Rainbow Electronics MAX1139 - Manual (Page 18)

MAX1136–MAX1139
10ms prior to conversion or powered continuously.
Wake-up is invisible when using an external reference or
V
DD as the reference.
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 MAX1137 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).
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 on AIN_/REF will be ignored by internal lim-
iter, which sets the highest available channel at AIN2 or
AIN10.
Internal Reference
The internal reference is 4.096V for the MAX1136/
MAX1138 and 2.048V for the MAX1137/MAX1139. 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] =
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.
External Reference
The external reference can range from 1V to V DD . For
maximum conversion accuracy, the reference must be
able to deliver up to 40µA and have an output imped-
ance of 500
Ω or less. If the reference has a higher out-
put impedance or is noisy, bypass it to GND as close to
AIN_/REF as possible with a 0.1µF capacitor.
Transfer Functions
Output data coding for the MAX1136–MAX1139 is bina-
ry in unipolar mode and two’s complement in bipolar
mode with 1LSB = (V
REF /2N) where ‘N’ is the number of
bits (10). Code transitions occur halfway between suc-
cessive-integer LSB values. Figure 12 and Figure 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
2.7V to 3.6V and 4.5V to 5.5V, Low-Power,
4-/12-Channel, 2-Wire Serial 10-Bit ADCs
18 ______________________________________________________________________________________
SEL2 SEL1 SEL0 REFERENCE VOLTAGE AIN_/REF
INTERNAL REFERENCE
STATE
0 0 X V DD Analog Input Always Off
0 1 X External Reference Reference Input Always Off
1 0 0 Internal Reference Analog Input Always Off
1 0 1 Internal Reference Analog Input Always On
1 1 0 Internal Reference Reference Output Always Off
1 1 1 Internal Reference Reference Output Always On
Table 6. Reference Voltage and AIN_/REF Format
111...111
OUTPUT CODE
FS = REF + GND
ZS = GND
FULL-SCALE
TRANSITION
111...110
100...010
100...001
100...000
011...111
011...110
011...101
000...001
000...000
0 1
512
INPUT VOLTAGE (LSB) (GND) FS -
1
LSB
2
1LSB =
V REF
1024
Figure 12. Unipolar Transfer Function