EVAL-AD7888CB(1999) Просмотр технического описания (PDF) - Analog Devices

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EVAL-AD7888CB
(Rev.:1999)

+2.7 V to +5.25 V, Micropower, 8-Channel, 125 kSPS, 12-Bit ADC in 16-Lead TSSOP

Analog Devices 

AD7888

TYPICAL CONNECTION DIAGRAM

Figure 8 shows a typical connection diagram for the AD7888.

Both AGND pins are connected to the analog ground plane of

the system. VREF is connected to a well decoupled VDD pin to

provide an analog input range of 0 V to VDD. The conversion

result is output in a 16-bit word with four leading zeroes fol-

lowed by the MSB of the 12-bit result. For applications where

power consumption is of concern, the automatic power down at

the end of conversion should be used to improve power perfor-

mance. See Modes of Operation section of the data sheet.

SUPPLY +2.7V

TO +5.25V

10␮F

0V TO

REF IN/

REF OUT

INPUT

0.1␮F

VDD REF IN/

REF OUT

AD7888

AIN1

AIN2

SCLK

DOUT

SERIAL

INTERFACE

AIN8

DIN

AGND

CS

AGND

␮C/␮P

Figure 8. Typical Connection Diagram

Analog Input

Figure 9 shows an equivalent circuit of the analog input structure

of the AD7888. The two diodes D1 and D2 provide ESD pro-

tection for the analog inputs. Care must be taken to ensure that

the analog input signal never exceeds the supply rails by more

than 200 mV. This will cause these diodes to become forward-

biased and start conducting current into the substrate. 20 mA is

the maximum current these diodes can conduct without causing

irreversible damage to the part. However, it is worth noting that

a small amount of current (1 mA) being conducted into the

substrate due to an overvoltage on an unselected channel, can

cause inaccurate conversions on a selected channel. The capaci-

tor C1 in Figure 9 is typically about 4 pF and can primarily be

attributed to pin capacitance. The resistor R1 is a lumped com-

ponent made up of the on resistance of a multiplexer and a switch.

This resistor is typically about 100 Ω. The capacitor C2 is the

ADC sampling capacitor and has a capacitance of 20 pF typically.

Note: The analog input capacitance seen when the track and

hold is in track mode is typically 38 pF, while in hold mode it is

typically 4 pF.

VDD

D1

C2

R1

20pF

VIN

C1

4pF

D2

CONVERSION PHASE – SWITCH OPEN

TRACK PHASE – SWITCH CLOSED

Figure 9. Equivalent Analog Input Circuit

For ac applications, removing high frequency components from

the analog input signal is recommended by use of an RC low-

pass filter on the relevant analog input pin. In applications

where harmonic distortion and signal to noise ratio are critical

the analog input should be driven from a low impedance source.

Large source impedances will significantly affect the ac perfor-

mance of the ADC. This may necessitate the use of an input

buffer amplifier. The choice of the op amp will be a function of

the particular application.

When no amplifier is used to drive the analog input the source

impedance should be limited to low values. The maximum

source impedance will depend on the amount of total harmonic

distortion (THD) that can be tolerated. The THD will increase

as the source impedance increases and performance will degrade.

Figure 10 shows a graph of the total harmonic distortion versus

analog input signal frequency for different source impedances.

–65

THD vs. FREQUENCY FOR DIFFERENT

SOURCE IMPEDANCES

–70 VDD = +5V

5V EXT REFERENCE

RIN = 1k⍀, CIN = 100pF

–75

RIN = 50⍀, CIN = 2.2nF

–80

–85

–90

0.15

RIN = 10⍀, CIN = 10nF

10.89

21.14

31.59

INPUT FREQUENCY – kHz

42.14

49.86

Figure 10. THD vs. Analog Input Frequency

Analog Input Selection

On power-up, the default AIN selection is AIN1. When return-

ing to normal operation from power-down, the AIN selected

will be the same one that was selected prior to power-down

being initiated. Table II below shows the multiplexer address

corresponding to each analog input from AIN1 to AIN8 for the

AD7888.

ADD2

0

0

0

0

1

1

1

1

Table II. Channel Configurations

ADD1

0

0

1

1

0

0

1

1

ADD0

0

1

0

1

0

1

0

1

Analog Input Channel

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

AIN7

AIN8

On-Chip Reference

The AD7888 has an on-chip 2.5 V reference. This reference can

be enabled or disabled by clearing or setting the REF bit in the

Control Register, respectively. If the on-chip reference is to be

used externally in a system, it must be buffered before it is ap-

plied elsewhere. If an external reference is applied to the device,

the internal reference is automatically overdriven. However, in

REV. 0

–9–

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