MAX1246_07 Datasheet, PDF (20/25 Pages) Maxim Integrated Products – 2.7V, Low-Power, 4-Channel, Serial 12-Bit ADCs in QSOP-16

+2.7V, Low-Power, 4-Channel,
Serial 12-Bit ADCs in QSOP-16
At VREF, the DC input resistance is a minimum of 18kΩ.
During conversion, an external reference at VREF must
deliver up to 350µA DC load current and have 10Ω or
less output impedance. If the reference has a higher
output impedance or is noisy, bypass it close to the
VREF pin with a 4.7µF capacitor.
Using the REFADJ input makes buffering the external
reference unnecessary. To use the direct VREF input,
disable the internal buffer by tying REFADJ to V_DD. In
power-down, the input bias current to REFADJ can be
as much as 25µA with REFADJ tied to V_DD. Pull
REFADJ to AGND to minimize the input bias current in
power-down.
Transfer Function
Table 7 shows the full-scale voltage ranges for unipolar
and bipolar modes.
The external reference must have a temperature coeffi-
cient of 4ppm/°C or less to achieve accuracy to within
1LSB over the 0°C to +70°C commercial temperature
range.
Figure 16 depicts the nominal, unipolar input/output
(I/O) transfer function, and Figure 17 shows the bipolar
input/output transfer function. Code transitions occur
halfway between successive-integer LSB values.
Output coding is binary, with 1LSB = 610µV (2.5V /
4096) for unipolar operation, and 1LSB = 610µV [(2.5V /
2 - -2.5V / 2) / 4096] for bipolar operation.
Layout, Grounding, and 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. Establish a single-point analog ground
(star ground point) at AGND, separate from the logic
ground. Connect all other analog grounds and DGND
to the star ground. No other digital system ground
should be connected to this ground. For lowest-noise
operation, the ground return to the star ground’s power
Table 7. Full Scale and Zero Scale
UNIPOLAR MODE
Full Scale
Zero Scale
VREF + COM
COM
OUTPUT CODE
11 . . . 111
11 . . . 110
11 . . . 101
FULL-SCALE
TRANSITION
00 . . . 011
00 . . . 010
00 . . . 001
00 . . . 000
01
(COM)
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INPUT VOLTAGE (LSB)
FS = VREF + COM
ZS = COM
1LSB = ^VREF
4096
FS
FS - 3/2LSB
Figure 16. Unipolar Transfer Function, Full Scale (FS) = VREF
+ COM, Zero Scale (ZS) = COM
supply should be low impedance and as short as
possible.
High-frequency noise in the V_DD power supply may
affect the high-speed comparator in the ADC. Bypass
the supply to the star ground with 0.1µF and 1µF
capacitors close to pin 1 of the MAX1246/MAX1247.
Minimize capacitor lead lengths for best supply-noise
rejection. If the power supply is very noisy, a 10Ω resis-
tor can be connected as a lowpass filter (Figure 18).
High-Speed Digital Interfacing with QSPI
The MAX1246/MAX1247 can interface with QSPI using
the circuit in Figure 19 (f_SCLK = 2.0MHz, CPOL = 0,
CPHA = 0). This QSPI circuit can be programmed to do a
conversion on each of the four channels. The result is
stored in memory without taxing the CPU, since QSPI
incorporates its own microsequencer.
The MAX1246/MAX1247 are QSPI compatible up to its
maximum external clock frequency of 2MHz.
Positive
Full Scale
VREF / 2
+ COM
BIPOLAR MODE
Zero
Scale
COM
Negative
Full Scale
-VREF / 2
+ COM
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