MAX1236 Datasheet, PDF (18/22 Pages) Maxim Integrated Products – 2.7V to 3.6V and 4.5V to 5.5V, Low-Power, 4-/12-Channel, 2-Wire Serial, 12-Bit ADCs

2.7V to 3.6V and 4.5V to 5.5V, Low-Power,
4-/12-Channel, 2-Wire Serial, 12-Bit ADCs
Table 6. Reference Voltage and AIN_/REF Format
SEL2
0
0
1
1
1
1
SEL1
0
1
0
0
1
1
SEL0
X
X
0
1
0
1
REFERENCE VOLTAGE
V_DD
External Reference
Internal Reference
Internal Reference
Internal Reference
Internal Reference
AIN_/REF
Analog Input
Reference Input
Analog Input
Analog Input
Reference Output
Reference Output
INTERNAL REFERENCE
STATE
Always Off
Always Off
Always Off
Always On
Always Off
Always On
Automatic shutdown results in dramatic power savings,
particularly at slow conversion rates and with internal
clock. For example, at a conversion rate of 10ksps, the
average supply current for the MAX1237 is 60µA (typ) and
drops to 6µA (typ) at 1ksps. At 0.1ksps the average sup-
ply current is just 1µA, or a minuscule 3µW of power con-
sumption, see Average Supply Current vs. Conversion
Rate in the Typical Operating Characteristics section).
Reference Voltage
SEL[2:0] of the setup byte (Table 1) control the reference
and the AIN_/REF configuration (Table 6). When
AIN_/REF is configured to be a reference input or refer-
ence output (SEL1 = 1), differential conversions on
AIN_/REF appear as if AIN_/REF is connected to GND
(see Note 2 and Table 4). Single-ended conversion in
scan mode AIN_/REF is ignored by the internal limiter,
which sets the highest available channel at AIN2 or
AIN10.
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 PC boards. Wire-wrap configurations are not
recommended since the layout should ensure proper
separation of analog and digital traces. Do not run ana-
log and digital lines parallel to each other, and do not
layout digital signal paths underneath the ADC pack-
age. Use separate analog and digital PC board 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 possible. Route digital signals far away from sensi-
tive analog and reference inputs.
Internal Reference
The internal reference is 4.096V for the MAX1236/
MAX1238 and 2.048V for the MAX1237/MAX1239. SEL1 of
the setup byte controls whether AIN_/REF is used for an
analog input or a reference (Table 6). When AIN_/REF is
configured to be an internal reference output (SEL[2:1] =
OUTPUT CODE
111...111
111...110
FS = REF + GND
ZS = GND
FULL-SCALE
TRANSITION
11), decouple AIN_/REF to GND with a 0.1µF capacitor.
Once powered up, the reference always remains on until
reconfigured. The reference should not be used to supply
current for external circuitry.
100...010
100...001
100...000
1
LSB
=
V_REF
4096
External Reference
The external reference can range from 1V to V_DD. For
maximum conversion accuracy, the reference must be
011...111
011...110
011...101
able to deliver up to 40µA and have an output imped-
ance of 500kΩ or less. If the reference has a higher out-
put impedance or is noisy, bypass it to GND as close to
000...001
000...000
AIN_/REF as possible with a 0.1µF capacitor.
Transfer Functions
Output data coding for the MAX1236–MAX1239 is bina-
ry in unipolar mode and two’s complement in bipolar
01
GND
2048
INPUT VOLTAGE (LSB)
Figure 12. Unipolar Transfer Function
FS
-
1
2
LSB
mode with 1 LSB = (V_REF / 2N) where “N” is the number
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