Datasheet

Increasing the gain increases the resolution of the ADC
(LSB size decreases), but reduces the differential input
voltage range. Calculate 1 LSB in unipolar mode using
the following equation:
where: V
REF
= V
REF+
- V
REF-.
For a gain of 1 and V
REF
= 2.5V, the full-scale voltage
in unipolar mode is 2.5V and 1 LSB ≈ 38.1µV. For a
gain of 4, the full-scale voltage in unipolar mode is
0.625V (V
REF
/GAIN) and 1 LSB ≈ 9.5µV. The differential
input voltage range in this example reduces from 2.5V
to 0.625V, and the resolution increases since the LSB
size decreases from 38.1µV to 9.5µV.
Calculate 1 LSB in bipolar mode using the following
equation:
where: V
REF
= V
REF+
- V
REF-.
Unipolar and Bipolar Modes
The B/U bit in the setup register (Table 9) configures
the MAX1415/MAX1416 for unipolar or bipolar transfer
functions. Figures 4 and 5 illustrate the unipolar and
bipolar transfer functions, respectively.
In unipolar mode, the digital output code is straight
binary. When AIN+ = AIN-, the outputs are at zero
scale, which is the lower endpoint of the transfer func-
tion. The full-scale endpoint is given by AIN+ - AIN- =
V
REF
/ GAIN, where V
REF
= V
REF+
- V
REF-
.
In bipolar mode, the digital output code is in offset
binary. Positive full scale is given by AIN+ - AIN- =
+V
REF
/ GAIN and negative full scale is given by AIN+ -
AIN- = -V
REF
/ GAIN. When AIN+ = AIN-, the outputs
are at zero scale, which is the midpoint of the bipolar
transfer function.
When the MAX1415/MAX1416 are in buffered mode, the
absolute and common-mode analog input voltage
ranges reduce to between (GND + 50mV) and (V
DD
-
1.5V). The differential input voltage range is not affected
in buffered mode.
1
65 536
2
(, )
LSB
V
GAIN
REF
=×
1
65 536 (, )
LSB
V
GAIN
REF
=
MAX1415/MAX1416
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADCs
______________________________________________________________________________________ 21
Table 5. Input Sampling Capacitor vs. Gain
GAIN
INPUT SAMPLING CAPACITOR (C
SAMP
)
(pF)
1 3.75
2 7.5
415
8–128 30
0123
0000 0000 0000 0000
0000 0000 0000 0001
0000 0000 0000 0010
0000 0000 0000 0011
FULL-SCALE TRANSITION
1 LSB =
V
REF
(GAIN) (65,536)
65,53565,533
DIFFERENTIAL INPUT VOLTAGE (LSB)
1111 1111 1111 1101
1111 1111 1111 1110
1111 1111 1111 1111
1111 1111 1111 1100
V
REF
/GAIN
V
REF
/GAIN
BINARY OUTPUT CODE
Figure 4. MAX1415/MAX1416 Unipolar Transfer Function
1 LSB =
V
REF
(GAIN) (65,536)
0+1-1
0000 0000 0000 0000
0000 0000 0000 0001
0000 0000 0000 0010
+32,767
BINARY OUTPUT CODE
+32,765
1111 1111 1111 1101
1111 1111 1111 1110
1111 1111 1111 1111
1000 0000 0000 0000
1000 0000 0000 0001
0111 1111 1111 1111
x 2
-32,768 -32,766
DIFFERENTIAL INPUT VOLTAGE (LSB)
V
REF
/GAIN V
REF
/GAIN
V
REF
/GAINV
REF
/GAIN
Figure 5. MAX1415/MAX1416 Bipolar Transfer Function