EVAL-CONTROLBRD22【PDF 11/18 页】Differential Input, Dual 2 MSPS, 12-Bit,

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Preliminary Technical Data

AD7322

VDD

Vin0

VSS

Figure 10. Equivalent Analog Input Circuit-(Single Ended)

Figure 10

VDD

Vin+

VSS

VDD

Vin-

VSS

Figure 11. Equivalent Analog Input Circuit-(Differential)

Figure 11

Care should be taken to ensure the Analog Input never exceeds

the V

and V

supply rails by more than 300 mV. This will

cause the diodes to become forward biased and start

conducting into either the V

or V

rails. These diodes can

conduct up to 10 mA without causing irreversible damage to

the part.

The Capacitor C1, in

and can primarily be attributed to pin capacitance. The resistor

R1, is a lumped component made up of the on-resistance of the

input multiplexer and the track-and-hold switch. The Capacitor

C2, is the sampling capacitor, its capacitance will vary

depending on the Analog input range selected.

and

is typically 4 pF

Track-and-Hold Section

The Track-and-Hold on the Analog Input of the AD7322 allows

the ADC to accurately convert an input sine wave of full scale

amplitude to 13-Bit accuracy. The input bandwidth of the

Track-and-Hold is greater than the Nyquist rate of the ADC ,

the AD7322 can handle frequencies up to 13 MHz.

The Track-and-Hold enters its tracking mode on the 15

SCLK

falling edge after the CS falling edge. The time required to

acquire an input signal will depend on how quickly the

sampling capacitor is charged. With zero source impedance 300

ns will be sufficient to acquire the signal to the 13-bit level.

The acquisition time required is calculated using the following

formula:

ACQ

= 10 x ((R

SOURCE

+ R) C)

where C is the Sampling Capacitance and R is the resistance

seen by the track-and-hold amplifier looking back on the input.

For the AD7322, the value of R will include the on-resistance of

the input multiplexer. The value of R is typically 300

. R

SOURCE

should include any extra source impedance on the Analog

input.

TYPICAL CONNECTION DIAGRAM

Figure 12

In this configuration the AGND pin is connected to the Analog

ground plane of the system. The DGND pin is connected to the

Digital ground plane of the system. The Analog Inputs on the

AD7322 can be configured to operate in Single Ended, True

Differential or Pseudo Differential Mode. The AD7322 can

operate with either the internal or an external reference. In

, the AD7322 is configured to operate with the internal

2.5V reference. A 470 nF decoupling capacitor is required when

operating with the internal reference.

Figure 12

Figure 12. Typical Connection Diagram

shows a typical connection diagram for the AD7322.

The V

pin can be connected to either a 3V or a 5V supply

voltage. The V

and V

are the dual supplies for the high

voltage analog input structures. The voltage on these pins must

be equal to or greater than the highest analog input range

selected on the analog input channels, see

information. The V

DRIVE

pin is connected to the supply voltage

of the microprocessor. The voltage applied to the V

DRIVE

input

controls the voltage at which the serial interface operates.

for more

Table 5

Rev. PrE | Page 11 of 18

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