Rainbow-electronics MAX1400 Bedienungsanleitung PDF herunterladen (Seite 22)

MAX1400

+5V, 18-Bit, Low-Power, Multichannel,

Oversampling (Sigma-Delta) ADC

22 ______________________________________________________________________________________

Input Buffers

The MAX1400 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)

9.3 4.5

9.3 4.5 3.02

7.2 3.9

5.0 3.1 2.3

8, 16, 32,

64, 128

2.7

3.0

EXT

= 0pF C

EXT

= 50pF C

EXT

= 100pF

0.94 0.53

0.94 0.53 0.14

0.87 0.50

0.79 0.45 0.12

0.13

PGA GAIN

0.14

EXT

= 500pF C

EXT

= 1000pF C

EXT

= 5000pF

EXTERNAL RESISTANCE R

EXT

(kΩ)

4.5 2.2

4.5 2.2 1.52

3.5 1.9

2.4 1.5 1.1

8, 16, 32,

64, 128

1.3

1.5

EXT

= 0pF C

EXT

= 50pF C

EXT

= 100pF

0.46 0.26

0.46 0.26 0.069

0.42 0.24

0.38 0.22 0.059

0.069

PGA GAIN

0.069

EXT

= 500pF C

EXT

= 1000pF C

EXT

= 5000pF

EXTERNAL RESISTANCE R

EXT

(kΩ)

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 ... 35 36

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