AD7785 datasheet(21/33 Pages) AD

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AD7785

Rev. 0 | Page 20 of 32

ADC CIRCUIT INFORMATION

OVERVIEW

The AD7785 is a low power ADC that incorporates 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.

The output rate of the AD7785 (fADC) is user-programmable. The

allowable update rates, along with their 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 as notches are placed at both 50 Hz and 60 Hz with these

update rates (see Figure 14.)

The AD7785 uses 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.

DOUT/RDY

DIN

SCLK

DVDD

SERIAL

INTERFACE

AND

CONTROL

LOGIC

Σ-Δ

ADC

AD7785

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

–20

–40

–60

–80

–100

120

100

FREQUENCY (Hz)

Figure 13. Filter Profile with Update Rate = 4.17 Hz

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