AD7450ARM datasheet(14/24 Pages) AD

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REV. PrJ

PRELIMINARY TECHNICAL DATA

–14–

AD7450

VDD

VIN+

VIN-

VDD

Figure 11. Equivalent Analog Input Circuit.

Conversion Phase - Switches Open

Track Phase - Switches Closed

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 12 shows a graph

of the THD versus analog input signal frequency for dif-

ferent source impedances.

TITLE

Figure 12.THD vs Analog Input Frequency for Various

Source Impedances TBD

Figure 13 shows a graph of THD versus analog input

frequency for VDD of 5V and 3V, while sampling at

1MHz and 833kHz with a SCLK of 18 MHz and 15MHz

respectively.

TIT LE

Figure 13.THD vs Analog Input Frequency for 3V and 5V

Supply Voltages TBD

DRIVING DIFFERENTIAL INPUTS

Differential operation requires that VIN+ and VIN- be si-

multaneously driven with two equal signals that are 180

out of phase.

The common mode must be set up exter-

nally and has a range which is determined by VREF, the

power supply and the particular amplifier used to drive the

analog inputs (see figure 8).

Differential modes of opera-

tion with either an ac or dc input, provide the best THD

performance over a wide frequency range.

Since not all

applications have a signal preconditioned for differential

operation, there is often a need to perform single ended to

differential conversion.

Differential Amplifier

An ideal method of applying dc differential drive to the

AD7450 is to use a differential amplifier such as the AD8138.

This part can be used as a single ended to differential

amplifier or as a differential to differential amplifier. In both

cases the analog input needs to be bipolar. It also provides

common mode level shifting and buffering of the bipolar

input signal. Figure 14 shows how the AD8138 can be used

as a single ended to differential amplifier.

The positive and

negative outputs of the AD8138 are connected to the respec-

tive inputs on the ADC via a pair of series resistors to

minimize the effects of switched capacitance on the front end

of the ADC. The RC low pass filter on each analog input is

recommended in ac applications to remove high frequency

components of the analog input. The architecture of the

AD8138 results in outputs that are very highly balanced over

a wide frequency range without requiring tightly matched

external components.

If the analog input source being used has no impedance then

all four resistors (Rg1, Rg2, Rf1, Rf2) should be the same. If

the source has a 50

impedance and a 50

termination for

example, the value of Rg2 should be increased by 25

balance this parallel impedance on the input and thus ensure

that both the positive and negative analog inputs have the

same gain (see figure 14).

The outputs of the amplifier are

perfectly matched, balanced differential outputs of identical

amplitude and are exactly 180

o out of phase.

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