AD7724 Analog Devices, AD7724 Datasheet - Page 12

• Manufacturer Part Number: AD7724
• Description: Dual, 7th-Order, Sigma-Delta Modulator
• Manufacturer: Analog Devices

Specifications of AD7724

• Resolution (bits): 15bit
• # Chan: 2
• Sample Rate: 30MSPS
• Interface: Ser
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: 2.5V p-p,Uni 2.5V
• Adc Architecture: Sigma-Delta Modulator
• Pkg Type: QFP

AD7724
Input Circuits
Figures 12 and 13 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 12 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 13.
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.
1.25V
AIN =
0.625V
AIN =
1k
1k
R
R
12pF
–
OP275
+
R
R
1/2
12pF
OP275
1/2
1k
1k
1k
1k
12pF
–
+
OP275
OP07
1/2
OP275
12pF
OP07
1k
1/2
+
–
1k
110nF
110nF
1nF
1nF
110nF
110nF
1nF
1nF
DIFFERENTIAL
INPUT = 2.5V p-p
VIN(+)
VIN(–)
VIN(–) BIAS
VOLTAGE = 2.5V
REF1
REF2
DIFFERENTIAL
INPUT = 2.5V p-p
COMMON-MODE
VOLTAGE = 2.5V
VIN(–)
VIN(+)
REF1
REF2
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 14. Consult the crystal manufacturer’s recommenda-
tion for the load capacitors.
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 15) 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.
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 16.) 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 MΩ 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.
SINEWAVE
INPUT
CIRCUITRY
CLOCK
120pF
22pF
XTAL
25–150
1M
1M
MCLK
MCLK
XTAL_OFF
XTAL1
XTAL2