REV. C

–12–

AD7713

Figure 2 gives similar information to that outlined in Table I. In this plot, the output rms noise is shown for the full range of avail-

able cutoff frequencies rather than for some typical cutoff frequencies as in Tables I and II. The numbers given in these plots are

typical values at 25

°C.

10

1000

10000

100

NOTCH FREQUENCY — Hz

10000

100

0.1

1000

10

1

GAIN OF 1

GAIN OF 2

GAIN OF 4

GAIN OF 8

Figure 2a. Plot of Output Noise vs. Gain and Notch

Frequency (Gains of 1 to 8)

CIRCUIT DESCRIPTION

The AD7713 is a sigma-delta A/D converter with on-chip digi-

tal filtering, intended for the measurement of wide dynamic

range, low frequency signals such as those in industrial control

or process 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 bidirec-

tional serial communications port.

The part contains three analog input channels, two program-

mable gain differential input and one programmable gain high-

level single-ended input. The gain range on both inputs is from

1 to 128. For the AIN1 and AIN2 inputs, this means that the

input can accept unipolar signals of between 0 mV to +20 mV

and 0 V to +2.5 V or bipolar signals in the range from

±20 mV

to

±2.5 V when the reference input voltage equals +2.5 V. The

input voltage range for the AIN3 input is +4

× V

is 0 V to + 10 V with the nominal reference of +2.5 V and a

gain of 1. The input signal to the selected analog input channel

is continuously sampled at a rate determined by the frequency

of the master clock, MCLK IN, and the selected gain (see

Table III). A charge balancing A/D converter (Sigma-Delta

Modulator) converts the sampled signal into a digital pulse train

whose duty cycle contains the digital information. The program-

mable gain function on the analog input is also incorporated in

this sigma-delta modulator with the input sampling frequency

filter processes the output of the sigma-delta modulator and up-

dates the output register at a rate determined by the first notch

frequency of this filter. The output data can be read from the

serial port randomly or periodically at any rate up to the output

register update rate. The first notch of this digital filter (and

hence its –3 dB frequency) can be programmed via an on-chip

control register. The programmable range for this first notch

frequency is from 1.952 Hz to 205.59 Hz, giving a programma-

ble range for the –3 dB frequency of 0.52 Hz to 53.9 Hz.

The basic connection diagram for the part is shown in Figure 3.

This shows the AD7713 in the external clocking mode with

NOTCH FREQUENCY — Hz

10

1000

10000

100

1000

10

0.1

100

1

GAIN OF 16

GAIN OF 32

GAIN OF 64

GAIN OF 128

Figure 2b. Plot of Output Noise vs. Gain and Notch

Frequency (Gain of 16 to 128)

from the analog +5 V supply. Some applications will have sepa-

cases the analog supply will exceed the +5 V digital supply (see

Power Supplies and Grounding section).

REF IN(+)

AIN1(+)

AIN1(–)

AIN3

AV

DD

AGND

DGND

MCLK IN

MCLK OUT

SCLK

SDATA

DRDY

TFS

RFS

REF IN(–)

SYNC

A0

ANALOG +5V

SUPPLY

10µF

0.1µF

0.1µF

AD7713

DIFFERENTIAL

ANALOG

INPUT

SINGLE–ENDED

ANALOG INPUT

ANALOG

GROUND

DIGITAL

GROUND

DATA

READY

RECEIVE

(READ)

SERIAL

DATA

SERIAL

CLOCK

TRANSMIT

(WRITE)

MODE

STANDBY

ADDRESS

INPUT

AIN2(+)

AIN2(–)

{

DIFFERENTIAL

ANALOG INPUT

+2.5V

REFERENCE

{

Figure 3. Basic Connection Diagram

The AD7713 provides a number of calibration options which

can be programmed via the on-chip control register. A calibra-

tion cycle may be initiated at any time by writing to this control

register. The part can perform self-calibration using the on-chip

calibration microcontroller and SRAM to store calibration

parameters. Other system components may also be included in

the calibration loop to remove offset and gain errors in the input

channel using the system calibration mode. Another option is a

background calibration mode where the part continuously per-

forms self-calibration and updates the calibration coefficients.

Once the part is in this mode, the user does not have to worry

about issuing periodic calibration commands to the device or

asking the device to recalibrate when there is a change in the

ambient temperature or power supply voltage.