Rainbow Electronics MAX1109 - Manual (Page 18)

MAX1108/MAX1109
External Reference
To use an external reference, set bit 2 (I/EREF) and bit
1 (REFSHDN) of control byte to 0 and connect the
external reference (V
REF between 1V and V DD ) directly
at the REF pin. The DC input impedance at REF is
extremely high, consisting of leakage current only (typi-
cally 10nA). During a conversion, the reference must
be able to deliver up to 20µA average load current and
have an output impedance of 1k
Ω
or less at the conver-
sion clock frequency. If the reference has higher output
impedance or is noisy, bypass it close to the REF pin
with a 0.1µF capacitor. MAX1109 has an internal refer-
ence of +4.096V. To use the device with supply volt-
ages below 4.5V, external reference mode is required.
With an external reference voltage of less than +2.048V
(MAX1108) or +4.096V (MAX1109) at REF, the increase
in the ratio of the RMS noise to the LSB value (FS / 256)
results in performance degradation and decreased
dynamic range.
Internal Reference
To use the internal reference, set bit 2 (I/EREF) and bit 1
(REFSHDN) of the control byte to 1 and bypass REF with
a 1µF capacitor to ground. The internal reference can be
powered down after a conversion by setting bit 1 (REF-
SHDN) of the control byte to 0. When using the internal
reference, use MAX1108 and MAX1109 with supply volt-
age below 4.5V and above 4.5V, respectively.
Transfer Function
Table 4 shows the full-scale voltage ranges for unipolar
and bipolar modes. Figure 12a depicts the nominal,
unipolar I/O transfer function, and Figure 12b shows the
bipolar I/O transfer function. The zero scale is deter-
mined by the input selection setting and is either COM,
GND, or CH1.
Code transitions occur at integer LSB values. Output
coding is straight binary for unipolar operation and
two’s complement for bipolar operation. With a +2.048V
reference, 1LSB = 8mV (V
REF / 256).
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 run
digital lines underneath the ADC package.
Figure 13 shows the recommended system-ground
connections. A single-point analog ground (star-ground
point) should be established at the A/D ground.
Connect all analog grounds to the star ground. No digi-
tal-system ground should be connected to this point.
The ground return to the power supply for the star
ground should be low impedance and as short as pos-
sible for noise-free operation.
High-frequency noise in the V DD power supply may
affect the comparator in the ADC. Bypass the supply to
the star ground with 0.1µF and 1µF capacitors close to
the V
DD pin of the MAX1108/MAX1109. Minimize
capacitor lead lengths for best supply-noise rejection. If
the power supply is very noisy, a 10
Ω
resistor can be
connected to form a lowpass filter.
Single-Supply, Low-Power,
2-Channel, Serial 8-Bit ADCs
18 ______________________________________________________________________________________
OUTPUT CODE
FULL-SCALE
TRANSITION
11111111
11111110
11111101
00000011
00000010
00000001
00000000
1 2 3 0 FS
FS - 1LSB
FS = V REF + COM
1LSB = V REF
256
INPUT VOLTAGE (LSB) (COM)
Figure 12a. Unipolar Transfer Function
01111111
OUTPUT CODE
01111110
00000010
00000001
00000000
11111111
11111110
11111101
10000001
10000000
-FS COM
INPUT VOLTAGE (LSB) +FS -
1
LSB
2
+FS =
V REF
+ COM
2
-FS =
-V REF
+ COM
2
COM =
V REF
2
1LSB =
V REF
256
Figure 12b. Bipolar Transfer Function