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AD7731

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CIRCUIT DESCRIPTION

The AD7731 is a sigma-delta A/D converter with on-chip digital

filtering, intended for the measurement of wide dynamic range,

low-frequency signals such as those in strain-gage, pressure

transducer, temperature measurement, industrial control or pro-

cess control applications. It contains a sigma-delta (or charge-

balancing) ADC, a calibration microcontroller with on-chip

static RAM, a clock oscillator, a digital filter and a bidirectional

serial communications port. The part consumes 13.5 mA of

power supply current with a standby mode which consumes

only 20

µA. The part operates from a single +5 V supply. The

clock source for the part can be provided via an external clock

or by connecting a crystal oscillator or ceramic resonator across

the MCLK IN or MCLK OUT pins.

The part contains three programmable-gain fully differential

analog input channels which can be reconfigured as five pseudo-

differential inputs. The part handles a total of seven different

input ranges on all channels which are programmed via the on-

chip registers. The differential unipolar ranges are: 0 mV to

+20 mV through 0 V to +1.28 V and the differential bipolar

ranges are:

±20 mV through ±1.28 V.

The AD7731 employs a sigma-delta conversion technique to

realize up to 24 bits of no missing codes performance. The

sigma-delta modulator converts the sampled input signal into a

digital pulse train whose duty cycle contains the digital informa-

tion. A digital low-pass filter processes the output of the sigma-

delta modulator and updates the data register at a rate that can

be programmed over the serial interface. The output data from

the part is accessed over this serial interface. The cutoff fre-

quency and output rate of this filter can be programmed via on-

chip registers. The output noise performance and peak-to-peak

resolution of the part varies with gain and with the output rate

as shown in Tables I to IV.

The analog inputs are buffered on-chip, allowing the part to

handle significant source impedances on the analog input. This

means that external R, C filtering (for noise rejection or RFI

interference reduction) can be placed on the analog inputs if

required. The common-mode voltage range for the analog in-

puts comes within 1.2 V of AGND and 0.95 V of AVDD. The

reference input is also differential and the common-mode range

here is from AGND to AVDD.

The AD7731 contains a number of hardware and software

events that set or reset status flags and bits in registers. Table

XVII summarizes which blocks and flags are affected by the

different events.

Table XVII. Reset Events

Set Registers

Mode

Filter

Analog

Reset Serial

Set

RDY

Set

STDY

Event

to Default

Bits

Reset

Power-Down

Interface

Pin/Bit

Bit

Power-On Reset

Yes

000

Yes

RESET Pin

Yes

000

Yes

STANDBY Pin

As Is

Yes

Mode 011 Write

011

Yes

SYNC Pin

As Is

Yes

Mode 000 Write

000

Yes

Conversion or

New

Initial

Yes

Cal Mode Write

Value

Reset

Clock 32 1s

As Is

Yes

Data Register Read

As Is

Yes

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