AD7731BR-REEL Analog Devices Inc, AD7731BR-REEL Datasheet - Page 25

• Manufacturer Part Number: AD7731BR-REEL
• Description: IC,Data Acquisition Signal Conditioner,3-CHANNEL,24-BIT,SOP,24PIN,PLASTIC
• Manufacturer: Analog Devices Inc

Specifications of AD7731BR-REEL

• Rohs Status: RoHS non-compliant
• Number Of Bits: 24
• Sampling Rate (per Second): 6.4k
• Data Interface: DSP, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 125mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SOIC (0.300", 7.50mm Width)
• For Use With: EVAL-AD7731EBZ - BOARD EVALUATION FOR AD7731

REV. A
The operation mode can be changed to achieve optimum per-
formance in various applications. The CHP bit should generally
be set to 0 when using the AD7731 in applications where higher
throughput rates are a concern or in applications where the
reduced rejection at the chopping frequency in chop mode is an
issue. The part should be operated with CHP = 1 when drift,
noise rejection and optimum EMI rejection are important crite-
ria in the application.
The output update rate of the AD7731 is programmed using the
SF bits of the Filter Register. With CHP = 0, the output update
is determined by the relationship:
where SF is the decimal equivalent of the data loaded to the SF
bits of the Filter Register and f
and is 1/16th of the master clock frequency.
With CHP = 1, the output update is determined by the relation-
ship:
where SF is the decimal equivalent of the data loaded to the SF
bits of the Filter Register and f
and is 1/16th of the master clock frequency.
Thus for a given SF word the output rate from the AD7731 is
three times faster with CHP = 0 than CHP = 1.
The various filter stages and options are discussed in the follow-
ing sections.
First Stage Filter/SKIP Mode Enabled (SKIP = 1)
With SKIP mode enabled, the only filtering on the part is the
first stage filter. The frequency response for this first stage filter
is shown in Figure 8. The response of this first stage filter is
similar to that of an averaging filter but with a sharper roll-off.
With CHP = 0, the output rate for the filter corresponds with
the positioning of the first notch of the filter’s frequency re-
sponse. Thus, for the plot of Figure 8 where the output rate is
600 Hz (f
the filter is at 600 Hz. With CHP = 1, the magnitude response
is the same as in Figure 8 but in this case, the output rate is
1/3rd the output rate so for the example shown in Figure 8 the
output data rate is 200 Hz. The notches of this sinc
quency response are repeated at multiples of the first notch. The
filter provides attenuation of better than 100 dB around these
notches. Programming a different cutoff frequency via SF0 –
SF11 does not alter the profile of the filter response; it simply
changes the location of the notches. The –3 dB frequency for both
Chop and Nonchop modes is defined as:
Nonchop Mode (SKIP = 1, CHP = 0)
With CHP = 0, the input chopping on the AD7731 is disabled
and any offset content in the samples to the first stage filter are
all of the same polarity. When using the part in SKIP mode, the
user can take the output from the AD7731 directly. Time to the
first output for the part is 3 1/Output Rate in this mode. Table
XVIII summarizes the settling time and subsequent throughput
rate for the various different modes.
REV. 0
CLK IN
Output Rate
Output Rate
= 4.9152 MHz and SF = 512), the first notch of
f
3 dB
f
0.262
MOD
f
MOD
MOD
MOD
3 SF
f
is the modulator frequency
is the modulator frequency
SF
MOD
1
1
CHP
SF
1
CHP
0
3
1
filter fre-
–25–
Chop Mode (SKIP = 1, CHP = 1)
With CHP = 1, the AD7731 alternatively reverses the ADC
inputs, producing an output which contains the channel offset
when not reversed and the negative of the offset when reversed.
As a result, when operating in SKIP mode, the user has to take
two subsequent outputs from the AD7731 and average them to
produce a valid output from the first stage filter. While operat-
ing in this mode gives the benefits of chopping without the
longer settling time associated with the 22-tap FIR filter, care
should be taken with input signals near positive full-scale or
negative full-scale (zero-scale in unipolar mode). Since the
calibration coefficients are generated for the averaged offset, and
not for the individual offsets represented in each sample, one of
the two samples in the pair may record an all 1s or all 0s read-
ing. If this happens it will result in an error in the averaged
reading. Time to first output for the part is 1/Output Rate in
this mode. However, since the user really needs two outputs to
derive a correct chopped result, the time to get two outputs for
averaging is 2
settling time and subsequent throughput rate for the various
different modes. If the user wants the benefits of chopping with-
out the longer settling time associated with the 22-tap FIR filter,
it is recommended that the part be used in FASTStep™ mode.
Second Stage Filter
With SKIP mode disabled, the second stage filter is included in
the signal processing. This second stage filter produces a differ-
ent response depending on the CHP and FAST bits.
Normal FIR Operation (SKIP = 0)
The normal mode of operation of the second stage filter is as a
22-tap low-pass FIR filter. This second stage filter processes the
output of the first stage filter and the net frequency response of
the filter is simply a product of the filter response of both filters.
The overall filter response of the AD7731 is guaranteed to have
no overshoot.
Figure 8. SKIP Mode Frequency Response (SKIP = 1,
SF = 512)
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
200
1/Output Rate. Table XVIII summarizes the
400
600
FREQUENCY – Hz
800
1000
1200 1400
AD7731
1600
1800