CS5521/22/23/24/28
perform a system gain calibration. In either case,
the calibration signals must be within the specified
calibration limits for each specific calibration step
(refer to the ‘System Calibration Specifications’ in
If a system gain
calibration is performed the following conditions
must be met:
External
Connections
+
AIN+
0V +
-
AIN-
-
X20
-
+
2) The 1’s density of the modulator must not be
greater than 80 percent (the input to the
∆Σ
modulator must not exceed the maximum input
which Table 1 specifies).
3) The input must not be so small relative to the
range chosen that the resulting gain register’s
content, decoded in decimal, exceeds
3.9999998 (see the discussion of operating lim-
its on input span under the
and
sections).
This requires the full scale input voltage to the
modulator to be at least 25 percent of the nom-
inal value.
The converter’s input ranges were chosen to guar-
antee gain calibration accuracy to 1 LSB
16
or 16
LSB
24
when system gain calibration is performed.
This is useful when a user wants to manually scale
the full scale range of the converter and maintain
accuracy. For example, if a gain calibration is per-
formed with a 2.5 V full scale voltage and a 1.25 V
input range is desired, the user can read the con-
tents of the gain register, shift the register contents
left by 1 bit, and then write the result back to the
gain register. This multiples the gain by 2.
Assuming a system can provide two known voltag-
es, the following equations allow the user to manu-
ally compute the calibration register’s values based
on two uncalibrated conversions (see note). The
offset and gain calibration registers are used to ad-
just a typical conversion as follows:
Rc = (Ru + Co) * Cg / 2
22
.
Calibration can be performed using the following
equations:
Co = (Rc0/G - Ru0)
Cg = 2
22
* G
where G = (Rc1 - Rc0)/(Ru1-Ru0).
Note:
Uncalibrated conversions imply that the gain and off-
set registers are at default {gain register = 0x400000
(Hex) and offset register = 0x000000 (Hex)}.
CM +
-
Figure 15. System Calibration of Offset (High Ranges)
External
Connections
+
AIN+
Full Scale +
-
-
CM
+
-
+
X20
-
AIN-
Figure 16. System Calibration of Gain (Low Ranges)
External
Connections
+
AIN+
Full Scale
+
-
-
CM
+
-
+
X20
-
AIN-
Figure 17. System Calibration of Gain (High Ranges)
1) Full-scale input must not saturate the 20X in-
strumentation amplifier, if the calibration is on
an input range where the instrumentation am-
plifier is involved.
DS317F2
33
CIRRUS [ CIRRUS LOGIC ]
