Using the buffered REFADJ input avoids external
buffering of the reference. To use the direct VREF input,
disable the internal buffer by tying REFADJ to V
DD
.
Transfer Function and Gain Adjust
Figure 15 depicts the nominal, unipolar input/output
(I/O) transfer function, and Figure 16 shows the bipolar
input/output transfer function. Code transitions occur
halfway between successive integer LSB values. Output
coding is binary with 1 LSB = 1.00mV (4.096V/4096) for
unipolar operation and 1 LSB = 1.00mV ((4.096V/2 -
-4.096V/2)/4096) for bipolar operation.
Figure 17, the MAX186 Reference-Adjust Circuit, shows
how to adjust the ADC gain in applications that use the
internal reference. The circuit provides ±1.5%
(±65LSBs) of gain adjustment range.
Layout, Grounding, Bypassing
For best performance, use printed circuit boards.
Wire-wrap boards are not recommended. Board layout
should ensure that digital and analog signal lines are
separated from each other. Do not run analog and digi-
tal (especially clock) lines parallel to one another, or
digital lines underneath the ADC package.
Figure 18 shows the recommended system ground
connections. A single-point analog ground (“star”
ground point) should be established at AGND, sepa-
rate from the logic ground. All other analog grounds
and DGND should be connected to this ground. No
other digital system ground should be connected to
this single-point analog ground. The ground return to
the power supply for this ground should be low imped-
ance and as short as possible for noise-free operation.
High-frequency noise in the V
DD
power supply may
affect the high-speed comparator in the ADC. Bypass
these supplies to the single-point analog ground with
0.1µF and 4.7µF bypass capacitors close to the
MAX186/MAX188. Minimize capacitor lead lengths for
best supply-noise rejection. If the +5V power supply is
very noisy, a 10Ω resistor can be connected as a low-
pass filter, as shown in Figure 18.
MAX186/MAX188
Low-Power, 8-Channel,
Serial 12-Bit ADCs
______________________________________________________________________________________ 19
OUTPUT CODE
FULL-SCALE
TRANSITION
11 . . . 111
11 . . . 110
11 . . . 101
00 . . . 011
00 . . . 010
00 . . . 001
00 . . . 000
123
0
FS
FS - 3/2LSB
FS = +4.096V
1LSB = FS
4096
INPUT VOLTAGE (LSBs)
011 . . . 111
011 . . . 110
000 . . . 010
000 . . . 001
000 . . . 000
111 . . . 111
111 . . . 110
111 . . . 101
100 . . . 001
100 . . . 000
-FS
0V
INPUT VOLTAGE (LSBs)
+FS - 1LSB
FS = +4.096
2
1LSB = +4.096
4096
+5V
510k
100k
24k
0.01µF
12
REFADJ
MAX186
Figure 17. MAX186 Reference-Adjust Circuit
Figure 15. MAX186/MAX188 Unipolar Transfer Function,
4.096V = Full Scale
Figure 16. MAX186/MAX188 Bipolar Transfer Function,
±4.096V/2 = Full Scale
