AD7731 Analog Devices, AD7731 Datasheet - Page 28

• Manufacturer Part Number: AD7731
• Description: Low Noise, High Throughput 24-Bit Sigma-Delta ADC
• Manufacturer: Analog Devices

Specifications of AD7731

• Resolution (bits): 24bit
• # Chan: 3
• 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: DIP,SOIC,SOP

AD7731
FASTStep™ Mode (SKIP = 0, FAST = 1)
The second mode of operation of the second stage filter is in
FASTStep™ mode which enables it to respond rapidly to step
inputs even when the second stage filter is in the loop. The
FASTStep™ mode is not relevant with SKIP mode enabled.
The FASTStep™ mode is enabled by placing a 1 in the FAST
bit of the Filter Register. If the FAST bit is 0, the part continues
to process step inputs with the normal FIR filter as the second
stage filter. With FASTStep™ mode enabled, the second stage
filter will continue to process steady state inputs with the filter
in its normal FIR mode of operation. However, the part is con-
tinuously monitoring the output of the first stage filter and com-
paring it with the second-previous output. If the difference
between these two outputs is greater than a predetermined
threshold (1% of full scale), the second stage filter switches to a
simple moving average computation. This also happens when a
change in channels takes place regardless of how close the volt-
ages on the two channels are. When the change is detected, the
STDY bit of the Status Register goes to 1.
The initial number of averages in the moving average computa-
tion is either 2 (chop enabled) or 1 (chop disabled). The num-
ber of averages will be held at this value as long as the threshold
is exceeded. Once the threshold is no longer exceeded (the step
on the analog input has settled), the number of outputs used to
compute the moving average output is increased. The first and
second outputs from the first stage filter where the threshold is
no longer exceeded is computed as an average by 2, then 4
outputs with an average of 4, 8 outputs with an average of 8 and
6 outputs with an average of 16. At this time, the second stage
filter reverts back to its normal FIR mode of operation. When
the second stage filter reverts back to the normal FIR, the STDY
bit of the Status Register goes to 0.
Figure 13 gives an indication of the different responses to a step
input with FASTStep™ mode enabled and disabled. The verti-
cal axis indicates the settling of the output to the input step
change while the horizontal axis shows how many outputs it
takes for that settling to occur. The positive input step change
occurs at a time coincident with the fifth output.
Figure 13. Step Response for FASTStep™ and Normal
Operation
20000000
15000000
10000000
5000000
0
0
5
NUMBER OF OUTPUTS
10
15
20
25
–28–
In FASTStep™ mode, the part has settled to the new value
much faster. For example, with CHP = 1, the FASTStep™
mode settles to its value in two outputs while the normal mode
settling takes 23 outputs. Between the second and 23rd output,
the FASTStep™ mode produces a settled result but with addi-
tional noise compared to the specified noise level for its operat-
ing conditions. This noise level starts at approximately 3 times
the final noise converging to FIR mode performance. The com-
plete settling time to where the part is back within the specified
noise number, is the same for FASTStep™ mode and for normal
mode. When switching channels, the profile of Figure 13 will
not be seen. Since the part is synchronized when a channel
change takes place, it will not produce an output until the filter
(either FASTStep™ or FIR) is settled. Table XVIII gives an
indication of the faster settling time benefits of FASTStep™
mode.
As can be seen from Table XVIII, the FASTStep™ mode gives
a much earlier indication of where the output channel is going
and what its new value is. This feature is very useful in scanning
multiple channels where the user does not have to wait for the
FIR settling time to see if a channel has changed value. In this
case, the part can be set up with CHP = 1, SKIP = 0 and FAST
= 1. This takes advantage of the low drift, better noise immunity
benefits of the CHOP mode. When a change in channels takes
place, the part enters FASTStep™ mode and provides an output
result in 2
Note, if the FAST bit is set and the part operated in single con-
version mode, the AD7731 will continue to output results until
the STDY bit goes to 0.
Table XVIII. Time to First and Subsequent Outputs Follow-
ing Channel Change
SKIP
0
0
1
1
0
0
1
2
This O/P is fully settled.
X = Don’t Care.
0
1
0
1
0
1
CHP
1/Output Rate.
FAST
0
0
X
X
1
1
2
Time
to First O/P
24
66
3
3
3
6
SF/f
SF/f
SF/f
SF/f
SF/f
SF/f
MOD
MOD
MOD
MOD
MOD
MOD
1
Time to
SF/f
3
SF/f
3
SF/f
3
Subsequent O/Ps
SF/f
SF/f
SF/f
MOD
MOD
MOD
MOD
MOD
MOD
REV. 0
REV. A