EVAL-AD7793EBZ1 datasheet(20/32 Pages) AD

20 / 32 page

AD7792/AD7793

Rev. B | Page 20 of 32

ADC CIRCUIT INFORMATION

OVERVIEW

The AD7792/AD7793 are low power ADCs that incorporate a

∑-Δ modulator, a buffer, reference, in-amp, and an on-chip

digital filter intended for the measurement of wide dynamic

range, low frequency signals such as those in pressure

transducers, weigh scales, and temperature measurement

applications.

The part has three differential inputs that can be buffered or

unbuffered. The device can be operated with the internal 1.17 V

reference, or an external reference can be used. Figure 12 shows

the basic connections required to operate the part.

DOUT/RDY

DIN

SCLK

DVDD

SERIAL

INTERFACE

AND

CONTROL

LOGIC

Σ-Δ

ADC

AD7792/AD7793

AIN2(+)

REFIN(+)

REFIN(–)

AIN2(–)

AVDD

GND

MUX

BAND GAP

REFERENCE

INTERNAL

CLOCK

CLK

GND

AVDD

IN-AMP

BUF

REFIN(+) REFIN(–)

VBIAS

AIN1(+)

AIN1(–)

THERMOCOUPLE

JUNCTION

RREF

IOUT2

Figure 12. Basic Connection Diagram

The output rate of the AD7792/AD7793 (fADC) is user-program-

mable. The allowable update rates, along with their corresponding

settling times, are listed in Table 16. 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 as notches are placed at both 50 Hz and 60 Hz

with these update rates. See Figure 14.

The AD7792/AD7793 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 is from 4.17 Hz to 12.5 Hz, a Sinc3 filter, along with

an averaging filter, is used. When the update rate is from

16.7 Hz to 39 Hz, a modified Sinc3 filter is used. This filter

provides 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 242 Hz. Finally, an integrate-only filter is used

when the update rate equals 470 Hz.

Figure 13 to Figure 16 show the frequency response of the

different filter types for several update rates.

–20

–40

–60

–80

–100

120

100

FREQUENCY (Hz)

Figure 13. Filter Profile with Update Rate = 4.17 Hz

–20

–40

–60

–80

–100

200

180

160

140

120

100

FREQUENCY (Hz)

Figure 14. Filter Profile with Update Rate = 16.7 Hz

–20

–40

–60

–80

–100

3000

2500

2000

1500

1000

500

FREQUENCY (Hz)

Figure 15. Filter Profile with Update Rate = 242 Hz

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