MAX1415 Datasheet, PDF (21/36 Pages) Maxim Integrated Products – 16-Bit, Low-Power, 2-Channel, Sigma-Delta ADCs

16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADCs
Table 5. Input Sampling Capacitor vs. Gain
GAIN
1
2
4
8–128
INPUT SAMPLING CAPACITOR (C_SAMP)
(pF)
3.75
7.5
15
30
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:
1 LSB =
V_REF
GAIN (65,536)
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:
1 LSB =
V_REF
×2
GAIN (65,536)
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.
1111 1111 1111 1111
1111 1111 1111 1110
1111 1111 1111 1101
1111 1111 1111 1100
VREF/GAIN
FULL-SCALE TRANSITION
1 LSB =
VREF
(GAIN) (65,536)
0000 0000 0000 0011
0000 0000 0000 0010
0000 0000 0000 0001
0000 0000 0000 0000
0
12 3
65,533
DIFFERENTIAL INPUT VOLTAGE (LSB)
65,535
Figure 4. MAX1415/MAX1416 Unipolar Transfer Function
1111 1111 1111 1111
1111 1111 1111 1110
1111 1111 1111 1101
1000 0000 0000 0001
1000 0000 0000 0000
0111 1111 1111 1111
VREF/GAIN
1 LSB =
VREF
x2
(GAIN) (65,536)
VREF/GAIN
0000 0000 0000 0010
0000 0000 0000 0001
0000 0000 0000 0000
-32,768
-32,766
-1 0 +1
+32,765 +32,767
DIFFERENTIAL INPUT VOLTAGE (LSB)
Figure 5. MAX1415/MAX1416 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.
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