Rainbow-electronics MAX1401 Manual do Utilizador descarregar pdf (Página 24)

MAX1401

+3V, 18-Bit, Low-Power, Multichannel,

Oversampling (Sigma-Delta) ADC

24 ______________________________________________________________________________________

Input Buffers

The MAX1401 provides a pair of input buffers to isolate

the inputs from the capacitive load presented by the

PGA/modulator (Figure 6). The buffers are chopper sta-

bilized to reduce the effect of their DC offsets and low-

frequency noise. Since the buffers can represent more

than 50% of the total analog power dissipation, they

may be shut down in applications where minimum

power dissipation is required and the capacitive input

load is not a concern. Disable the buffers in applications

where the inputs must operate close to AGND or V+.

When used in buffered mode, the buffers isolate the

inputs from the sampling capacitors. The sampling-

related gain error is dramatically reduced in this mode.

A small dynamic load remains from the chopper stabi-

lization. The multiplexer exhibits a small input leakage

current of up to 10nA. With high source resistances,

this leakage current may result in a DC offset.

Table 13c. R

EXT

, C

EXT

Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0)

Mode; 4x Modulator Sampling Frequency (MF1, MF0 = 10 ); X2CLK = 0; f

CLKIN

2.4576MHz

Table 13d. R

EXT

, C

EXT

Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0)

Mode; 8x Modulator Sampling Frequency (MF1, MF0 = 11); X2CLK = 0; f

CLKIN

2.4576MHz

EXT

MUX

MUXOUT

ADCIN

AMP

SAMPLE

Figure 6. Analog Input, Buffered Mode (BUFF = 1)

7.0 3.4

7.0 3.4 2.32

5.5 3.0

3.8 2.4 1.7

8, 16, 32,

64, 128

2.1

2.3

EXT

= 0pF C

EXT

= 50pF C

EXT

= 100pF

0.71 0.40

0.71 0.40 0.11

0.66 0.38

0.60 0.34 0.09

0.10

PGA GAIN

0.11

EXT

= 500pF C

EXT

= 1000pF C

EXT

= 5000pF

EXTERNAL RESISTANCE, R

EXT

(kΩ)

3.4 1.7

3.4 1.7 1.12

2.7 1.4

1.8 1.2 0.85

8, 16, 32,

64, 128

1.0

1.1

EXT

= 0pF C

EXT

= 50pF C

EXT

= 100pF

0.35 0.20

0.35 0.20 0.05

0.32 0.18

0.29 0.17 0.04

0.05

PGA GAIN

0.05

EXT

= 500pF C

EXT

= 1000pF C

EXT

= 5000pF

EXTERNAL RESISTANCE, R

EXT

(kΩ)

1 2 ... 19 20 21 22 23 24 25 26 27 28 29 ... 35 36

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