Rainbow Electronics MAX11645 - Manual (Page 19)

MAX11644/MAX11645
Low-Power, 1-/2-Channel, I
2
C, 12-Bit ADCs
in Ultra-Tiny 1.9mm x 2.2mm Package
______________________________________________________________________________________ 19
Transfer Functions
Output data coding for the MAX11644/MAX11645 is
binary in unipolar mode and two’s complement in bipo-
lar mode with 1 LSB = (V
REF /2 N ) where N is the number
of bits (12). Code transitions occur halfway between
successive-integer LSB values. Figures 12 and 13
show the input/output (I/O) transfer functions for unipo-
lar and bipolar operations, respectively.
Layout, Grounding, and Bypassing
Only use PCBs. Wire-wrap configurations are not rec-
ommended since the layout should ensure proper sep-
aration of analog and digital traces. Do not run analog
and digital lines parallel to each other, and do not lay-
out digital signal paths underneath the ADC package.
Use separate analog and digital PCB ground sections
with only one star point (Figure 14) connecting the two
ground systems (analog and digital). For lowest noise
operation, ensure the ground return to the star ground’s
power supply is low impedance and as short as possi-
ble. Route digital signals far away from sensitive analog
and reference inputs.
High-frequency noise in the power supply (V DD ) could
influence the proper operation of the ADC’s fast compara-
tor. Bypass V
DD to the star ground with a network of two
parallel capacitors, 0.1μF and 4.7μF, located as close as
possible to the MAX11644/MAX11645 power-supply pin.
Minimize capacitor lead length for best supply noise
rejection, and add an attenuation resistor (5
Ω) in series
with the power supply if it is extremely noisy.
Definitions
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the values
on an actual transfer function from a straight line. This
straight line can be either a best straight-line fit or a line
drawn between the endpoints of the transfer function,
once offset and gain errors have been nullified. The
MAX11644/MAX11645’s INL is measured using the
endpoint.
Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
an actual step width and the ideal value of 1 LSB. A
DNL error specification of less than 1 LSB guarantees
no missing codes and a monotonic transfer function.
Aperture Jitter
Aperture jitter (t AJ ) is the sample-to-sample variation in
the time between the samples.
Aperture Delay
Aperture delay (t AD ) is the time between the falling
edge of the sampling clock and the instant when an
actual sample is taken.
MAX11644
MAX11645
OUTPUT CODE
FULL-SCALE
TRANSITION 11 . . . 111
11 . . . 110
11 . . . 101
00 . . . 011
00 . . . 010
00 . . . 001
00 . . . 000
1 2 3 0 FS
FS - 3/2 LSB
FS = V REF
ZS = GND
INPUT VOLTAGE (LSB)
1 LSB =
V REF
4096
Figure 12. Unipolar Transfer Function
011 . . . 111
011 . . . 110
000 . . . 010
000 . . . 001
000 . . . 000
111 . . . 111
111 . . . 110
111 . . . 101
100 . . . 001
100 . . . 000
- FS 0
INPUT VOLTAGE (LSB)
OUTPUT CODE
ZS = 0
+FS - 1 LSB
FS =
V REF
2
-FS =
-V REF
2
MAX11644
MAX11645
1 LSB =
V REF
4096
Figure 13. Bipolar Transfer Function