Datasheet

AD7731
–22–
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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: 0mV 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 AV
DD
. The
reference input is also differential and the common-mode range
here is from AGND to AV
DD
.
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 Yes Yes Yes Yes
RESET Pin Yes 000 Yes No Yes Yes Yes
STANDBY Pin No As Is Yes Yes No Yes Yes
Mode 011 Write No 011 Yes Yes No Yes Yes
SYNC Pin No As Is Yes No No Yes Yes
Mode 000 Write No 000 Yes No No Yes Yes
Conversion or No New Initial No No Yes Yes
Cal Mode Write Value Reset
Clock 32 1s No As Is No No Yes Yes Yes
Data Register Read No As Is No No No Yes No
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