AD7765 Analog Devices, AD7765 Datasheet - Page 25

• Manufacturer Part Number: AD7765
• Description: 24-Bit, 156 kSPS, 112 dB Sigma-Delta ADC with On-Chip Buffers and Serial Interface
• Manufacturer: Analog Devices

Specifications of AD7765

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 40MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: 6.5 V p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

CLOCKING THE AD7765
The AD7765 requires an external low jitter clock source. This
signal is applied to the MCLK pin. An internal clock signal
(ICLK) is derived from the MCLK input signal. The ICLK
controls the internal operation of the AD7765. The maximum
ICLK frequency is
For output data rates equal to those used in audio systems, a
12.288
output data rates of 96 kHz and 48 kHz are achievable with this
ICLK frequency.
MCLK JITTER REQUIREMENTS
The MCLK jitter requirements depend on a number of factors
and are given by
where:
OSR = oversampling ratio = f
f
SNR (dB) = target SNR.
Example 1
This example can be taken from Table 6, where:
ODR = 156.25 kHz.
f
f
SNR = 104 dB.
This is the maximum allowable clock jitter for a full-scale,
78.625 kHz input tone with the given ICLK and output
data rate.
Example 2
This second example can also be taken from Table 6, where:
ODR = 48 kHz.
f
f
SNR = 109 dB.
IN
ICLK
IN
ICLK
IN
= maximum input frequency.
(max) = 78.625 kHz.
(max) = 19.2 kHz.
= 20 MHz.
= 12.288 MHz.
ICLK = MCLK/2
t
t
j
j
(
(
rms
rms
MHz ICLK frequency can be used. As shown in Table 6,
)
)
=
=
2
2
×
×
π
π
×
×
20 MHz.
78
f
IN
.
625
×
OSR
10
128
×
10
SNR
To generate the ICLK,
ICLK
3
20
×
/ODR.
(
10
dB
. 5
)
35
=
102
.
29
ps
Rev. A | Page 25 of 32
The input amplitude also has an effect on these jitter figures.
For example, if the input level is 3 dB below full scale, the
allowable jitter is increased by a factor of √2, increasing the first
example to 144.65 ps rms. This happens when the maximum
slew rate is decreased by a reduction in amplitude.
Figure 43 and Figure 44 illustrate this point, showing the maxi-
mum slew rate of a sine wave of the same frequency but with
different amplitudes.
Figure 44. Maximum Slew Rate of the Same Frequency Sine Wave as in
t
–0.5
–0.5
–1.0
–1.0
j
1.0
0.5
1.0
0.5
(
rms
0
0
)
=
Figure 43. Maximum Slew Rate of a Sine Wave
2
×
Figure 43 with an Amplitude of 1 V p-p
π
×
19
with an Amplitude of 2 V p-p
2 .
256
×
10
3
×
10
. 5
45
=
470
ps
AD7765