filter Profile with Update Rate = 16.7 Hz

AD7798 Datasheet(PDF) 18 Page - Analog Devices

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