CS5505/6/7/8
CS5505/6/7/8
Understanding Converter Calibration
8000H (16-bit) or 80000H (20-bit) and multi-
plies the LSB size by two. This means that the
bipolar measurement range is not calibrated from
full scale positive to full scale negative. Instead
it is calibrated from the bipolar zero scale point
to full scale positive. The slope factor is then
extended below bipolar zero to accommodate the
negative input signals. The converter can be
used to convert both unipolar and bipolar signals
by changing the BP/UP pin. Recalibration is not
required when switching between unipolar and
bipolar modes.
Calibration can be performed at any time. A
calibration sequence will minimize offset errors
and set the gain slope scale factor. The delta-
sigma modulator in the converter is a differential
modulator. To calibrate out offset error, the
converter internally connects the modulator dif-
ferential inputs to an internal VREF- voltage and
measures the 1’s density output from the modu-
lator. It stores the digital code representation for
this 1’s density in SRAM and remembers this
code as being the zero scale point for the A/D
conversion. The converter then connects the
negative modulator differential input to the
VREF- input and the positive modulator differ-
ential input to the VREF+ voltage. The 1’s
density output from the modulator is then re-
corded. The converter uses the digital
representation of this 1’s density along with the
digital code for the zero scale point and calcu-
lates a gain scale factor. The gain scale factor is
stored in SRAM and used for calculating the
proper output codes during conversions.
Converter Performance
The CS5505/6/7/8 A/D converters have excellent
linearity performance. Calibration minimizes the
errors in offset and gain. The CS5505/7 devices
have no missing code performance to 16-bits.
The CS5506/8 devices have no missing code
performance to 20-bits. Figure 7 illustrates the
DNL of the 16-bit CS5505. The converters
achieve Common Mode Rejection (CMR) at dc
of 105 dB typical, and CMR at 50 and 60 Hz of
120 dB typical.
The states of A0, A1 and BP/UP are ignored
during calibration but should remain stable
throughout the calibration period to minimize
noise.
The CS5505/6/7/8 can experience some drift as
temperature changes. The CS5505/6/7/8 use
chopper-stabilized techniques to minimize drift.
Measurement errors due to offset or gain drift
can be eliminated at any time by recalibrating
the converter.
When conversions are performed in unipolar
mode or in bipolar mode, the converter uses the
same calibration factors to compute the digital
output code. The only difference is that in bipo-
lar mode the on-chip microcontroller offsets the
computed output word by a code value of
+1
+1/2
0
-1/2
-1
0
32,768
65,535
Codes
Figure 7. CS5505 Differential Nonlinearity plot.
DDSS599FF45
1177
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