AD7450ARM datasheet(13/24 Pages) AD

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PRELIMINARY TECHNICAL DATA

–13–

AD7450

TITLE

Figure 8. Input Common Mode Range (CM) versus VREF

(Vdd = 5V and VREF (max) = 2.5V)

-1

0.75

1.25

1.5

1.75

2.25

2.5

VREF

2.8

0.9

Figure 9. Input Common Mode Range versus VREF

(Vdd = 5V and VREF (max) = 2.5V) when Driving VIN+ and VIN-

with the AD8138 Differential Amplifier

Figure 10 shows examples of the inputs to VIN+ and VIN-

for different values of VREF for VDD = 5 V. It also gives

the maximum and minimum common mode voltages for

each reference value according to figure 8.

Reference = 0.625 V (VREFmax/4)

0.625 V peak to peak

Common Mode (CM)

CMmin=0.275 V

CMmax=3.8 V

Reference = 1.25 V (VREFmax/2)

1.25 V peak to peak

Common Mode (CM)

CMmin=0.85 V

CMmax=3.55 V

Reference = 2.5 V (VREFmax)

2.5 V peak to peak

Common Mode (CM)

CMmin=2 V

CMmax=3 V

Figure 10. Examples of the Analog Inputs to VIN+ and VIN-

for Different Values of VREF for VDD = 5 V.

Analog Input Structure

Figure 11 shows the equivalent circuit of the analog input

structure of the AD7450.

The four diodes provide ESD

protection for the analog inputs.

Care must be taken to

ensure that the analog input signals never exceed the sup-

ply rails by more than 200mV.

This will cause these

diodes to become forward biased and start conducting into

the substrate.

These diodes can conduct up to 10mA with-

out causing irreversible damage to the part.

The capacitors C1, in figure 11 are typically 4pF and can

primarily be attributed to pin capacitance.

The resistors

are lumped components made up of the on-resistance of

the switches. The value of these resistors is typically about

100 .

The capacitors, C2, are the ADC’s sampling ca-

pacitors and have a capacitance of 16pF typically.

For ac applications, removing high frequency components

from the analog input signal is recommended by the use of

an RC low-pass filter on the relevant analog input pins.

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 performance of the ADC.

This may necessitate the use of an input buffer amplifier.

The choice of the opamp will be a function of the particu-

lar application.

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