Digital Filter; Sinc 4 Filter (Chop Disabled) - Analog Devices AD7195 Manual

AD7195

DIGITAL FILTER

The AD7195 offers a lot of flexibility in the digital filter. The
device has four filter options. The device can be operated
3 4
with a sinc or sinc filter, chop can be enabled or disabled, and
zero latency can be enabled. The option selected affects the
output data rate, settling time, and 50 Hz/60 Hz rejection. The
following sections describe each filter type, indicating the
available output data rates for each filter option. The filter res-
ponse along with the settling time and 50 Hz/60 Hz rejection
is also discussed.
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SINC FILTER (CHOP DISABLED)
When the AD7195 is powered up, the sinc
by default and chop is disabled. This filter gives excellent noise
performance over the complete range of output data rates. It
also gives the best 50 Hz/60 Hz rejection, but it has a long
settling time.
CHOP
Figure 24. Sinc
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Sinc Output Data Rate/Settling Time
The output data rate (the rate at which conversions are available
on a single channel when the ADC is continuously converting)
is equal to
f = f /(1024 × FS[9:0])
ADC CLK
where:
f is the output data rate.
ADC
f is the master clock (4.92 MHz nominal).
CLK
FS[9:0] is the decimal equivalent of Bit FS9 to Bit FS0 in the
mode register.
The output data rate can be programmed from 4.7 Hz to
4800 Hz; that is, FS[9:0] can have a value from 1 to 1023.
The settling time for the sinc
t = 4/f
SETTLE ADC
When a channel change occurs, the modulator and filter are
reset. The settling time is allowed to generate the first conver-
sion after the channel change. Subsequent conversions on this
channel occur at 1/f
ADC
CHANNEL A
CHANNEL
CONVERSIONS
CH A CH A CH A
Figure 25. Sinc
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4
filter is selected
ADC
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MODULATOR SINC /SINC
4
Filter (Chop Disabled)
4
filter is equal to
.
CHANNEL B
CH B CH B
f
1/
ADC
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Channel Change
When conversions are performed on a single channel and a
step change occurs, the ADC does not detect the change in
analog input. Therefore, it continues to output conversions
at the programmed output data rate. However, it is at least four
conversions later before the output data accurately reflect the
analog input. If the step change occurs while the ADC is
processing a conversion, then the ADC takes five conversions
after the step change to generate a fully settled result.
ANALOG
INPUT
ADC
OUTPUT
f
1/
ADC
Figure 26. Asynchronous Step Change in Analog Input
The 3 dB frequency for the sinc
f = 0.23 × f
3dB
Table 29 gives some examples of the relationship between the
values in Bits FS[9:0] and the corresponding output data rate
and settling time.
Table 29. Examples of Output Data Rates and the
Corresponding Settling Time
FS[9:0] Output Data Rate (Hz)
480 10
96 50
80 60
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Sinc Zero Latency
Zero latency is enabled by setting the single bit (Bit 11) in the
mode register to 1. With zero latency, the complete settling time
is allowed for each conversion. Therefore, the conversion time
when converting on a single channel or when converting on
several channels is constant. The user does not need to consider
the effects of channel changes on the output data rate.
The output data rate equals
f = 1/t
ADC
where:
f is the output data rate.
ADC
f is the master clock (4.92 MHz nominal).
CLK
FS[9:0] is the decimal equivalent of Bit FS9 to Bit FS0 in the
mode register.
CH B
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filter is equal to
ADC
Settling Time (ms)
400
80
66.6
= f /(4 × 1024 × FS[9:0])
SETTLE CLK
FULLY
SETTLED