mx7705 Maxim Integrated Products, Inc., mx7705 Datasheet - Page 19

• Manufacturer Part Number: mx7705
• Description: Mx7705 16-bit, Low-power, 2-channel, Sigma-delta Adc
• Manufacturer: Maxim Integrated Products, Inc.

Table 5. Input Sampling Capacitor vs. Gain
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
For a gain of one and V
age in unipolar mode is 2.5V and 1 LSB ≈ 38.1µV. For a
gain of four, the full-scale voltage in unipolar mode is
0.625V (V
tial input voltage range in this example reduces from
2.5V to 0.625V, and the resolution increases, since the
LSB size decreased from 38.1µV to 9.5µV.
Calculate 1 LSB in bipolar mode using the following
equation:
where V
The B/U bit in the setup register (Table 9) configures
the MX7705 for unipolar or bipolar transfer functions.
Figures 4 and 5 illustrate the unipolar and bipolar trans-
fer 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
In bipolar mode, the digital output code is in offset
binary. Positive full scale is given by AIN+ - AIN- =
+V
AIN- = -V
are at zero scale, which is the midpoint of the bipolar
transfer function.
REF
8–128
GAIN
REF
1
2
4
/ GAIN, where V
/ GAIN and negative full scale is given by AIN+ -
REF
REF
REF
REF
INPUT SAMPLING CAPACITOR (C
= V
= V
/ GAIN) and 1 LSB ≈ 9.5µV. The differen-
1
/ GAIN. When AIN+ = AIN-, the outputs
REF+
REF+
LSB
1
______________________________________________________________________________________
LSB
Unipolar and Bipolar Modes
=
- V
- V
REF
GAIN
=
REF-.
REF-.
REF
GAIN
= V
V
REF
( ,
65 536
REF+
= 2.5V, the full-scale volt-
V
3.75
7.5
REF
15
30
( ,
65 536
- V
)
REF-
×
)
2
.
SAMP
16-Bit, Low-Power, 2-Channel,
) (pF)
When the MX7705 is in buffered mode, the absolute and
common-mode analog input voltage ranges reduce to
between (GND + 50mV) and (V
input voltage range is not affected in buffered mode.
Figure 4. MX7705 Unipolar Transfer Function
Figure 5. MX7705 Bipolar Transfer Function
1111 1111 1111 1111
1111 1111 1111 1110
1111 1111 1111 1101
1111 1111 1111 1100
0000 0000 0000 0011
0000 0000 0000 0010
0000 0000 0000 0001
0000 0000 0000 0000
1111 1111 1111 1111
1111 1111 1111 1110
1111 1111 1111 1101
1000 0000 0000 0001
1000 0000 0000 0000
0111 1111 1111 1111
0000 0000 0000 0010
0000 0000 0000 0001
0000 0000 0000 0000
-32,768
Sigma-Delta ADC
0
1 LSB =
1
1 LSB =
-32,766
2
(GAIN) (65,536)
V
(GAIN) (65,536)
REF
3
/ GAIN
V
REF
V
REF
DIFFERENTIAL INPUT VOLTAGE (LSB)
DIFFERENTIAL INPUT VOLTAGE (LSB)
-1
x 2
V
FULL-SCALE TRANSITION
DD
REF
0
/ GAIN
+1
- 1.5V). The differential
V
REF
+32,765
65,533
/ GAIN
+32,767
65,535
19