AD7796 datasheet(16/24 Pages) AD

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AD7796/AD7797

Data Sheet

Rev. B | Page 16 of 24

ADC CIRCUIT INFORMATION

OVERVIEW

The AD7796/AD7797 are low power ADCs that incorporate a

Σ-∆ modulator, in-amp, and an on-chip digital filter intended

for measuring wide dynamic range, low frequency signals, such

as those in pressure transducers and weigh scales.

Each device has one differential input that is buffered. Figure 10

shows the basic connections required to operate the device.

AD7796/AD7797

AIN(+)

AIN(–)

INTERNAL

CLOCK

AVDD

DVDD

REFIN(–)

REFIN(+)

DOUT/RDY

DIN

SCLK

MUX

GND

CLK

Σ-Δ

ADC

VDD

×128

TEMP

SENSOR

SERIAL

INTERFACE

AND

CONTROL

LOGIC

IN+

IN–

OUT–

OUT+

Figure 10. Basic Connection Diagram

The output rate of the AD7796/AD7797 (fADC) is user-

programmable. 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 12).

The AD7796/AD7797 use slightly different filter types,

depending on the output update rate used to optimize the

rejection of quantization noise and device noise. When the

update rate is 4.17 Hz to 12.5 Hz, a Sinc3 filter and an averaging

filter are used. When the update rate is 16.7 Hz to 33.2 Hz, a

modified Sinc3 filter is used. This filter gives simultaneous

50 Hz/60 Hz rejection when the update rate equals 16.7 Hz.

A Sinc4 filter is used when the update rate is from 50 Hz to

123 Hz. Figure 11 to Figure 13 show the frequency response

of the different filter types for some of the update rates.

–20

–40

–60

–80

–100

120

100

FREQUENCY (Hz)

Figure 11. Filter Profile with Update Rate = 4.17 Hz

–20

–40

–60

–80

–100

200

180

160

140

120

100

FREQUENCY (Hz)

Figure 12. Filter Profile with Update Rate = 16.7 Hz

–20

–40

–60

–80

–100

1000

900

800

700

600

500

400

300

200

100

FREQUENCY (Hz)

Figure 13. Filter Profile with Update Rate = 50 Hz

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