MX7705 datasheet(19/33 Pages) MAXIM

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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 VREF = VREF+ - VREF-.

For a gain of one and VREF = 2.5V, the full-scale volt-

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 (VREF / GAIN) and 1 LSB

≈ 9.5µV. The differen-

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 VREF = VREF+ - VREF-.

Unipolar and Bipolar Modes

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- =

VREF / GAIN, where VREF = VREF+ - VREF-.

In bipolar mode, the digital output code is in offset

binary. Positive full scale is given by AIN+ - AIN- =

+VREF / GAIN and negative full scale is given by AIN+ -

AIN- = -VREF / GAIN. When AIN+ = AIN-, the outputs

are at zero scale, which is the midpoint of the bipolar

transfer function.

When the MX7705 is in buffered mode, the absolute and

common-mode analog input voltage ranges reduce to

between (GND + 50mV) and (VDD - 1.5V). The differential

input voltage range is not affected in buffered mode.

65 536

)

LSB

GAIN

REF

=×

65 536

)

LSB

GAIN

REF

16-Bit, Low-Power, 2-Channel,

Sigma-Delta ADC

______________________________________________________________________________________

Table 5. Input Sampling Capacitor vs. Gain

GAIN

INPUT SAMPLING CAPACITOR (CSAMP) (pF)

3.75

7.5

415

8–128

0000 0000 0000 0000

0000 0000 0000 0001

0000 0000 0000 0010

0000 0000 0000 0011

FULL-SCALE TRANSITION

1 LSB =

VREF

(GAIN) (65,536)

65,535

65,533

DIFFERENTIAL INPUT VOLTAGE (LSB)

1111 1111 1111 1101

1111 1111 1111 1110

1111 1111 1111 1111

1111 1111 1111 1100

VREF / GAIN

Figure 4. MX7705 Unipolar Transfer Function

1 LSB =

VREF

(GAIN) (65,536)

0+1

-1

0000 0000 0000 0000

0000 0000 0000 0001

0000 0000 0000 0010

+32,767

+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)

VREF / GAIN

Figure 5. MX7705 Bipolar Transfer Function

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