EVAL-AD7328CB【PDF 17/36 页】8-Channel, Software-Selectable True Bipo

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AD7328

Rev. A | Page 17 of 36

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Figure 30. Equivalent Analog Input Circuit (Differential)

Care should be taken to ensure that the analog input does

not exceed the V

and V

supply rails by more than 300 mV.

Exceeding this value causes the diodes to become forward

biased and to start conducting into either the V

supply rail

or V

supply rail. These diodes can conduct up to 10 mA

without causing irreversible damage to the part.

In Figure 29 and Figure 30, Capacitor C1 is typically 4 pF and

can primarily be attributed to pin capacitance. Resistor R1 is a

lumped component made up of the on-resistance of the input

multiplexer and the track-and-hold switch. Capacitor C2 is the

sampling capacitor; its capacitance varies depending on the

analog input range selected (see the Specifications section).

Track-and-Hold Section

The track-and-hold on the analog input of the AD7328 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 AD7328

can handle frequencies up to 22 MHz.

The track-and-hold enters its tracking mode on the 14

SCLK

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

an input signal depends on how quickly the sampling capacitor

is charged. With zero source impedance, 305 ns is sufficient to

acquire the signal to the 13-bit level. The acquisition time for

the ±10 V, ±5 V, and 0 V to +10 V ranges to settle to within

± LSB is typically 200 ns.

The acquisition time required is calculated using the following

formula:

ACQ

= 10 × ((

SOURCE

)

where

is the sampling capacitance, and

is the resistance

seen by the track-and-hold amplifier looking at the input.

For the AD7328, the value of R includes the on-resistance of the

input multiplexer and is typically 300 Ω. R

SOURCE

should include

any extra source impedance on the analog input.

The AD7328 enters track mode on the 14

SCLK rising edge.

When the AD7328 is run at a throughput rate of 1 MSPS with a

20 MHz SCLK signal, the ADC has approximately

1.5 SCLK +

QUIET

to acquire the analog input signal. The ADC goes back into

hold mode on the CS falling edge.

As the V

supply voltage is reduced, the on-resistance of

the input multiplexer increases. Therefore, based on the equation

for t

ACQ

, it is necessary to increase the acquisition time provided

to the AD7328 and hence decrease the overall throughput rate.

Figure 31 shows that if the throughput rate is reduced when

operating with minimum V

and V

supplies, the specified

THD performance is maintained.

–50

–95

±V

SUPPLIES (V)

–55

–60

–65

–70

–75

–80

–85

–90

500kSPS

750kSPS

1MSPS

= V

= 5V

INTERNAL REFERENCE

= 25°C

= 10kHz

±5V RANGE

SE MODE

Figure 31. THD vs. ±V

Supply Voltage at 500 kSPS, 750 kSPS, and 1 MSPS

Unlike other bipolar ADCs, the AD7328 does not have a

resistive analog input structure. On the AD7328, the bipolar

analog signal is sampled directly onto the sampling capacitor.

This gives the AD7328 high analog input impedance. The

analog input impedance can be calculated from the following

formula:

= 1/(

)

where

is the sampling frequency, and

is the sampling

capacitor value.

depends on the analog input range chosen (see the

Specifications section). When operating at 1 MSPS, the analog

input impedance is typically 75 kΩ for the ±10 V range. As the

sampling frequency is reduced, the analog input impedance

further increases. As the analog input impedance increases, the

current required to drive the analog input therefore decreases.

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