CS5102A-BL Cirrus Logic Inc, CS5102A-BL Datasheet - Page 26

• Manufacturer Part Number: CS5102A-BL
• Description: A/D Converter (A-D) IC
• Manufacturer: Cirrus Logic Inc

Specifications of CS5102A-BL

• Input Channels Per Adc: 2
• Mounting Type: Surface Mount
• No. Of Channels: 2
• Power Rating: 44mW
• Supply Voltage Min: 4.5V
• Peak Reflow Compatible (260 C): No
• Sample Rate: 20kSPS
• Supply Voltage Max: 5.5V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

7. CS5101A & CS5102A PERFORMANCE
7.1
The self-calibration scheme utilized in the
CS5101A and CS5102A features a calibration res-
olution of 1/4 LSB, or 18-bits. This ideally yields
DNL of 1/4 LSB, with code widths ranging from 3/4
to 5/4 LSBs.
Traditional laser-trimmed ADCs have significant
differential nonlinearities. Appearing as wide and
narrow codes, DNL often causes entire sections of
the transfer function to be missing. Although their
affect is minor on S/(N+D) with high amplitude sig-
nals, DNL errors dominate performance with low-
level signals. For instance, a signal 80 dB below
full-scale will slew past only 6 or 7 codes. Half of
those codes could be missing with a conventional
16-bit ADC which achieves only 14-bit DNL.
The most common source of DNL errors in con-
ventional ADCs is bit weight errors. These can
arise due to accuracy limitations in factory trim sta-
tions, thermal or physical stresses after calibration,
and/or drifts due to aging or temperature variations
in the field. Bit-weight errors have a drastic effect
on a converter's AC performance. They can be an-
alyzed as step functions superimposed on the in-
put signal. Since bits (and their errors) switch in
and out throughout the transfer curve, their effect
is signal dependent. That is, harmonic and inter-
modulation distortion, as well as noise, can vary
with different input conditions.
Differential nonlinearities in successive-approxi-
mation ADCs also arise due to dynamic errors in
the comparator. Such errors can dominate if the
converter's throughput/sampling rate is too high.
The comparator will not be allowed sufficient time
to settle during each bit decision in the successive-
approximation algorithm. The worst-case codes for
dynamic errors are the major transitions (1/2 FS;
1/4, 3/4 FS; etc.). Since DNL effects are most crit-
26
Differential Nonlinearity
ical with low-level signals, the codes around mid-
scale (1/2 FS) are most important. Yet those codes
are worst-case for dynamic DNL errors!
With all linearity calibration performed on-chip to
18-bits, the CS5101A and CS5102A maintain ac-
curate bit weights. DNL errors are dominated by
residual calibration errors of
dynamic errors in the comparator. Furthermore, all
DNL effects on S/(N+D) are buried by white broad-
band noise. (See Figures 19 and 21).
Figure 13 illustrates the DNL histogram plot of a
typical CS5101A at 25 °C. Figure 14 illustrates the
DNL of the CS5101A at 138 °C ambient after cali-
bration at 25 °C ambient. Figure 15 and Figure 16
illustrate the DNL of the CS5102A at 25 °C and
138 °C ambient, respectively. A histogram test is a
statistical method of deriving an A/D converter's
differential nonlinearity. A ramp is input to the A/D
and a large number of samples are taken to ensure
a high confidence level in the test's result. The
number of occurrences for each code is monitored
and stored. A perfect A/D converter would have all
codes of equal size and therefore equal numbers
of occurrences. In the histogram test a code with
the average number of occurrences will be consid-
ered ideal (DNL = 0). A code with more or less oc-
currences than average will appear as a DNL of
greater or less than zero LSB. A missing code has
zero occurrences, and will appear as a DNL of -1
LSB.
Figures 17 and 18 illustrate the code width distribu-
tion of the DNL plots shown in Figure 13 and
Figure 15 respectively. The DNL error distribution
plots indicate that the CS5101A and CS5102A cal-
ibrate the majority of their codes to tighter toler-
ance than the DNL plots in Figures 13 and 15
appear to indicate.
CS5101A CS5102A
1/4 LSB rather than
DS45F6