REV. A

AD7724

–12–

Input Circuits

Figures 28 and 29 show two simple circuits for bipolar mode

operation. Both circuits accept a single-ended bipolar signal

source and create the necessary differential signals at the input

to the ADC.

The circuit in Figure 28 creates a 0 V to 2.5 V signal at the

VIN(+) pins to form a differential signal around an initial bias

voltage of 1.25 V. For single-ended applications, best THD

performance is obtained with VIN(–) set to 1.25 V rather than

2.5 V. The input to the AD7724 can also be driven differen-

tially with a complementary input as shown in Figure 29.

In this case, the input common-mode voltage is set to 2.5 V.

The 2.5 V p-p full-scale differential input is obtained with a

1.25 V p-p signal at each input in antiphase. This configuration

minimizes the required output swing from the amplifier circuit

and is useful for single supply applications.

12pF

1k

1k

1/2

OP275

AIN =

1.25V

+

–

12pF

1k

110nF

R

1nF

VIN(–)

VIN(+)

REF1

REF2

110nF

DIFFERENTIAL

INPUT = 2.5V p-p

VIN(–) BIAS

VOLTAGE = 2.5V

OP07

R

1nF

1/2

OP275

+

–

1k

Figure 28. Single-Ended Analog Input for Bipolar Mode

Operation

12pF

1k

AIN =

0.625V

1k

1k

12pF

1k

1/2

OP275

110nF

R

R

1nF

VIN(–)

1nF

1/2

OP275

VIN(+)

DIFFERENTIAL

INPUT = 2.5V p-p

COMMON-MODE

VOLTAGE = 2.5V

REF1

OP07

REF2

110nF

Figure 29. Single-Ended-to-Differential Analog Input

Circuit for Bipolar Mode Operation

The 1 nF capacitors at each input store charge to aid the

amplifier settling as the input is continuously switched. A

resistor in series with the drive amplifier output and the 1 nF

input capacitor may also be used to create an antialias filter.

Clock Generation

The AD7724 contains an oscillator circuit to allow a crystal or

an external clock signal to generate the master clock for the

ADC. The connection diagram for use with the crystal is shown

in Figure 30. Consult the crystal manufacturer's recommenda-

tion for the load capacitors.

1M

XTAL

MCLK

Figure 30. Crystal Oscillator Connection

An external clock must be free of ringing and have a minimum

rise time of 5 ns. Degradation in performance can result as high

edge rates increase coupling that can generate noise in the sam-

pling process. The connection diagram for an external clock

source (Figure 31) shows a series damping resistor connected

between the clock output and the clock input to the AD7724.

The optimum resistor will depend on the board layout and the

impedance of the trace connecting to the clock input.

CLOCK

CIRCUITRY

MCLK

25–150

Figure 31. External Clock Oscillator Connection

A low phase clock should be used to generate the ADC sam-

pling clock because sampling clock jitter effectively modulates

the input signal and raises the noise floor. The sampling clock

generator should be isolated from noisy digital circuits, grounded

and heavily decoupled to the analog ground plane.

A sine wave can also be used to provide the clock (Figure 32.) A

sine wave with a voltage swing between 0.4 V p-p and 4 V p-p is

needed. XTAL_OFF is tied low and a 1 M

Ω resistor is needed

between XTAL1 and XTAL2. A 22 pF capacitor is connected

in parallel with this resistor. The sine wave is ac coupled to

XTAL1 using a 120 pF capacitor. The use of a sine wave to

generate the clock eliminates the need for a square wave clock

source which introduces noise.

1M

XTAL1

XTAL2

XTAL_OFF

22pF

SINEWAVE

INPUT

120pF

Figure 32. Using a Sine Wave Input as a Clock Source