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AD7707 Datasheet(PDF) 32 Page - Analog Devices

Part No. AD7707
Description  3-Channel 16-Bit, Sigma-Delta ADC
Download  53 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7707 Datasheet(HTML) 32 Page - Analog Devices

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AD7707
Rev. B | Page 31 of 52
MD0 bits in the setup register return to 0, 0. This gives the
earliest indication that the calibration sequence is complete. The
DRDY line goes high when calibration is initiated and does not
return low until there is a valid new word in the data register.
The duration time from the calibration command being issued
to DRDY going low is 4 × 1/output rate as the part performs a
normal conversion on the analog input voltage before DRDY
goes low. If DRDY is low before (or goes low during) the
calibration command write to the setup register, it may take up
to one modulator cycle (MCLK IN/128) before DRDY goes
high to indicate that calibration is in progress. Therefore, DRDY
should be ignored for up to one modulator cycle after the last
bit is written to the setup register in the calibration command.
In the unipolar mode, the system calibration is performed between
the two endpoints of the transfer function; in the bipolar mode,
it is performed between midscale (zero differential voltage) and
positive full scale.
The fact that the system calibration is a two-step calibration offers
another feature. After the sequence of a full system calibration
has been completed, additional offset or gain calibrations can
be performed by themselves to adjust the system zero reference
point or the system gain. Calibrating one of the parameters,
either system offset or system gain, does not affect the other
parameter.
System calibration can also be used to remove any errors from
source impedances on the analog input when the part is used in
unbuffered mode. A simple R, C antialiasing filter on the front
end may introduce a gain error on the analog input voltage, but
the system calibration can be used to remove this error.
SPAN AND OFFSET LIMITS ON THE LOW LEVEL
INPUT CHANNELS, AIN1 AND AIN2
Whenever a system calibration mode is used, there are limits on
the amount of offset and span that can be accommodated. The
overriding requirement in determining the amount of offset
and gain that can be accommodated by the part is the require-
ment that the positive full-scale calibration limit is <1.05 ×
VREF/gain. This allows the input range to go 5% above the
nominal range. The built-in headroom in the AD7707’s analog
modulator ensures that the part will still operate correctly with a
positive full-scale voltage that is 5% beyond the nominal.
The input span in both the unipolar and bipolar modes has a
minimum value of 0.8 × VREF/gain and a maximum value of 2.1
× VREF/gain. However, the span (which is the difference between
the bottom of the AD7707’s input range and the top of its input
range) has to take into account the limitation on the positive
full-scale voltage. The amount of offset that can be accommo-
dated depends on whether the unipolar or bipolar mode is
being used. Once again, the offset has to take into account the
limitation on the positive full-scale voltage. In unipolar mode,
there is considerable flexibility in handling negative offsets. In
both unipolar and bipolar modes, the range of positive offsets
that can be handled by the part depends on the selected span.
Therefore, in determining the limits for system zero-scale and
full-scale calibrations, the user must ensure that the offset range
plus the span range does exceed 1.05 × VREF/gain. This is best
illustrated with the following examples.
If the part is used in unipolar mode with a required span of 0.8 ×
VREF/gain, the offset range the system calibration can handle is
from −1.05 × VREF/gain to +0.25 × VREF/gain. If the part is used
in unipolar mode with a required span of VREF/gain, the offset
range the system calibration can handle is from −1.05 × VREF/gain
to +0.05 × VREF/gain. Similarly, if the part is used in unipolar
mode and required to remove an offset of 0.2 × VREF/gain, the
span range the system calibration can handle is 0.85 × VREF/gain.
AD7707 LOW LEVEL
INPUT CHANNEL
INPUT RANGE
(0.8 × VREF/GAIN TO
2.1 × VREF/GAIN)
UPPER LIMIT ON
AD7707 INPUT VOLTAGE
NOMINAL ZERO-
SCALE POINT
OFFSET CALIBRATIONS MOVE
INPUT RANGE UP OR DOWN
LOWER LIMIT ON
AD7707 INPUT VOLTAGE
–1.05 × VREF/GAIN
0V DIFFERENTIAL
GAIN CALIBRATIONS EXPAND
OR CONTRACT THE
AD7707 INPUT RANGE
1.05 × VREF/GAIN
Figure 17. Span and Offset Limits for Low Level Input Channels,
AIN1 and AIN2
If the part is used in bipolar mode with a required span of
±0.4 × VREF/gain, the offset range the system calibration can
handle is from –0.65 × VREF/gain to +0.65 × VREF/gain. If the
part is used in bipolar mode with a required span of ±VREF/gain,
then the offset range that the system calibration can handle is
from −0.05 × VREF/gain to +0.05 × VREF/gain. Similarly, if the
part is used in bipolar mode and required to remove an offset of
±0.2 × VREF/gain, the span range the system calibration can handle
is ±0.85 × VREF/gain. Figure 17 shows a graphical representation of
the span and offset limits for the low level input channels.
SPAN AND OFFSET LIMITS ON THE HIGH LEVEL
INPUT CHANNEL AIN3
The exact same reasoning
for low level input channels can
applied to the high level input channel. When using the high
level channel, the attenuator provides an attenuation factor of 8.
All span and offset limits should be multiplied by a factor of 8.
Therefore, the range of input span in both the unipolar and
bipolar modes has a minimum value of 6.4 × VREF/gain and a
maximum value of 16.8 × VREF/gain. The offset range plus the
span range cannot exceed 8.4 × VREF/gain.


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