Cirrus Logic AN150 - Manual (Page 8)

AN150
8 AN150REV2
What is the difference between a “self” cali-
bration and a “system” calibration?
A self calibration uses voltages that are available to
the converter to perform calibrations, and does not
take into account any system-level effects. The
converter performs a self offset calibration by dis-
connecting the AIN+ and AIN- inputs of the speci-
fied channel, and shorting them to the common-
mode internal to the ADC. The converter then does
a conversion and computes a value for the offset
register. A self gain calibration is performed by dis-
connecting the AIN+ and AIN- inputs and connect-
ing them to the VREF+ and VREF- inputs
respectively. The converter then performs a con-
version and calculates the gain register value from
that conversion. Self calibrations are only valid in
the 1X gain range with a voltage reference of 1 to
2.5 V. A self calibration of offset is possible in the
other gain ranges by using the Input Short (IS) bit
in the Configuration Register. This bit can be set to
‘1’, and a system offset calibration can be per-
formed on the appropriate channel. The IS bit must
be set back to ‘0’ for normal operation of the con-
verter.
System calibrations rely on the correct voltage lev-
els being applied to the voltage inputs during the
calibration operation. For a system offset calibra-
tion, the desired “zero” point should be applied to
the AIN+ and AIN- inputs before the calibration
command is sent and throughout the calibration
process. Typically, this point is zero volts, but the
converters can calibrate out ±100% of the nominal
input range in bipolar mode, and ±90% of the nom-
inal input range in unipolar mode. During a system
gain calibration, the desired full-scale signal should
be applied to the voltage inputs of the converter.
The CS5531/32/33/34 can calibrate the gain slope
with input voltages that are anywhere between 3%
and 110% of the nominal full-scale voltage.
How accurate is the converter without cali-
bration?
The converter’s gain settings are not factory
trimmed, so if the converter is not calibrated, the
absolute gain accuracy is typically ±1%. The track-
ing error between the different PGIA gain settings
(2X - 64X) is typically about ±0.3%. If absolute ac-
curacy is required, the converter should be calibrat-
ed for both offset and gain in the specific ranges
where it is needed.
What are the advantages of using the on-chip
calibration registers?
The on-chip calibration registers allow the convert-
er to be easily interfaced to a simple, low-cost, 8-bit
microcontroller without a lot of software overhead.
The subtraction operation used by the offset regis-
ter and the multiplication operation used by the
gain register can both be performed inside the con-
verter for fast, precise results when using even a
very simple microcontroller. The internal registers
also provide the user an easy means to use a variety
of different calibration techniques for more accura-
cy.
Why is there no offset DAC in these convert-
ers?
The high dynamic range of the CS553x family of
ADCs eliminates the need for an offset DAC. The
offset register can perform the same function that
an offset DAC would normally do in other ADCs.
For example, a typical 2 mV/V bridge has a maxi-
mum output of 10 mV with a 5 V excitation supply.
Using the 64X gain range in unipolar mode, there
is still approximately 29 mV of headroom that can
accommodate sensor or system offsets. Performing
a system-level calibration or employing gain scal-
ing techniques allow the user to adjust the input
range of the converter to a 0 to 10 mV range after
removing any offset that is present. Using the 7.5
samples/s word rate, the dynamic range of these
converters allows them to still achieve 17 bits of