20-Bit, Pin-Programmable,

Low Power Sigma-Delta ADC

AD7781

Rev. 0

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FEATURES

Pin-programmable filter response

Update rate: 10 Hz or 16.7 Hz

Pin-programmable in-amp gain

Pin-programmable power-down and reset

Status function

Internal clock oscillator

Internal bridge power-down switch

Current

115 μA typical (gain = 1)

330 μA typical (gain = 128)

Simultaneous 50 Hz/60 Hz rejection

Power supply: 2.7 V to 5.25 V

−40°C to +105°C temperature range

Independent interface power supply

Packages

14-lead, narrow body SOIC

16-lead TSSOP

2-wire serial interface (read-only device)

SPI compatible

Schmitt trigger on SCLK

APPLICATIONS

Weigh scales

Pressure measurement

Industrial process control

Portable instrumentation

FUNCTIONAL BLOCK DIAGRAM

A

G = 1

OR 128

20-BIT Σ-Δ

ADC

DOUT/RDY

GND

AIN(+)

AIN(–)

SCLK

FILTER

REFIN(+)

GAIN

REFIN(–)

INTERNAL

CLOCK

AD7781

PDRST

BPDSW

Figure 1.

Table 1.

Parameter

Gain = 128

Gain = 1

Output Data Rate

10 Hz

16.7 Hz

10 Hz

16.7 Hz

RMS Noise

C Grade

44 nV

65 nV

2.4 μV

2.7 μV

B Grade

55 nV

90 nV

2.4 μV

2.7 μV

P-P Resolution

C Grade

17.6

17.1

18.8

18.7

B Grade

17.3

16.6

18.8

18.7

Settling Time

300 ms

120 ms

300 ms

120 ms

GENERAL DESCRIPTION

The AD7781 is a complete, low power front-end solution for

bridge sensor products, including weigh scales, strain gages,

and pressure sensors. It contains a precision, low power, 20-bit

sigma-delta (Σ-Δ) ADC, an on-chip, low noise programmable

gain amplifier (PGA), and an on-chip oscillator.

Consuming only 330 μA, the AD7781 is particularly suitable for

portable or battery-operated products where very low power is

required. The AD7781 also has a power-down mode that allows

the user to switch off the power to the bridge sensor and power

down the AD7781 when not converting, thus increasing the

battery life of the product.

For ease of use, all the features of the AD7781 are controlled by

dedicated pins. Each time that a data read occurs, eight status bits

are appended to the 20-bit conversion. These status bits contain a

pattern sequence that can be used to confirm the validity of the

serial transfer.

The on-chip PGA has a gain of 1 or 128, supporting a full-scale

differential input of ±5 V or ±39 mV. The device has two filter

response options. The filter response at the 16.7 Hz update rate

provides superior dynamic performance. The settling time is

120 ms at this update rate. At the 10 Hz update rate, the filter

response provides better than −45 dB of stop-band attenuation.

In load cell applications, this stop-band rejection is useful to

reject low frequency mechanical vibrations of the load cell. The

settling time is 300 ms at this update rate. Simultaneous 50 Hz/

60 Hz rejection occurs at both the 10 Hz and 16.7 Hz update rates.

The AD7781 operates with a power supply from 2.7 V to 5.25 V.

It is available in a narrow body, 14-lead SOIC package and in a

16-lead TSSOP package.