REV. 0

AD7782

–7–

Typical Performance Characteristics

8

7

0

6

5

4

3

2

1

9

TPC 1. Noise Distribution Histogram

ADC CIRCUIT INFORMATION

Overview

The AD7782 incorporates an analog multiplexer with a

Σ-∆ ADC

channel, on-chip programmable 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 transducer, 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 channels

are configured as two fully differential inputs. Buffering the input

channels means that the part can accommodate significant source

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

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

inputs if required. The device requires an external reference of

+2.5 V nominal. Figure 3 shows the basic connections required

to operate the part.

AD7782

REFIN(+)

REFIN(–)

AIN1(–)

AIN1(+)

AIN2(–)

AIN2(+)

XTAL1

XTAL2

DOUT/

RDY

CS

SCLK

GND

POWER

SUPPLY

10 F

0.1 F

CONTROLLER

32.768kHz

CRYSTAL

6k

10k

IN–

OUT+

OUT–

IN+

Figure 3. Basic Connection Diagram

× ×

()

32 768

10

69

8

3

3

./

while the settling time equals:

t

f

t

SETTLE

ADC

ADC

=









=×

2

2

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

the AD7782. 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.

FREQUENCY – Hz

0

0

10

406080

100

–40

–80

–120

–140

–160

–20

–60

–100

20

30

50

70

90

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)

2.5

0

1.0

3.0

2.5

2.0

1.5

3.5

5.0

4.5

4.0

2.0

1.5

1.0

0.5

3.0

160mV RANGE

2.56V RANGE

TPC 2. RMS Noise vs. Reference Input