Maxim MX7705 Manual do Utilizador descarregar pdf (Página 19)

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 one and V

REF

= 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 (V

REF

/ 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 V

REF

= V

REF+

- V

REF-.

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- =

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- =

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 MX7705 is in buffered mode, the absolute and

common-mode analog input voltage ranges reduce to

between (GND + 50mV) and (V

- 1.5V). The differential

input voltage range is not affected in buffered mode.

65 536

(, )

LSB

GAIN

REF

=×

65 536

(, )

LSB

GAIN

REF

MX7705

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

Sigma-Delta ADC

______________________________________________________________________________________ 19

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 =

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

REF

/ GAIN

REF

/ GAIN

BINARY OUTPUT CODE

Figure 4. MX7705 Unipolar Transfer Function

1 LSB =

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)

REF

/ GAIN V

REF

/ GAIN

REF

/ GAINV

REF

/ GAIN

Figure 5. MX7705 Bipolar Transfer Function

1 2 ... 14 15 16 17 18 19 20 21 22 23 24 ... 32 33

Sigma-Delta ADC 1

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