AD7730L Analog Devices, AD7730L Datasheet - Page 28

• Manufacturer Part Number: AD7730L
• Description: CMOS, 24-Bit Low Power Sigma-Delta ADC for Bridge Transducer Applications
• Manufacturer: Analog Devices

Specifications of AD7730L

• Resolution (bits): 24bit
• # Chan: 2
• Sample Rate: 5MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p,(Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOIC,SOP

AD7730/AD7730L
Because of this effect, care should be taken in choosing an out-
put rate that is close to the line frequency in the application. If
the line frequency is 50 Hz, an output update rate of 50 Hz
should not be chosen as it will significantly reduce the AD7730’s
line frequency rejection (the 50 Hz will appear as a dc effect
with only 6 dB attenuation). Choosing an output rate of 55 Hz
will result in a 6 dB—attenuated aliased frequency of 5 Hz with
only a further 25 dB attenuation based on the filter profile. This
number is based on the filter roll-off and Figure 11 can be used
as a reference by dividing the frequency scale by a factor of 4.
Choosing 57 Hz as the output rate will give better than 90 dB
attenuation of the aliased line frequency which appears as a
7 Hz signal. Similarly, multiples of the line frequency should be
avoided as the output rate because harmonics of the line fre-
quency will not be fully attenuated. The programmability of the
AD7730’s output rate should allow the user to readily choose an
output rate that overcomes this issue. An alternative is to use
the part in nonchop mode.
Figure 13 shows the frequency response for the AD7730 with
the second stage filter set for normal FIR operation, chop mode
disabled, the decimal equivalent of the word in the SF bits set to
1536 and a master clock frequency of 4.9152 MHz. The response
is analogous to that of Figure 11, with the three-times-larger SF
word producing the same 200 Hz output rate. Once again, the
response will scale proportionally with master clock frequency.
The response is shown from dc to 100 Hz. The rejection at
50 Hz
Figure 13. Detailed Full Frequency Response of AD7730
(Second Stage Filter as Normal FIR, Chop Disabled)
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
1 Hz, and 60 Hz
0
0
10
20
30
FREQUENCY – Hz
1 Hz is better than 88 dB.
40
50
60
70
80
90
100
–28–
The –3 dB frequency for the frequency response of the AD7730
with the second stage filter set for normal FIR operation and
chop mode enabled, is determined by the following relationship:
In this case, f
tion is greater than 64.5 dB, is determined by:
In this case, f
Figure 14 shows the frequency response for the same set of
conditions as for Figure 13, but in this case the response is
shown out to 600 Hz. This plot is comparable to that of Figure
12. The most notable difference is the absence of the peaks in
the response at 200 Hz and 400 Hz. As a result, interference at
these frequencies will be effectively eliminated before being
aliased back to dc.
Figure 14. Expanded Full Frequency Response of AD7730
(Second Stage Filter as Normal FIR, Chop Disabled)
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
3 dB
3 dB
50 100 150 200 250 300 350 400
= 7.8 Hz and the stop band, where the attentua-
= 28 Hz.
f
f
STOP
3dB
0.039
0.14
FREQUENCY – Hz
f
f
CLK IN
CLK IN
16
16
SF
SF
450
1
1
500 550 600
REV. A