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AD7783
(Rev.:RevB)

Read-Only, Pin Configured 24-Bit Σ-Δ ADC with Excitation Current Sources

Analog Devices 

Typical Performance Characteristics–AD7783

9

3.0

8

2.5

؎2.56V RANGE

7

6

2.0

5

VDD = 5V

1.5

VREF = 2.5V

TA = 25؇C

4

3

1.0

؎160mV RANGE

2

0.5

1

0

0

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

VREF (V)

TPC 1. Noise Distribution Histogram

TPC 2. RMS Noise vs. Reference Input

ADC CIRCUIT INFORMATION

Overview

The AD7783 incorporates a S-D ADC channel, on-chip pro-

grammable gain amplifier, and on-chip digital filtering intended

for the measurement of wide dynamic range, low frequency

signals such as those in weigh-scale, strain gage, pressure trans-

ducer, or temperature measurement applications.

This ADC input is buffered and can be programmed to have an

input voltage range of ± 160 mV or ± 2.56 V. The input channel

is configured as a fully differential input. Buffering the input

channel means that the part can accommodate significant source

impedances on the analog input and that R, C filtering (for

noise rejection or RFI reduction) can be placed on the analog

input if required. The device requires an external reference of

2.5 V nominal. Figure 3 shows the basic connections required

to operate the part.

POWER

SUPPLY

10␮F

0.1␮F

IN+

OUT–

IN–

OUT+

10k⍀

6k⍀

VDD

AD7783

AIN(+)

AIN(–)

CS

IOUT1

IOUT2

REFIN(+)

REFIN(–)

DOUT/RDY

SCLK

XTAL1

XTAL2

GND

␮CONTROLLER

32.768kHz

CRYSTAL

Figure 3. Basic Connection Diagram

The output rate of the AD7783 (fADC) equals

( ) fADC = 32.768 ¥ 103 / 69 ¥ 8 ¥ 3

while the settling time equals

tSETTLE

=

Ê

ËÁ

2ˆ

fADC ¯˜

= 2 ¥ tADC

Normal-mode rejection is the major function of the digital filter

on the AD7783. Simultaneous 50 Hz and 60 Hz rejection of

better than 60 dB is achieved as notches are placed at both 50 Hz

and 60 Hz. Figure 4 shows the filter rejection.

0

–20

–40

–60

–80

–100

–120

–140

–160

0 10 20 30 40 50 60 70 80 90 100

FREQUENCY (Hz)

OUTPUT DATA RATE = 19.8Hz

INPUT BANDWIDTH = 4.74Hz

50Hz REJECTION = –66dB, 50Hz ؎1Hz REJECTION = –60dB

60Hz REJECTION = –117dB, 60Hz ؎1Hz REJECTION = –94dB

Figure 4. Filter Profile (Filter Notches at Both

50 Hz and 60 Hz)

REV. B

–7–

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